Magnetic Effect of Current and Electromagnetic Induction ICSE Class 10

What Magnetic Effect of Current and Electromagnetic Induction is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Physics. Magnetic Effect of Current and Electromagnetic Induction is in the 4. Electricity and Magnetism (iv) part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Physics (Science Paper 1) syllabus, examination year 2027

Key concepts

Oersted's experiment and the field round a straight wire

Oersted found that a compass needle near a wire is deflected when a current flows: a current produces a magnetic field. Reversing the current reverses the deflection.

Around a long straight wire the field lines are concentric circles centred on the wire, in a plane perpendicular to it. The field is stronger closer to the wire and for a larger current.

Right-hand thumb rule: hold the wire in the right hand with the thumb along the current; the curled fingers give the direction of the field lines.

Worked example

A vertical wire carries current upwards through a horizontal card. Seen from above, what is the direction of the field lines? What happens if the current is increased?

Try it first, then show the solutionSolution

1. Thumb upwards: fingers curl anticlockwise as seen from above.
2. A larger current gives a stronger field: field lines closer together, larger compass deflection.

Answer: Anticlockwise circles; stronger field

Field of a loop and a solenoid; the clock rule

In a circular loop, the field lines near the centre are nearly straight and perpendicular to the loop. A long coil (solenoid) gives a field like a bar magnet's: nearly uniform inside, with N and S poles at the ends.

Clock rule: look at one end of the coil. If the current flows anticlockwise, that end is a North pole; if clockwise, it is a South pole.

The field is stronger with more current, more turns, and a soft iron core.

Outside a magnet, field lines go from N to S.

Worked example

Looking at end A of a solenoid, the current flows clockwise. What is the polarity of A and of the other end B?

Try it first, then show the solutionSolution

1. Clockwise current seen at an end: South pole.
2. So A is S and B is N.

Answer: A: South; B: North

Electromagnets and permanent magnets

An electromagnet is a soft iron core inside a solenoid. It is a magnet only while current flows. Soft iron is used because it is easily magnetised and loses its magnetism as soon as the current stops; steel keeps its magnetism and is used for permanent magnets.

Comparison:

• Electromagnet: temporary; strength can be changed (by current or turns); polarity can be reversed; can be made very strong.
• Permanent magnet: magnetism permanent; fixed strength and polarity; usually weaker.

Uses: electric bells, cranes for lifting scrap iron, relays and circuit breakers, loudspeakers, MRI scanners, removing iron splinters.

Worked example

Why is the core of an electromagnet made of soft iron and not steel?

Try it first, then show the solutionSolution

1. Soft iron is magnetised strongly while current flows.
2. It loses its magnetism almost completely when the current is switched off, so the magnet can be switched on and off; steel would stay magnetised.

Answer: Soft iron magnetises and demagnetises easily.

Force on a conductor and Fleming's left-hand rule

A current-carrying conductor placed across a magnetic field experiences a force. The force is largest when the conductor is perpendicular to the field and zero when parallel.

Fleming's left-hand rule: stretch the thumb, forefinger and middle finger of the left hand mutually at right angles. Forefinger → field (N to S), middle finger → current, thumb → motion (force).

The force increases with the field strength, the current and the length of conductor in the field. A thicker rod of the same material has lower resistance, so on the same supply it carries more current and feels a larger force.

Worked example

A horizontal wire carries current from west to east in a vertical magnetic field pointing downwards. In which direction is the force on the wire?

Try it first, then show the solutionSolution

1. Left hand: forefinger down (field), middle finger east (current).
2. Thumb then points north.

Answer: Towards the north

The DC motor (parts and energy transfer)

A DC motor converts electrical energy into mechanical (rotational) energy.

Main parts:

• armature coil: a rectangular coil of many turns on a soft iron core, free to rotate;
• field magnet: strong permanent magnet (or electromagnet) providing the field;
• split-ring commutator: reverses the current in the coil every half turn so it keeps rotating in the same direction;
• carbon brushes: press on the commutator and connect the coil to the battery.

The two sides of the coil feel opposite forces (Fleming's left-hand rule), forming a couple that turns the coil.

The speed of rotation increases with the current, the number of turns in the coil, the strength of the magnetic field (and the area of the coil).

Worked example

State the energy conversion in a DC motor and two ways to make it rotate faster.

Try it first, then show the solutionSolution

1. Electrical energy → mechanical energy.
2. Increase the current; increase the number of turns of the coil (or use a stronger magnet).

Answer: Electrical → mechanical; more current, more turns or stronger field

Electromagnetic induction

When the magnetic field through a coil changes, an emf, and so a current, is induced in the coil. Moving a magnet into or out of a coil, moving the coil, or switching the current in a nearby coil on or off all do this. No current flows when there is no relative motion or change.

The induced current is larger when the change is faster, the coil has more turns, and the magnet is stronger.

Direction: the induced current always flows so as to oppose the change producing it (Lenz's law). For a straight conductor moving across a field, use Fleming's right-hand rule: forefinger → field, thumb → motion, middle finger → induced current.

Worked example

The north pole of a magnet is pushed quickly into a coil connected to a galvanometer. What happens (a) as it moves in, (b) when it stops inside, (c) when it is pulled out?

Try it first, then show the solutionSolution

1. (a) The galvanometer deflects: a current is induced. The coil's near end becomes a N pole, opposing the approach.
2. (b) No change, so no current: no deflection.
3. (c) Deflection in the opposite direction.

Answer: Deflection; no deflection; opposite deflection

AC generator and AC vs DC

An AC generator converts mechanical energy into electrical energy by rotating a coil in a magnetic field.

Main parts: armature coil; field magnet; two slip rings (one fixed to each end of the coil, so the output alternates); carbon brushes pressing on the slip rings and leading to the external circuit.

The induced current increases with the speed of rotation, the number of turns, the area of the coil and the field strength.

Household AC in India has frequency 50 Hz (and voltage 220 V).

Advantages of AC over DC: it can be stepped up or down easily with transformers; it can be transmitted over long distances with less energy loss (at high voltage, low current); AC generators are simpler and cheaper to build.

Worked example

Name the parts of an AC generator that make its output alternating, and state its energy conversion.

Try it first, then show the solutionSolution

1. Slip rings: each end of the coil stays connected to the same brush, so the current direction alternates as the coil rotates.
2. Mechanical energy → electrical energy.

Answer: Slip rings; mechanical → electrical

Transformers

A transformer changes the voltage of an alternating supply. It has two coils, the primary and the secondary, wound on a laminated soft iron core. AC in the primary makes a changing magnetic field in the core, which induces an emf in the secondary. (It does not work on steady DC.)

For an ideal transformer:
$\frac{V_s}{V_p}=\frac{N_s}{N_p}=\frac{I_p}{I_s}$.

• Step-up: $N_s>N_p$ (turns ratio $\frac{N_s}{N_p}>1$); voltage raised, current lowered. Used at power stations.
• Step-down: $N_s<N_p$ (turns ratio < 1); voltage lowered, current raised. Used in phone chargers, doorbells, local substations.

The coil carrying the larger current is made of thicker wire (less resistance, less heating): the secondary in a step-down, the primary in a step-up transformer.

Worked example

A transformer has 2000 turns in the primary and 100 in the secondary. The primary is connected to 220 V AC. Find the secondary voltage and name the type. If the secondary supplies 2 A, find the primary current (ideal transformer).

Try it first, then show the solutionSolution

1. $V_s=220\times \frac{100}{2000}=11$ V: step-down.
2. $V_p{I}_p=V_s{I}_s$: $I_p=\frac{11\times 2}{220}=0.1$ A.

Answer: 11 V; step-down; 0.1 A

Physics in real life: Magnetic Effect of Current and Electromagnetic Induction

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Transformer: Step-Up and Step-Down
• Electromagnetic Induction and the AC Generator
• Force on a Current-Carrying Conductor and the DC Motor

Magnetic Effect of Current and Electromagnetic Induction formula sheet

Transformer (ideal)

$\frac{V_s}{V_p}=\frac{N_s}{N_p}=\frac{I_p}{I_s}$

Step-up: Nₛ > Nₚ; step-down: Nₛ < Nₚ.

Power in an ideal transformer

$V_p{I}_p=V_s{I}_s$

Input power = output power.

Household AC

$f=50\text{Hz},V=220\text{V}$

India.

Force on a conductor (qualitative)

$F\propto B,I,l$

Maximum when perpendicular to the field.

Magnetic Effect of Current and Electromagnetic Induction: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Magnetic Effect of Current and Electromagnetic Induction

The question patterns to expect, and the method that works for each.

Tricks to remember Magnetic Effect of Current and Electromagnetic Induction

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Left for Motors, Right for generators

  LM, RG: left hand for motors (force), right hand for generators (induced current).

  Use it when: Choosing Fleming's rule.

• FBI on the left hand

  Thumb = Force (motion), first finger = B (field), middle = I (current).

  Use it when: Fleming's left-hand rule.

• Clock rule

  Clockwise seen from an end = South; anticlockwise = North.

  Use it when: Polarity of coils.

• Split for motor, slip for generator

  Split ring keeps a motor turning one way; slip rings give an alternating output.

  Use it when: Motor vs AC generator parts.

Magnetic Effect of Current and Electromagnetic Induction: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 10-mark question split into 3 + 3 + 4 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Magnetic Effect of Current and Electromagnetic Induction

The slips examiners see most often, and what to write instead.

• ✗Mistake: Motor factors confused with generator factors

  CISCE examiners' comments, 2024: for a DC motor's speed, candidates confused number of coils with number of turns, wrote 'magnet' instead of strength of the field, and added induced current or area of cross-section; for an AC generator, others gave motor factors.

  ✓Do this: Motor speed: current, turns, field strength. Generator output: speed of rotation, turns, area, field strength.

  Usually costs: 1 mark each.

• ✗Mistake: Wrong transformer type or thicker coil

  CISCE examiners' comments, 2024: candidates misread the windings, called a step-down transformer step-up, and chose the primary as thicker because it has more turns.

  ✓Do this: Count the turns; the thicker wire is on the coil that carries the larger current (secondary in step-down).

  Usually costs: 2 marks.

• ✗Mistake: Wrong rule named

  CISCE examiners' comments, 2024 and 2025: for a compass near a wire candidates named Fleming's rules or the clock rule instead of the right-hand thumb rule; for a rod in a field they named the right-hand rule or Faraday's or Lenz's law instead of Fleming's left-hand rule.

  ✓Do this: Field of a wire: right-hand thumb rule. Force on a current: left-hand rule. Induced current: right-hand rule. Polarity of a coil: clock rule.

  Usually costs: 1 mark.

• ✗Mistake: Missing the indirect effect of thickness

  CISCE examiners' comments, 2025: replacing a rod by a thicker one, candidates said 'no effect' because area is not in F = BIl, not seeing that lower resistance means more current and a larger force.

  ✓Do this: Ask what the change does to I, B or l indirectly.

  Usually costs: 2 marks.

• ✗Mistake: Polarity and field-line direction

  CISCE examiners' comments, 2025: candidates did not know the clock rule, changed the orientation of the given bar, and drew field lines in the wrong direction.

  ✓Do this: Keep the figure as given; apply the clock rule at each end; draw lines from N to S outside.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Voltage instead of frequency; vague terms

  CISCE examiners' comments, 2025: candidates gave 220 V when the frequency of AC was asked, wrote 'alternate current', and gave vague answers about the turns ratio of a step-up transformer.

  ✓Do this: AC in India: 50 Hz (220 V). Step-up turns ratio Nₛ/Nₚ > 1.

  Usually costs: 1 mark each.

Practice questions on Magnetic Effect of Current and Electromagnetic Induction

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   The frequency of the AC household supply in India is:
   (a) 50 Hz
   (b) 60 Hz
   (c) 220 Hz
   (d) 100 Hz

   Hint

   Not the voltage.

   Answer

   (a)

2. Q2 · 1 mark · MCQBoard level

   The part of a DC motor that reverses the current in the coil every half rotation is the:
   (a) brush
   (b) slip ring
   (c) split-ring commutator
   (d) field magnet

   Hint

   Split, not slip.

   Answer

   (c)

3. Q3 · 1 mark · MCQHarder board level

   For a step-up transformer, the turns ratio $\frac{N_s}{N_p}$ is:
   (a) less than 1
   (b) equal to 1
   (c) more than 1
   (d) zero

   Hint

   More secondary turns.

   Answer

   (c)

4. Q4 · 1 mark · Assertion-reasonBoard level

   Assertion (A): A current is induced in a coil only while a magnet is moving relative to it.
   Reason (R): A current is induced only when the magnetic field through the coil changes.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   No change, no current.

   Answer

   (c)

5. Q5 · 2 marks · Very short answerHarder board level

   State two factors that affect the magnitude of the induced current in an AC generator.

   Hint

   Speed, turns, area, field.

   Answer

   Any two: speed of rotation of the coil; number of turns; area of the coil; strength of the magnetic field.

6. Q6 · 2 marks · Short answerHarder board level

   Which produces a varying magnetic field when it flows through a conductor, AC or DC? Why?

   Hint

   A varying current gives a varying field.

   Answer

   Alternating current: its magnitude and direction keep changing, so the field it produces changes too; steady DC gives a constant field.

7. Q7 · 3 marks · Long answerBoard level

   (a) Describe Oersted's experiment. (b) State the right-hand thumb rule.

   Hint

   Compass near a wire.

   Answer

   (a) A compass placed below a wire deflects when current flows and deflects the other way when the current is reversed: a current produces a magnetic field. (b) Hold the wire in the right hand with the thumb along the current; the curled fingers show the direction of the field lines.

8. Q8 · 3 marks · Long answerHarder board level

   A transformer reduces 220 V to 11 V and its secondary has 50 turns. Find the primary turns. If the secondary current is 4 A, find the primary current (ideal).

   Hint

   Nₚ = Nₛ × Vₚ/Vₛ.

   Answer

   $N_p=50\times \frac{220}{11}=1000$; $I_p=\frac{11\times 4}{220}=0.2$ A.

9. Q9 · 4 marks · Long answerHOTS / competency

   A copper rod connected to a cell lies across the poles of a horseshoe magnet and moves when the circuit is closed. (a) Name the rule for its direction of motion. (b) The rod is replaced by a thicker copper rod of the same length. Will the force increase, decrease or stay the same? Justify. (c) State two other ways to increase the force.

   Hint

   Thicker rod, less resistance.

   Answer

   (a) Fleming's left-hand rule. (b) Increase: the thicker rod has lower resistance, so more current flows from the same cell, and the force increases with current. (c) Use a stronger magnet; increase the current (e.g. more cells); increase the length of rod in the field.

10. Q10 · 3 marks · Long answerHarder board level

    Give three differences between a DC motor and an AC generator.

    Hint

    Energy, ring, rule.

    Answer

    Motor: electrical → mechanical; generator: mechanical → electrical. Motor uses a split-ring commutator; AC generator uses slip rings. Motor's direction follows Fleming's left-hand rule; generator's induced current follows Fleming's right-hand rule.

11. Q11 · 1 mark · MCQHOTS / competency

    A bar magnet is dropped with its N pole downwards through a horizontal copper ring. As the magnet approaches the ring, the upper face of the ring behaves as:
    (a) a north pole
    (b) a south pole
    (c) no pole
    (d) alternately N and S

    Hint

    The induced current opposes the approach.

    Answer

    (a)

12. Q12 · 4 marks · Long answerHOTS / competency

    Electricity from a generator at 11 kV is stepped up to 220 kV for transmission. (a) What is the turns ratio $\frac{N_s}{N_p}$? (b) By what factor does the current fall for the same power? (c) By what factor does the heating loss $I^{2}R$ in the cables fall? (d) Why can this not be done with DC?

    Hint

    Ratio 20.

    Answer

    (a) $\frac{220}{11}=20$. (b) Current falls by a factor of 20. (c) Loss falls by $20^2=400$. (d) Transformers need a changing magnetic field, which only AC provides.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Who showed that a current produces a magnetic field?Show answer

   Oersted.

2. 2.Shape of field lines round a straight wire?Show answer

   Concentric circles.

3. 3.Rule for the polarity of a coil?Show answer

   Clock rule.

4. 4.Energy conversion in a DC motor?Show answer

   Electrical → mechanical.

5. 5.Energy conversion in an AC generator?Show answer

   Mechanical → electrical.

6. 6.Frequency of AC in India?Show answer

   50 Hz.

7. 7.Transformer relation?Show answer

   Vₛ/Vₚ = Nₛ/Nₚ = Iₚ/Iₛ.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you match each rule to its use?
• ☐Can you mark poles with the clock rule?
• ☐Can you label a DC motor and an AC generator?
• ☐Do you know factors for motor speed vs generator output?
• ☐Can you identify step-up/step-down and the thicker coil?
• ☐Can you state three advantages of AC?

Magnetic Effect of Current and Electromagnetic Induction: frequently asked questions