1. Introduction to Magnetic Effects of Electric Current
The magnetic effect of electric current is the production of a magnetic field around a conductor when current flows through it. Electricity and magnetism are closely linked: a current always creates a magnetic field, and a changing magnetic field can create (induce) a current.
You already know that a magnet attracts iron and that a compass needle always settles in the north–south direction. The big discovery in this chapter is that a compass needle also turns when a wire carrying current is brought near it. In other words, electric current produces a magnetic field exactly like a magnet does.
Oersted’s Experiment (1820)
Hans Christian Oersted discovered this connection. In the classic experiment:
Set-up: a compass needle is placed on a table below a straight wire that runs north–south and is connected to a battery through a key.
Observation 1: when no current flows, the needle points along the wire in the usual north–south direction.
Observation 2: as soon as the key is closed and current flows, the compass needle deflects away from the north–south direction.
Observation 3: when the direction of current is reversed, the needle deflects in the opposite direction.
Conclusion: a current-carrying conductor produces a magnetic field around it. The direction of this field depends on the direction of the current.
This showed for the first time that electricity and magnetism are two sides of the same phenomenon electromagnetism. Today this principle runs almost every device around us.
Everyday applications of electromagnetism
- Electric bell, electric doorbell and electromagnetic cranes — use an electromagnet to attract a metal striker or to lift heavy scrap.
- Electric motor (fans, mixers, water pumps, washing machines, toys) — a current in a magnetic field produces motion.
- Electric generator and bicycle dynamo — motion in a magnetic field produces current (electromagnetic induction).
- Loudspeakers and headphones — a changing current makes a coil and diaphragm vibrate to produce sound.
- Relays and circuit breakers / MCBs — a small current switches a stronger circuit.
- MRI machines and transformers — magnetic fields used in medicine and power distribution (used in Class 10 only qualitatively).
2. Magnetic Field and Magnetic Field Lines
The magnetic field is the region around a magnet (or a current-carrying conductor) in which a magnetic force can be experienced. It is a vector quantity — it has both a magnitude (strength) and a direction at every point.
- Direction of the magnetic field: the direction in which the north pole of a freely suspended tiny compass needle points when placed at that point. By convention, field lines outside a bar magnet run from the north pole to the south pole.
- Strength of the magnetic field: stronger where the field lines are crowded together (for example, near the poles of a bar magnet) and weaker where they are spread apart.
Magnetic field lines are imaginary closed curves drawn to represent the magnetic field. The tangent to a field line at any point gives the direction of the magnetic field at that point, and the closeness (density) of the lines tells us about the field strength.
Properties of magnetic field lines
- They form continuous closed curves — they never begin or end at an isolated point.
- Outside a bar magnet they run from the north pole to the south pole.
- Inside the bar magnet they run from the south pole to the north pole, so the complete curve is closed.
- Two field lines never intersect. If they did, the tangent at the crossing point would give two different directions for the field at the same point, which is impossible.
- Closer field lines mean a stronger field; spread-out lines mean a weaker field.
- They are crowded near the poles (strongest field) and spread out far from the magnet (weak field).
The lines are imaginary — they only help us picture a real physical quantity, the magnetic field B. When you are asked to draw them, show cramped lines near the poles and wide, smooth, closed curves far away, with arrowheads that point from N to S outside the magnet.
3. Magnetic Field Due to a Current-Carrying Straight Conductor
When current flows through a straight wire, a magnetic field is set up all around the wire. Experiment (with a compass or iron filings) shows that the magnetic field lines around a straight conductor form concentric circles with the wire at their centre.
- The field lines are complete circles lying in planes perpendicular to the wire.
- The direction of the field (the sense of the circles) is given by the right-hand thumb rule.
- The field is stronger closer to the wire (circles crowded near the wire) and weaker far away.
- Qualitatively, the field strength (a) increases when the current is increased and (b) decreases as the distance from the wire increases. (These are learned qualitatively in Class 10, without the full formula.)
Hold the straight conductor in your right hand so that the thumb points in the direction of the conventional current. Then the curled fingers point in the direction of the magnetic field lines around the conductor.
Imagine gripping the wire in your right hand with the thumb along the current. Do not use your left hand — the rule works only with the right hand. For a horizontal wire, rotate the whole picture in your mind so that the thumb still lies along the current before you read off the field direction.
4. Magnetic Field Due to a Current-Carrying Circular Loop
If a straight wire is bent into a circular loop, the magnetic field lines around it bend too. The field lines pass through the loop, come out of one face, curve around the outside and enter the other face — like the field of a short bar magnet with two poles.
- Field at the centre: it is perpendicular to the plane of the loop at the centre. Its direction is given by the same right-hand grip: curl the fingers of your right hand in the direction of the current in the loop, and the extended thumb points in the direction of the magnetic field at the centre.
- The face of the loop from which field lines emerge (where the current appears anticlockwise) behaves like the north pole; the opposite face (current appears clockwise) behaves like the south pole.
- Effect of increasing the current: the magnetic field (and its strength) around the loop increases.
- Effect of increasing the number of turns: if the same wire is wound as a coil with several turns, each turn adds its field at the centre, so the total magnetic field is stronger. (This is the idea behind the solenoid of the next section.)
The magnetic field at the centre of a circular coil increases with the current and with the number of turns. If the radius of the loop is reduced, the field at the centre also becomes stronger (qualitatively), because the field lines are more crowded.
5. Solenoid and Its Magnetic Field
A solenoid is a coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder (helix). It is a long loop of wire, i.e., a current-carrying circular loop stretched along an axis.
When current flows through a solenoid, it produces a magnetic field whose field-line pattern is almost identical to that of a bar magnet of similar size.
- One end of the solenoid behaves as the north pole and the other as the south pole; the field-line pattern outside looks like that of a bar magnet.
- Inside the solenoid: the field lines are parallel and closely spaced (except near the ends), so the magnetic field inside a long solenoid is nearly uniform — same strength and direction over a large central region.
- Polarity by the right-hand rule: grip the solenoid with your right hand so that the fingers curl in the direction of the current; the thumb then points towards the north pole end of the solenoid.
- Alternative check (viewing a face): if the current appears anticlockwise when you look at a face, that face is the north pole; if it appears clockwise, that face is the south pole.
- The field is strengthened by increasing the number of turns or by increasing the current (qualitative).
The right-hand thumb rule and the right-hand grip are the same idea: fingers go along the current, thumb gives the field (or pole) direction. Students who use the left hand every time flip the polarity or field direction — always use the right hand.
6. Electromagnet
An electromagnet is a temporary magnet produced by a current-carrying coil wound around a soft iron core. It behaves as a strong magnet only while the current flows; when the current stops, the core loses its magnetism.
Construction
- A soft iron core (a bar or a horse-shoe shaped piece of soft iron).
- An insulated copper wire wound as many turns around the core (the coil).
- A battery (cell) and a switch (key) connected to the two ends of the coil.
Working
When the key is closed, current flows through the coil and the soft iron core becomes strongly magnetized, attracting nearby iron objects (nails, pins, scrap). When the key is opened, the current stops and the core almost completely loses its magnetism at once. This temporary magnetism is why we call it an electromagnet.
Soft iron is easily magnetized and easily demagnetized (it is strongly attracted but does not retain magnetism). Steel retains magnetism even after the current stops (it becomes a permanent magnet), so steel is not used for electromagnets. The core can also be switched on/off because of soft iron, which is the whole point of an electromagnet.
Factors that strengthen an electromagnet
- More turns in the coil: more turns of wire around the core give a stronger field.
- Larger current: increasing the current strengthens the magnetic field.
- Nature of the core: soft iron is a far better core than air, wood or copper; a soft iron core makes the magnet much stronger (it concentrates the field lines).
Applications of electromagnets
- Electric bell and electric doorbell — the electromagnet pulls a hammer onto the gong (see application examples).
- Lifting magnets (electromagnetic cranes) in scrap yards — a strong horse-shoe electromagnet lifts and drops heavy iron and steel scrap.
- Relays — a small current in the relay coil switches a large current in another circuit.
- Circuit breakers (MCBs) and magnetic locks, loudspeakers and some medical equipment also use electromagnets.
7. Force on a Current-Carrying Conductor in a Magnetic Field
A current-carrying conductor placed in a magnetic field experiences a force (it may be pushed sideways). The force appears because the magnetic field of the current interacts with the external magnetic field of the magnet.
- The direction of the force depends on the direction of the current and the direction of the magnetic field. Reversing either one reverses the force; reversing both reverses the force twice (back to the original side, but with reversed field and current both).
- The force is maximum when the conductor is placed perpendicular to the magnetic field.
- The force is zero when the conductor is parallel to the magnetic field (the two fields do not interact to push the wire sideways).
- The direction of the force is found with Fleming’s left-hand rule below.
Hold the thumb, forefinger (index finger) and middle finger of your left hand so that all three are mutually perpendicular. Then:
Forefinger → direction of the magnetic field (from N to S).
Middle finger → direction of the current (conventional, positive to negative through the conductor).
Thumb → direction of the force (or motion).
Take a conductor in which the current flows towards the east (right) and the magnetic field points into the page. With the left hand: forefinger into the page (field), middle finger right (current) → the thumb points up. So the conductor is pushed upward. Reverse the current and the conductor is pushed downward.
Fleming’s left-hand rule is used where a current in a field produces motion (the motor effect). Fleming’s right-hand rule is used where motion in a field produces current (the generator/induction effect). Keep this pairing fixed.
8. Electric Motor
An electric motor is a device that converts electrical energy into mechanical energy (motion).
Principle: a current-carrying coil placed in a magnetic field experiences a force and rotates. This is the motor effect (Fleming’s left-hand rule).
Main parts (construction)
- Armature: a rectangular coil (ABCD) wound around a soft-iron core, mounted on a rotating axle. The core concentrates the field and increases the force.
- Field magnet: a strong magnet (or horse-shoe magnet) that supplies the magnetic field in which the armature rotates.
- Split-ring commutator: two halves of a copper ring, insulated from each other and from the axle, fixed on the axle. It reverses the direction of current in the coil.
- Brushes: two carbon pieces that press against the rotating commutator and carry current from the battery into the coil.
- Battery (source of current) and connecting wires.
Working (step by step)
- Current enters the coil ABCD through the brush and commutator. Suppose the current flows D → C → B → A, so side BC carries current downward and side AD carries current upward.
- By Fleming’s left-hand rule, the two vertical sides lying in the field experience forces at right angles to the field: side AD is pushed out of the page and side BC is pushed into the page. The top and bottom sides are parallel to the field, so they feel no force.
- These two opposite forces form a couple (a turning pair of equal opposite forces) that makes the coil rotate about the axle.
- After the coil turns a half rotation, side BC and side AD have swapped their positions in front of the poles. At that instant the split halves of the commutator change contact: the half touching the right brush now touches the left brush. This reverses the direction of current in the coil.
- Once more Fleming’s left-hand rule gives forces that keep turning the coil in the same direction. The reversing of current after every half rotation is what makes the coil rotate continuously instead of coming to rest.
Without the split-ring commutator the coil would turn by about half a revolution and then be pulled back (it would oscillate about the vertical position). Reversing the current every half rotation keeps the turning direction the same, so the shaft spins continuously.
Applications of the electric motor
- Ceiling and table fans, exhaust fans
- Mixers, grinders, blenders, washing machines
- Water pumps, coolers
- Electric toys, toy cars
- Hair dryers, vacuum cleaners, and many other domestic appliances
9. Electromagnetic Induction
The production of an electric current in a closed circuit by a changing magnetic field (by relative motion of a magnet and a coil) is called electromagnetic induction. The current produced is called the induced current.
The dynamic experiment: connect the ends of a coil (a solenoid) to a sensitive galvanometer (G), and push a bar magnet towards the coil or pull it away. The galvanometer needle deflects, showing that a current is produced in the coil. Key observations:
- The current appears only while the magnet is moving — when the magnet is at rest the needle returns to zero.
- The deflection increases when the motion is faster and when a stronger magnet is used.
- Reversing the direction of motion (pull instead of push) reverses the direction of the induced current (needle swings the other way).
- The same effect is produced when the coil is moved and the magnet is held still — it is the relative motion that matters.
- Faraday discovered this effect in 1831 and used the same effect to light a small bulb.
The induced current exists only during the motion. A steady field does not induce a current in a stationary closed coil; it is the change in the magnetic field over time that produces the current.
Applications of electromagnetic induction
- The electric generator (dynamo) works on this principle.
- Induction cooktops heat cookware by induced currents.
- Electromagnetic braking and microphones (a vibrating diaphragm moves a coil near a magnet to generate a small signal).
Hold the thumb, forefinger and middle finger of your right hand mutually perpendicular. Then:
Thumb → direction of motion of the conductor.
Forefinger → direction of the magnetic field (N to S).
Middle finger → direction of the induced current.
10. Electric Generator
An electric generator is a device that converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction: a coil rotating in a magnetic field has a changing magnetic field through it, so a current is induced in it.
Main parts
- Armature: a rectangular coil (ABCD) mounted on an axle, placed between the poles of a strong field magnet (a U-shaped magnet, or an electromagnet).
- Field magnet: provides the magnetic field; the coil rotates inside it.
- Slip rings: two separate brass rings fixed on the axle, connected to the two ends of the coil.
- Carbon brushes: press against the slip rings and carry the induced current from the rotating rings to the stationary external circuit.
- External circuit: the appliance (e.g. a bulb) connected across the brushes.
Working (step by step)
- The axle is driven by some external agent (steam, falling water, wind, or a bicycle wheel), so the coil rotates in the magnetic field.
- As the coil turns, the magnetic field through it keeps changing, so an induced current flows through the coil by electromagnetic induction.
- The induced current reaches the slip rings through the coil ends, the brushes pick it up, and it flows through the external circuit, lighting the bulb.
- The direction of the induced current reverses after every half rotation of the coil, so the generator produces alternating current.
A generator turns mechanical energy into electrical energy (use Fleming’s right-hand rule). A motor turns electrical energy into mechanical energy (use Fleming’s left-hand rule). The two devices look similar in construction but work in opposite ways.
11. AC and DC
An electric current whose direction reverses periodically is called alternating current. The mains electricity in our homes is AC. In India it is supplied at 220 V, 50 Hz: the current reverses direction 100 times per second (two reversals per cycle).
A current that flows only in one direction is called direct current. It is produced by cells, batteries, solar cells and DC (dynamo) generators.
Why mains electricity is transmitted as AC
- AC can be stepped up to very high voltages for transmission and stepped down close to our homes using transformers. At high voltage and low current, energy lost as heat in the wires is much smaller. DC cannot be stepped up or down easily.
- AC generators (alternators) are simpler to build than DC generators.
Household supply in India: 220 V, 50 Hz AC. The frequency 50 Hz means the current completes 50 cycles each second.
12. Domestic Electric Circuits and Safety
In our homes, electrical energy is distributed through three wires: the live (or phase) wire, the neutral wire, and the earth wire. Potential difference between live and neutral is 220 V. The earth wire is connected to the ground (earth).
- All electric gadgets (bulbs, fans, sockets) are connected in parallel, so each gets the full main voltage (220 V) and each can be switched independently. The current divides according to need. A fuse is placed in series with the appliance in the live wire.
- Parallel connection means that if one appliance fails, the others keep working.
- In India the live wire is usually red, the neutral black and the earth wire green (or yellow). Colour conventions for plug and socket bars follow this pattern.
Fuse — the safety device
- A fuse is a short, thin piece of an alloy of tin and lead (low melting point) placed in the live wire before the appliance.
- If the current rises above a safe value (e.g. because of overloading or a short circuit), the fuse wire heats up, melts and breaks the circuit, protecting the appliance and the wiring from damage.
- In small domestic installations the fuse is often fixed in a porcelain carrier on the distribution board.
- The fuse is rated at a current slightly higher than the maximum current the appliance draws. Fuses must be of the correct rating — a very thick fuse wire will not melt in time and gives no protection.
Earthing — protects you from shock
- The metal body of heavy appliances (refrigerator, washing machine, electric iron, toaster) is connected to earth through the earth wire (third pin of the plug).
- If, due to a fault, the live wire touches the metal body, the metal body becomes live. With earthing, the current flows to earth through the earth wire instead of through a person touching the appliance, avoiding a severe shock.
- In a 3-pin plug the earth pin is longer and thicker than the other two so that earthing connects first and disconnects last.
Overloading and short circuit
- Short circuit: when the live wire and neutral wire touch each other directly, an extremely large current flows and the wires heat greatly — this can cause a fire. The fuse blows and disconnects the supply.
- Overloading: connecting too many appliances to a single socket draws excess current; the wire overheats. The fuse protects against this too.
- Precautions: do not overload sockets, use correct fuse ratings, use fused plugs, keep wires away from water, and never touch switches with wet hands.
13. Important Rules to Remember
| Rule | What it states | Used for | Hand & fingers |
|---|---|---|---|
| Right-hand thumb rule | Hold a straight conductor in the right hand with the thumb along the direction of current; the curled fingers give the direction of the magnetic field lines. | Direction of magnetic field around a straight current-carrying wire | Right hand; thumb = current |
| Fleming’s left-hand rule | If the forefinger shows the direction of the magnetic field and the middle finger the direction of the current, the thumb shows the direction of the force (motion). | Direction of force on a current-carrying conductor (motor) | Left hand; three fingers mutually perpendicular |
| Fleming’s right-hand rule | If the thumb shows the direction of motion of the conductor and the forefinger the direction of the magnetic field, the middle finger shows the direction of the induced current. | Direction of induced current (generator/induction) | Right hand; three fingers mutually perpendicular |
Left → motor, Right → generator. Or remember the magnetic field is always along the forefinger in both Fleming rules; only the thumb/middle finger swap roles.
14. Important Definitions
| Term | Definition |
|---|---|
| Magnetic field | The region surrounding a magnet (or a current-carrying conductor) in which a magnetic force can be detected. |
| Magnetic field line | An imaginary line along which a (hypothetical) north pole would move in a magnetic field; the tangent at any point gives the direction of the field there. |
| Electromagnet | A soft iron core with an insulated coil of copper wire wound around it, which behaves as a magnet only while current flows through the coil. |
| Solenoid | A coil of many circular turns of insulated copper wire wound closely, in which the turns resemble touching circles. |
| Electromagnetic induction | The production of an electric current in a closed circuit because of a changing magnetic field (relative motion of a coil and a magnet). |
| Induced current | The current that flows in a closed coil when the magnetic field around it changes. |
| Electric motor | A device that converts electrical energy into mechanical energy using the force on a current-carrying coil in a magnetic field. |
| Electric generator (dynamo) | A device that converts mechanical energy into electrical energy using electromagnetic induction. |
| Split-ring commutator | Two halves of a copper ring, insulated from each other and from the axle, which reverse the direction of current in the armature coil of a motor after every half rotation. |
| Slip rings | Two separate brass rings that connect the rotating armature coil of a generator to the stationary external circuit through brushes. |
| Alternating current (AC) | Current that reverses its direction periodically; in India the mains supply is AC at 220 V and 50 Hz. |
| Direct current (DC) | Current that flows in only one direction, e.g. from a cell, battery or solar cell. |
| Electric fuse | A short piece of a low-melting-point alloy placed in series with the live wire; it melts and breaks the circuit when the current exceeds its rating. |
| Earthing (grounding) | Connecting the metal body of an appliance to the earth through the earth wire, so that if a live wire touches the body, the current flows to the earth and no shock reaches the user. |
15. Important Diagrams at a Glance
| Fig. | Diagram | What to include |
|---|---|---|
| Fig. 1 | Magnetic field lines of a bar magnet | Closed curves, N to S outside, S to N inside, crowded near poles, no two lines intersect |
| Fig. 2 | Field around a straight conductor | Concentric circles; ⊙ (out of page) or × (into page); direction with thumb rule |
| Fig. 3 | Right-hand thumb rule | Thumb = current, curled fingers = field direction |
| Fig. 4 | Field at the centre of a circular loop | Straight line perpendicular to the plane of the loop through its centre; direction by thumb rule |
| Fig. 5 | Solenoid with its field | Many close turns, nearly uniform field inside, N and S poles identified |
| Fig. 6 | Electromagnet | Soft iron core, coil, switch, cell, iron pieces being picked up |
| Fig. 7 | Fleming’s left-hand rule | Three mutually perpendicular fingers; fore = field, middle = current, thumb = force |
| Fig. 8 | Electric motor | Armature coil ABCD, magnet with N and S, split-ring commutator, brushes, battery |
| Fig. 9 | Faraday’s induction experiment | Coil + galvanometer, bar magnet being pushed/pulled, labelled N/S |
| Fig. 10 | Fleming’s right-hand rule | Thumb = motion, fore = field, middle = induced current (right hand) |
| Fig. 11 | Electric generator | Armature coil ABCD, magnet, slip rings + brushes, external circuit (bulb) |
| Fig. 12 | AC versus DC graphs | Sinusoidal wave for AC; straight horizontal line for DC; axes labelled current vs time |
In every diagram label the N and S poles, the direction of the current with arrows, and the direction of the force or field lines. In motor/generator figures also label the commutator/slip rings and the brushes.
16. Solved Examples
Work each one on your own first, then compare. Always state the rule being used before writing the direction -- that is exactly how the examination expects you to reason.
Magnetic Field and Field Lines
State the direction of magnetic field lines outside and inside a bar magnet.
Rule: Field lines run from the north pole to the south pole outside a magnet, and from south to north inside the magnet, so the lines form closed loops.
Answer: outside the magnet N → S; inside the magnet S → N.
Why do magnetic field lines never intersect each other?
Explanation: At the point of intersection there would be two directions of the magnetic field at the same point, which is impossible -- a free magnetic pole (or a compass needle) can experience the field in only one direction.
Answer: two field directions cannot exist at one point, so field lines never intersect.
A compass needle is placed near the north pole of a bar magnet. Which way does the needle’s north end point?
Explanation: Like poles repel. The north pole of the compass needle is repelled by the magnet’s north pole, so the needle’s north end points away from the magnet’s north pole (towards the magnet’s south end).
Answer: away from the magnet’s north pole.
List three important properties of magnetic field lines.
Points: (1) They are closed curves (N to S outside, S to N inside). (2) They never intersect. (3) Lines are crowded where the field is strong, spaced where it is weak; the tangent gives the field direction.
Answer: strength shown by crowding; direction shown by tangent; closed loops that never cross.
Where is the magnetic field of a bar magnet strongest on a sheet of paper near the magnet, and why?
Explanation: The field (and the crowding of field lines) is greatest at the two poles, because in a bar magnet the field lines emerge and re-enter most densely at the magnetic poles.
Answer: at the poles.
Right-Hand Thumb Rule and Straight Conductor
State the right-hand thumb rule.
Statement: If a current-carrying straight conductor is held in the right hand so that the thumb points in the direction of the current, the fingers curling around the conductor give the direction of the magnetic field lines.
Answer: thumb = current; curled fingers = direction of field lines.
A horizontal current-carrying wire lies on a table in the north–south direction, with the current flowing from south to north. A compass needle is placed just above the wire. In which direction does the needle deflect?
Rule applied: right-hand thumb rule.
Solution: Point the right thumb along the current (north). At a point just above the wire the curled fingers point towards the east.
Answer: the needle deflects towards the east.
For a straight conductor carrying current into the plane of the paper, draw the pattern of the magnetic field lines.
Explanation: The field lines are concentric circles around the conductor. For current into the page, with the thumb pointing into the page, the fingers curl clockwise as seen from the reader’s side.
Answer: concentric circles, clockwise around the × (into-the-page) current.
Two parallel conductors carry electric current in the same direction. Do they attract or repel?
Explanation: The field of the first conductor acts on the current of the second, and by Fleming’s left-hand rule the force on the second is towards the first conductor (and vice versa by symmetry). Parallel currents in the same direction attract.
Answer: they attract each other.
What changes if the current in one of two parallel conductors is reversed?
Explanation: The force on the second conductor now reverses, so the two conductors push each other apart. Antiparallel currents repel.
Answer: the force reverses and the conductors repel.
Circular Loop and Solenoid
State the direction of the magnetic field at the centre of a circular loop carrying current, when the current appears anticlockwise as seen from above.
Rule applied: right-hand thumb rule on the loop (or the clock rule).
Solution: For anticlockwise current (viewed from above), the field at the centre is directed out of the page, towards the viewer, along the axis of the loop.
Answer: perpendicular to the plane of the loop, out of the page (towards the viewer).
How can the strength of the magnetic field at the centre of a current-carrying loop be increased?
Method: (1) Increase the number of turns in the coil. (2) Increase the current through the loop. The field at the centre is directly proportional to both.
Answer: increase the number of turns and/or the current.
Define a solenoid and state the rule used to identify its magnetic polarity.
Definition: A solenoid is a coil of many circular turns of insulated copper wire wound in the shape of a long cylinder, the turns being close and almost touching.
Clock rule: looking at any end of the solenoid, if the current appears anticlockwise, that end is the north pole; if clockwise, it is the south pole. (Equivalently use the right-hand thumb rule on the winding.)
Answer: anticlockwise-current face → N pole; clockwise-current face → S pole.
List three ways to increase the strength of an electromagnet.
Method: (1) Use more turns of the coil. (2) Increase the electric current. (3) Use a soft iron core (better than an air core or a steel core, since soft iron loses its magnetism when the current stops).
Answer: more turns, larger current, soft iron core (within limits such as not saturating).
Force on a Conductor, Electric Motor
A current-carrying conductor lies in a magnetic field directed towards the right of the page. The current flows out of the page towards the viewer. Find the direction of the force.
Rule applied: Fleming’s left-hand rule.
Solution: Forefinger → field (right); middle finger → current (towards the viewer); the thumb points upward.
Answer: the force acts upward.
The current in a conductor flows downward and the magnetic field points left (towards the west). Find the direction of the force.
Rule applied: Fleming’s left-hand rule.
Solution: Forefinger → field (left); middle finger → current (down); the thumb points into the page.
Answer: the force acts into the page.
Why is a split-ring commutator used in a DC motor?
Explanation: The coil of a motor experiences a force that would make it rotate one way for only half a turn; without a commutator it would come to rest at the vertical position. The split-ring commutator reverses the direction of the current in the coil after every half rotation, so the force, and hence the rotation, continues in the same direction.
Answer: to reverse the direction of current in the coil after every half rotation, keeping the rotation continuous.
State the principle on which an electric motor works, and name the energy conversion that takes place in it.
Principle: A current-carrying coil placed in a magnetic field experiences a force (a couple) and therefore rotates.
Energy conversion: electrical energy → mechanical energy.
Answer: force on a current-carrying coil in a magnetic field; electrical → mechanical energy.
Electromagnetic Induction and the Generator
State the discovery of Faraday and the key condition for electromagnetic induction.
Discovery (1831): when a magnet is moved towards or away from a closed coil, a current is induced in the coil.
Condition: there must be relative motion between the magnet and the coil, i.e. the magnetic field through the coil must change with time.
Answer: induced current flows only while the magnetic field through the coil is changing.
How does the induced current change when the magnet is moved towards a coil faster, and why?
Explanation: A faster push changes the magnetic field through the coil more rapidly, producing a larger induced current (larger deflection of the galvanometer). Reversing the direction of motion reverses the direction of the induced current.
Answer: a faster motion gives a larger induced current; the direction reverses with the motion.
A conductor is moved downward in a magnetic field directed into the page. Using Fleming’s right-hand rule, find the direction of the induced current.
Rule applied: Fleming’s right-hand rule.
Solution: Thumb → motion (down); forefinger → field (into the page); the middle finger points towards the right, which is the direction of the induced current.
Answer: the induced current flows towards the right.
What is the function of the slip rings and brushes in an electric generator?
Explanation: The slip rings rotate with the armature coil, while the stationary carbon brushes press against them. Together they carry the induced current from the rotating coil to the stationary external circuit without twisting the wires.
Answer: slip rings + brushes transfer the induced current from the rotating coil to the external circuit.
State the energy conversion in (i) a generator and (ii) a motor.
(i) Generator: mechanical energy → electrical energy (electromagnetic induction).
(ii) Motor: electrical energy → mechanical energy (force on a current-carrying coil).
Answer: generator mechanical → electrical; motor electrical → mechanical.
Competency-Style Questions
A conductor is dragged due south in a uniform magnetic field directed due east. Use Fleming’s right-hand rule to find the direction of the induced current.
Rule applied: Fleming’s right-hand rule.
Solution: Thumb → motion (south); forefinger → field (east); the middle finger points out of the page (towards the viewer), which is the direction of the induced current.
Answer: the induced current flows out of the page towards the viewer.
A coil of a solenoid is wound so that, looking at the near end, the current appears to flow clockwise. Identify the pole at that end, and state the rule.
Rule applied: the clock rule for a solenoid.
Solution: Clockwise current (seen from the near end) → the near end behaves as the south pole. Anticlockwise current would have given a north pole.
Answer: the near end is the south pole (clockwise face → S).
If the soft iron core of an electromagnet is replaced by a steel core, why does the magnet no longer switch off properly?
Explanation: Soft iron loses its magnetism almost instantly when the current stops, so the electromagnet can be switched on and off. Steel, once magnetised, retains strong residual magnetism and would keep attracting the objects even after the current is cut off.
Answer: steel retains magnetism (it is not magnetically soft), so a steel core cannot be demagnetised easily.
17. Application-Based Examples
These questions connect magnetic fields, motors, generators and safety devices to things we use every day.
Original — CBSE Exam-Style
Why is an electromagnet used in a scrapyard crane instead of a permanent magnet?
Answer:
An electromagnet can be switched on to pick up (attract) scrap-iron pieces and switched off to drop them. A permanent magnet cannot be switched off, so it would be impossible to release the load.
Original — CBSE Exam-Style
Why is a fuse connected in the live wire and not in the neutral or earth wire?
Answer:
The fuse must cut off the live supply. Placed in the live wire, when it melts the whole circuit becomes dead (no live potential reaches the appliance), so a person touching the appliance cannot get a shock. A fuse in the neutral would not disconnect the live wire from the appliance.
Original — CBSE Exam-Style
Why do domestic electric circuits with many appliances get overloaded, and what are the consequences?
Answer:
Connecting too many appliances to a single circuit (parallel connection) draws a total current far above the designed value. The wires and insulation heat up (I²R losses), which can damage the wiring and even cause a fire. The fuse blows or the circuit breaker trips to protect the circuit.
Original — CBSE Exam-Style
A washing machine is provided with a three-pin plug. Why is the earth wire (third pin) necessary for such an appliance?
Answer:
The metal body of a washing machine could become live if the live wire touches it due to a fault. The earth wire safely conducts that current to the ground, so no current flows through the user who touches the body, preventing a severe electric shock.
Original — CBSE Exam-Style
A toy car runs on a cell (DC). A student connects the toy motor to the mains (AC at 220 V, 50 Hz). What problem would arise?
Answer:
The toy motor is designed for a low voltage (a few volts of DC), not 220 V of AC. Applying 220 V would blow the motor (excessive current). Also, the direction of current in a DC motor that relies on a unique split-ring arrangement would need correct DC polarity; AC at the wrong voltage would damage or burn the motor.
Original — CBSE Exam-Style
Why is a CD ceiling fan fitted with a capacitor still called a "motor", and how does it differ from a generator?
Answer:
A fan is a motor because it converts electrical energy (mains AC) into the rotational (mechanical) energy of the fan blades. A generator does the opposite: it converts mechanical energy into electrical energy. In a power plant an alternator (AC generator) converts the mechanical energy of a rotating turbine into the electricity we use.
Original — CBSE Exam-Style
What precaution should be taken when replacing a blown fuse, and why must the correct rating be used?
Answer:
Switch off the mains, then replace the fuse with one of the correct (same) rating. A fuse of too high a rating would not melt in time, so an over-current could overheat the wiring before the fuse acts; a fuse of too low a rating would melt even during normal operation, causing repeated nuisance trips.
Original — CBSE Exam-Style
An induction cooktop does not heat the glass surface it sits on, yet heats the metallic vessel placed on it. Using your knowledge of electromagnetic induction, explain why only the metal vessel heats up.
Answer:
The cooktop generates a changing magnetic field through a coil below its glass top. This changing field induces currents (eddy/induced currents) in the metallic vessel placed above it; the induced currents heat the vessel by Joule heating. The glass and the cooktop body contain no good conductor (glass is an insulator), so no induced currents flow in them and they do not heat up.
18. Competency and Reasoning Questions
These problems test your ability to apply the magnetic rules in new situations. Always pick the correct rule first (thumb rule / LHR / RHR).
Original — CBSE Exam-Style
A current-carrying conductor is placed in a magnetic field that points towards the right of the page, and the current flows out of the page towards the viewer. Find the direction of the force.
Answer:
Fleming’s left-hand rule: forefinger → field (right), middle finger → current (towards the viewer), so the thumb points upward. The conductor is pushed upward.
Original — CBSE Exam-Style
A conductor is moved downward perpendicular to a magnetic field directed into the page. Find the direction of the induced current.
Answer:
Fleming’s right-hand rule: thumb → motion (down), forefinger → field (into the page), so the middle finger points towards the right. The induced current flows towards the right.
Original — CBSE Exam-Style
A horizontal conducting wire lies on a table in the north–south direction and carries current from south to north. State the directions of the magnetic field at a point directly above and at a point directly below the wire.
Answer:
Apply the right-hand thumb rule with the thumb pointing north (direction of current). At a point directly above the wire the field is directed towards the east, and at a point directly below the wire it is directed towards the west (the field lines are concentric circles, with opposite directions above and below).
Original — CBSE Exam-Style
In a solenoid, the end at which the current appears anticlockwise (seen from that end) behaves as which pole? Justify.
Answer:
That end behaves as the north pole. The current in the anticlockwise coil (seen from that end) gives, by the right-hand thumb rule on the turns, a magnetic field emerging out of that face; a face from which the field emerges behaves as a north pole.
Original — CBSE Exam-Style
If the direction of current and the direction of magnetic field are both reversed together, what happens to the direction of force on a current-carrying conductor in a field?
Answer:
It remains the same. Reversing the current reverses the force; reversing the field also reverses the force; reversing both returns the force to its original direction. (Check with Fleming’s left-hand rule by turning both fingers opposite.)
Original — CBSE Exam-Style
A student replaces a steel core in an electromagnet with a soft iron core. How does the behaviour of the electromagnet change?
Answer:
With a soft iron core the magnet (a) becomes stronger for the same current, because soft iron concentrates the magnetic field more than air or steel, and (b) loses its magnetism as soon as the current is switched off. Soft iron is therefore used in electromagnets that must be switched on and off repeatedly.
19. Case-Based Learning
Read each passage carefully, then answer the questions that follow using the physics of this chapter.
Original — CBSE Exam-Style
A ceiling fan contains an electric motor. When the mains switch is turned on, AC at 220 V flows through the motor coil which is mounted on the shaft of the fan and placed in the magnetic field of the motor.
Q1. What energy conversion takes place in the fan motor?
Answer:
Electrical energy → mechanical (rotational) energy of the blades.
Q2. Why does the motor coil need to be located in a magnetic field?
Answer:
A current-carrying coil placed in a magnetic field experiences a force (couple), which causes the coil and the blades attached to it to rotate. Without the field there would be no turning force.
Q3. What is the advantage of winding the coil around a soft iron core in the motor?
Answer:
The soft iron core concentrates the magnetic field, so a larger force (and hence a stronger turning effect) is obtained for the same current.
Q4. If the fan operates on 220 V AC and we instead feed it 220 V DC, it still rotates. Why does a DC-supplied AC motor tend to overheat, whereas the electric generator opposite construction is fine?
Answer:
For a general AC induction motor the construction assumes alternating current; driving it on DC causes excessive current through the coil (low reactance at DC) which produces extra heat (I²R). The important point for this chapter is that a motor converts electrical to mechanical energy, and the reverse device, the generator, converts mechanical to electrical energy.
Original — CBSE Exam-Style
An electric bell works using an electromagnet. A soft iron strip (armature) is pulled by the electromagnet, striking the gong, and the contact that powers the electromagnet opens, so the strip springs back and re-makes the contact.
Q1. Why cannot a permanent magnet be used in the bell?
Answer:
The electromagnet must attract the armature only when the button is pressed. A permanent magnet would attract the strip all the time; only a switchable electromagnet gives the intermittent action needed for the ringing to repeat.
Q2. Why is the core of the electromagnet made of soft iron rather than steel?
Answer:
Soft iron magnetises instantly when the current flows and (crucially) demagnetises quickly when the current stops, letting the armature spring back and re-make the contact. Steel would retain magnetism and the bell would not work properly.
Q3. When the electro-magnet pulls the armature, the contacts break. What happens next?
Answer:
The current stops, the electromagnet loses its magnetism, the armature springs back by its own elasticity, and the contact re-closes, making the electromagnet attract the armature again. This make-and-break action repeats rapidly, producing the continuous ringing.
Original — CBSE Exam-Style
A bicycle dynamo pressed against the front wheel produces electricity for the headlamp. A small wheel rubs on the tyre, rotating a magnet near a coil.
Q1. On which principle does the dynamo work?
Answer:
Electromagnetic induction: the rotating magnet creates a changing magnetic field through the nearby coil, inducing a current in the coil.
Q2. When we ride faster, the lamp glows brighter. Why?
Answer:
Faster rotation changes the magnetic field through the coil more rapidly, so the induced current (and hence lamp brightness) is larger.
Q3. Is the current produced by this simple dynamo AC or DC?
Answer:
AC — as the magnet rotates past the coil, the induced current reverses direction every half turn, so the current alternates. (Bicycle dynamos typically produce alternating current.)
Q4. State the energy conversions that happen from the pedalling to the headlamp light.
Answer:
Muscular (mechanical) energy of pedalling → rotational energy of the wheel → electrical energy in the coil (electromagnetic induction) → light (and heat) energy of the bulb.
Original — CBSE Exam-Style
A current-carrying conductor placed perpendicular to a magnetic field experiences a force. The force is maximum when the conductor is perpendicular to the field and zero when it is parallel to the field.
Q1. Name the rule that gives the direction of this force.
Answer:
Fleming’s left-hand rule (motor rule): forefinger → field, middle finger → current, thumb → force/motion.
Q2. The field points upward and the current flows from west to east along the table. Find the direction of the force.
Answer:
LHR: forefinger → field (up), middle finger → current (east), so the thumb points into the page. The force acts into the page.
Q3. Why is the force zero when the conductor is parallel to the magnetic field?
Answer:
The interaction that pushes a current-carrying wire sideways requires the current to have a component perpendicular to the field. When the conductor is parallel to the field the current direction is along the field, so no sideways force is produced.
Q4. What happens to the direction of force if only the direction of current is reversed?
Answer:
The force reverses direction (it acts in the opposite direction) while its magnitude stays the same.
Original — CBSE Exam-Style
Faraday connected a coil to a sensitive galvanometer and pushed a bar magnet into it. The needle deflected while the magnet moved, returned to zero when the magnet stopped, and deflected the other way when the magnet was pulled out.
Q1. What name is given to the current that flows while the magnet moves?
Answer:
Induced current, produced by electromagnetic induction.
Q2. Why does no current flow when the magnet is held stationary inside the coil?
Answer:
The induced current appears only while the magnetic field through the coil is changing. When the magnet is still, the field through the coil is steady, so there is no induced current.
Q3. Why does the needle deflect in the opposite direction when the magnet is pulled out?
Answer:
Reversing the direction of motion reverses the direction in which the field through the coil changes, which reverses the direction of the induced current.
Q4. How can the deflection be increased in this experiment?
Answer:
By moving the magnet faster, by using a stronger magnet, or by using a coil with more turns. (All three make the field change through the coil more rapidly/larger.)
20. Important Comparison Tables
Quick side-by-side revision of the most frequently confused pairs.
| Point of comparison | Electric motor | Electric generator |
|---|---|---|
| Converts | Electrical energy → mechanical energy | Mechanical energy → electrical energy |
| Principle | Force on a current-carrying coil in a magnetic field | Electromagnetic induction |
| Rule used | Fleming’s left-hand rule | Fleming’s right-hand rule |
| Ring device | Split-ring commutator (reverses current) | Slip rings (carry current out) |
| Input / output | Takes current in, gives rotation out | Needs rotation in, gives current out |
| Examples | Fans, mixers, electric toys | Dynamo, power-station alternator |
| Point of comparison | Fleming’s left-hand rule | Fleming’s right-hand rule |
|---|---|---|
| Used in | Motor effect (force on a current-carrying conductor) | Generator effect (induced current) |
| Thumb shows | Direction of force / motion | Direction of motion of the conductor |
| Forefinger shows | Direction of magnetic field | Direction of magnetic field |
| Middle finger shows | Direction of current | Direction of induced current |
| Remember | Left → motor | Right → generator |
| Point of comparison | Alternating current (AC) | Direct current (DC) |
|---|---|---|
| Direction of flow | Reverses periodically | Flows in one direction only |
| Frequency | In India: 50 Hz (reverses 100 times/s) | 0 Hz (no alternation) |
| Source | AC generator, mains supply | Cell, battery, solar cell, DC generator |
| Voltage transformation | Easily stepped up/down with transformers | Not easily transformed |
| Use | Long-distance transmission, house supply | Batteries, portable electronics |
| Point of comparison | Electromagnet | Permanent magnet |
|---|---|---|
| Magnetised by | Electric current in a coil | Manufactured (or magnetised) material |
| Can be switched off | Yes — no current, no magnetism | No |
| Polarity | Can be reversed by reversing the current | Fixed |
| Strength | Can be made very strong (big current, many turns) | Limited |
| Core | Soft iron (loses magnetism quickly) | Steel or magnetic material |
| Examples of use | Electric bell, crane, door lock | Speaker, compass, refrigerator door |
| Point of comparison | Solenoid | Bar magnet |
|---|---|---|
| Magnetic field | Produced only while current flows | Permanent (always present) |
| Strength | Can be varied with current / turn count | Fixed |
| Poles | N and S ends (reversible by reversing current) | Fixed N and S ends |
| Field pattern | Outside the solenoid it resembles that of a bar magnet | The classical bar-magnet pattern |
| Core | Air if no core; with a soft iron core it becomes an electromagnet | Solid magnetic material |
| Point of comparison | Magnetic field lines | Electric current (in a wire) |
|---|---|---|
| Nature | Closed curves around a magnet/conductor | Flow of charge along a conductor |
| Cause | Magnetic poles or moving charges (currents) | Potential difference across a conductor |
| Direction | N → S outside a magnet | Positive (conventional) to negative terminal |
| Detected by | Compass needle, iron filings | Ammeter, effects (heating, magnetic, chemical) |
| SI unit | Field strength: tesla (or gauss) | Current: ampere (A) |
21. Common Mistakes to Avoid
| Mistake | Why it happens | Correction |
|---|---|---|
| Using the wrong Fleming rule | Mis-remembering which hand goes where | Left hand = motor (force); right hand = generator (induced current). Field is always the forefinger in both |
| Thumb of the right-hand thumb rule pointing along the field | Swapping the two quantities | Thumb always points along the direction of current; the curled fingers give the field |
| Treating field lines as starting lines that end | Comparing with electric field lines | Magnetic field lines are closed loops (N → S outside, S → N inside the magnet) |
| Guessing solenoid polarity | Not applying the clock rule | Anticlockwise current seen from an end → that end is N; clockwise → S |
| Thinking a conductor parallel to the field experiences a force | Remembering only "field acts on current" | Force is maximum perpendicular to the field and zero when parallel to it |
| Believing current flows while the magnet is held still in the coil | Confusing field strength with change of field | Induced current needs the field through the coil to be changing; a still magnet gives no current |
| Writing that a generator produces DC | Household mains is AC, forgetting the rest | An AC generator gives alternating current; a DC generator uses a commutator to give DC |
| Swapping the commutator and the slip rings | The two names look similar | Motor → split-ring commutator (reverses current); generator → slip rings (carry current out) |
| Getting the above/below field direction backwards | Forgetting the circle of field lines | For current N→S, field above the wire is east and below it is west; reverse the current and both flip |
| Thinking the earth wire carries the operating current | Not knowing its role | The earth wire carries current only during a fault (when the live wire touches the metal body), safely to the ground |
22. Exam Tips
- Always name the rule first: right-hand thumb rule, Fleming’s left-hand rule or Fleming’s right-hand rule. Examiners award marks for the rule, the finger assignment, and then the final direction — in that order.
- State the finger assignments explicitly: e.g. "forefinger → field (into the page), middle finger → current (towards the right), so the force acts upward." Each labelled step earns marks.
- Draw direction symbols clearly: a dot (⊙) for "out of the page/towards you" and a cross (×) for "into the page/away". Misreading these symbols is a common reason for losing these marks.
- For motor/generator diagrams label everything: armature coil (ABCD), N and S poles, arrows for field and current, the split-ring commutator or slip rings, brushes, and the battery or external circuit.
- Remember the key values: mains supply in India 220 V, 50 Hz; the current direction reverses 100 times per second; field strongest at the poles.
- Distinguish the devices: motor = electrical to mechanical (LHR); generator = mechanical to electrical (RHR). Given only a description, decide the device first, then the rule.
- Answers about safety: mention the fuse (series, live wire), earthing (third pin, metal body) and overloading/short-circuit clearly with the reason for each.
- Competency-type prompts: if a direction is asked, identify current/field/motion first, then apply the correct rule; if "what happens if reversed" appears, reverse only the quantity named in the question.
23. Quick Revision (5–10 Minutes)
| Concept | Key point | Rule / value |
|---|---|---|
| Magnetic field lines | Closed curves; never intersect; crowed where the field is strong | N → S outside a magnet |
| Field around a straight conductor | Concentric circles centred on the wire | Right-hand thumb rule |
| Field of a circular loop | Straight line perpendicular to the plane through the centre | Clock rule / thumb rule |
| Solenoid | Graph of a bar magnet outside; uniform field inside | Anticlockwise face → N pole |
| Electromagnet | Soft iron core + current-carrying coil; switchable | Stronger: more turns, more current |
| Force on a conductor in a field | Max when perpendicular; zero when parallel | Fleming’s left-hand rule |
| Electric motor | Electrical → mechanical; split-ring commutator | Fleming’s left-hand rule |
| Electromagnetic induction | Current induced by a changing magnetic field | Fleming’s right-hand rule |
| Electric generator | Mechanical → electrical; slip rings | Fleming’s right-hand rule |
| AC / DC | AC reverses direction periodically; DC flows one way | India: 220 V, 50 Hz |
| Fuse | Series, live wire; melts above rated current | Protects against overloading/short circuit |
| Earthing | Metal body to earth via third pin | Prevents electric shock |
One-line pointers
- Left hand → motor; right hand → generator; thumb → current in the thumb rule.
- The field is along the forefinger in both Fleming rules.
- Induced current flows only while the magnetic field through the coil is changing.
- Household appliances are connected in parallel so each gets the full 220 V.
- The fuse is always in the live wire; the earth wire is connected to the metal body.
- Soft iron (not steel) is used in electromagnets because it demagnetises as soon as current stops.
- Reversing either the current or the field reverses the force; reversing both keeps it the same.
24. Self-Practice Questions
MCQ
The SI unit of magnetic field (magnetic flux density) is (a) tesla (b) weber (c) gauss (d) ampere.
MCQ
A magnetic compass needle settles in the north–south direction because (a) the Moon attracts it (b) the Earth has a magnetic field (c) the needle is magnetised by sunlight (d) of air currents.
MCQ
Magnetic field lines of a bar magnet (a) start at the south pole (b) are closed loops passing through the magnet (c) are straight lines only (d) can intersect at the poles.
MCQ
The direction of the magnetic field around a straight current-carrying conductor is given by the (a) Fleming left-hand rule (b) Fleming right-hand rule (c) right-hand thumb rule (d) right-hand screw rule for solenoids only.
MCQ
The mains supply in India is (a) 220 V DC (b) 110 V AC at 50 Hz (c) 220 V AC at 50 Hz (d) 220 V AC at 60 Hz.
MCQ
A current-carrying conductor experiences the maximum force in a magnetic field when it is (a) parallel to the field (b) perpendicular to the field (c) at 45° to the field (d) anywhere inside the field.
MCQ
The device that converts mechanical energy into electrical energy is the (a) electric motor (b) electric generator (c) electric fuse (d) electromagnet.
MCQ
Looking into one end of a solenoid, the current appears anticlockwise. That end behaves as the (a) north pole (b) south pole (c) neutral point (d) depends on the number of turns.
MCQ
A split-ring commutator is used in a (a) DC motor (b) AC generator (c) transformer (d) voltmeter.
MCQ
The frequency of the domestic AC supply in India is (a) 50 Hz (b) 60 Hz (c) 100 Hz (d) 25 Hz.
VSA
State Fleming’s left-hand rule.
VSA
Name one device that works on the principle of electromagnetic induction.
VSA
What happens to the direction of the force on a current-carrying conductor if only the direction of the magnetic field is reversed (current unchanged)?
VSA
State the right-hand thumb rule for a straight conductor.
VSA
Name the safety device placed in series in the live wire that protects a household circuit from overloading and short-circuiting.
SA I
Why do two parallel current-carrying conductors attract each other when the currents flow in the same direction?
SA I
Draw the magnetic field lines around a straight conductor carrying current into the plane of the paper, showing the direction of the field.
SA I
Why is soft iron (and not steel) used as the core of an electromagnet?
SA I
State the principle of an electric motor. What is the function of the split-ring commutator in it?
SA I
Why is the fuse connected in the live wire and not in the neutral wire?
SA II
A horizontal conductor carries current towards the north. The magnetic field is directed into the plane of the paper. Find the direction of the force on the conductor and name the rule used.
SA II
A conductor is moved towards the east through a uniform magnetic field directed towards the south. Using Fleming’s right-hand rule, find the direction of the induced current.
SA II
State the energy conversion in (i) an electric motor (ii) an electric generator, and name the rule used in each.
SA II
Write three differences between alternating current (AC) and direct current (DC).
SA II
Why is the galvanometer deflection larger when a magnet is pushed into a coil faster? Name the quantity that increases.
Reasoning
A straight conductor on a table carries current from west to east. Give the directions of the magnetic field at a point directly above and at a point directly below the conductor.
Reasoning
An electric motor and an electric generator are often described as "opposite devices". Why? Give one energy-conversion difference.
Reasoning
If the current through a solenoid is doubled and the number of turns is halved, what happens to the strength of the magnetic field of the solenoid?
Case-based
A rectangular coil carrying current is placed in the magnetic field between the pole pieces of a strong magnet. The coil rotates as soon as the current flows.
(i) Which energy conversion takes place?
(ii) Which rule gives the direction of rotation?
(iii) What would happen if the direction of the current were reversed?
Case-based
A student connects a coil to a galvanometer and pushes a strong bar magnet towards the coil, first slowly and then quickly.
(i) In which case does the galvanometer deflect more, and why?
(ii) What does the student observe when the magnet is held stationary inside the coil?
(iii) What does the student observe when the magnet is pulled out, and why?
25. Answer Key and Hints
| Q | Answer / Hint |
|---|---|
| 1 | (a) tesla (gauss is an older CGS unit) |
| 2 | (b) the Earth has a magnetic field |
| 3 | (b) closed loops passing through the magnet (N → S outside, S → N inside) |
| 4 | (c) right-hand thumb rule |
| 5 | (c) 220 V AC at 50 Hz |
| 6 | (b) perpendicular to the field (zero when parallel) |
| 7 | (b) electric generator |
| 8 | (a) north pole (anticlockwise face → N) |
| 9 | (a) DC motor (reverses current every half rotation) |
| 10 | (a) 50 Hz |
| 11 | Holding the forefinger, middle finger and thumb of the left hand mutually perpendicular: forefinger = magnetic field, middle finger = current, thumb = force/motion |
| 12 | Electric generator / dynamo (also: transformer, induction cooktop principle) |
| 13 | It reverses (reversing the field alone reverses the force) |
| 14 | Hold the conductor in the right hand with the thumb pointing along the current; the curled fingers give the direction of the magnetic field lines |
| 15 | Electric fuse |
| 16 | The field of each conductor acts on the current of the other; by Fleming’s left-hand rule, same-direction parallel currents attract |
| 17 | Concentric circles around the wire; for current into the page the field is clockwise as seen by the reader (right-hand thumb rule) |
| 18 | Soft iron loses its magnetism as soon as the current stops, so the electromagnet can be switched on/off; steel retains magnetism and would stay magnetised |
| 19 | Principle: a current-carrying coil placed in a magnetic field rotates. Commutator reverses the current direction in the coil every half rotation so the coil keeps turning the same way |
| 20 | So that when the fuse melts the live supply to the appliance is cut completely, making the circuit dead and safe (a fuse in the neutral would leave the live wire at mains potential) |
| 21 | Force acts towards the west; Fleming’s left-hand rule (forefinger = field into page, middle = current north → thumb = west) |
| 22 | Induced current acts downwards (into the table); Fleming’s right-hand rule (thumb = east, forefinger = south → middle = down) |
| 23 | (i) motor: electrical → mechanical, LHR (ii) generator: mechanical → electrical, RHR |
| 24 | Direction: AC reverses periodically, DC is unidirectional; source: AC generator/mains vs cell/battery; frequency: AC has 50 Hz in India, DC has none; voltage stepping: AC easily stepped up/down |
| 25 | Faster motion changes the magnetic field through the coil more rapidly, giving a larger induced current (a larger rate of change of flux); the deflection is larger |
| 26 | Thumb rule with current east: at a point directly above the conductor the field is towards the south; directly below it is towards the north |
| 27 | Motor converts electrical energy to mechanical; generator converts mechanical to electrical — exact opposites; the Fleming rules are mirror images (left/right hands) |
| 28 | Class 10 level: field strength depends on the product (turns × current) → doubling current and halving turns leaves the strength the same |
| 29 | (i) electrical → mechanical (ii) Fleming’s left-hand rule (iii) the coil rotates in the opposite direction |
| 30 | (i) quicker push → larger deflection (faster change of field) (ii) no deflection — field is steady (iii) needle deflects in the opposite direction because reversing the motion reverses the direction of the induced current |