Electro-magnetism

ICSE Class 10 · Physics · Chapter 10

What this chapter covers

Magnetic effect of a current: Oersted's experiment
Magnetic field due to a current in a straight wire
Magnetic field due to a current in a loop
Electromagnets
Force on a current-carrying conductor: Fleming's left-hand rule
The DC electric motor
Electromagnetic induction
Fleming's right-hand rule
The AC generator
AC versus DC
Transformer

Key definitions

Oersted's experiment is the demonstration that a wire carrying an electric current deflects a magnetic compass needle placed near it.

Electromagnetic induction is the phenomenon in which an electromotive force, and hence a current in a closed circuit, is produced in a conductor whenever the magnetic field linked with it changes.

Key points

A current-carrying conductor produces a magnetic field around it; Oersted's experiment shows this by the deflection of a compass needle.
Field lines around a straight wire are concentric circles in a plane perpendicular to the wire; the field is stronger near the wire and with a larger current.
The right-hand thumb rule gives the direction of the field lines: thumb along the current, fingers curl along the field.
A current loop acts like a flat magnet; at its centre the field lines are nearly straight and perpendicular to the loop. An anticlockwise current (as seen) makes the near face a north pole.
An electromagnet is a soft iron core inside a current-carrying coil; it is a strong, temporary magnet whose strength and polarity can be controlled.
Fleming's left-hand rule: Field (forefinger), Current (middle finger), Force or motion (thumb). It is the principle of the electric motor.
A DC motor (coil, magnet, split-ring commutator, brushes) converts electrical energy into mechanical energy.
Electromagnetic induction is the production of an emf when the magnetic field linked with a conductor changes.
Fleming's right-hand rule: Field (forefinger), Motion (thumb), induced Current (middle finger). It is the principle of the generator.
An AC generator (coil, magnet, slip rings, brushes) converts mechanical energy into electrical energy.
Household AC in India is 50 Hz at about 220–230 V; the current reverses direction 100 times per second.
AC is preferred for supply because transformers can step its voltage up and down, which lets power be transmitted with little loss.
A transformer works only on AC. In a step-up transformer the secondary has more turns and thinner wire; in a step-down transformer the secondary has fewer turns and thicker wire. V_s/V_p = N_s/N_p.

Questions answered

Describe Oersted's experiment and state the conclusion drawn from it.

A compass needle placed below a straight wire running north-south deflects when a current is passed through the wire, deflects the other way when the current is reversed, and returns to north-south when the current stops. The experiment shows that a current-carrying conductor produces a magnetic field around it.

Define a magnetic field line and give one reason why two field lines cannot cross.

A magnetic field line is an imaginary curve whose tangent at any point gives the direction of the magnetic field there. If two lines crossed, a compass needle at the crossing point would have to point in two directions at once, which is impossible.

State the right-hand thumb rule. A vertical wire carries current downwards; seen from above, do the field lines run clockwise or anticlockwise?

If the wire is held in the right hand with the thumb along the current, the curled fingers give the direction of the field lines. A downward current (into the page, seen from above) gives clockwise field lines.

State two ways in which the magnetic field at the centre of a circular current loop can be increased.

Increase the current, or increase the number of turns (making the radius smaller also works).

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

Soft iron is magnetised strongly by a small field and loses its magnetism almost completely when the current is switched off, so the magnet can be turned on and off. Steel would keep its magnetism after the current stopped.

Give three differences between an electromagnet and a permanent magnet.

An electromagnet is magnetic only while current flows, its strength can be changed by changing the current, and its polarity can be reversed by reversing the current. A permanent magnet keeps its magnetism, has a fixed strength and has fixed poles.

State Fleming's left-hand rule. Name one device whose action depends on it.

With the forefinger, middle finger and thumb of the left hand mutually at right angles, if the forefinger shows the field and the middle finger the current, the thumb shows the force on the conductor. The DC electric motor (also the loudspeaker).

Name the main parts of a DC motor and state its energy transfer. What is the function of the split ring?

Coil (armature), field magnet, split-ring commutator, brushes and a DC source. It converts electrical energy into mechanical energy. The split ring reverses the current in the coil every half rotation so that the coil keeps turning in one direction.

Past board exam questions

ICSE 2027 sample paper · 2 marks

A copper wire is wound around a U-shape soft iron bar, and the two ends of the wire are A and B. An iron pin is brought near the arrangement. First Source P and then Source Q are connected across AB, each operating independently. Source P is an alternating current source (symbol: ~ in a circle) and Source Q is a cell (d.c. source with + and − terminals). (a) State True or False: Source P as well as Source Q, when connected across AB, can attract the iron pin. (b) Justify your answer to (a) with a suitable reason.

Show answer

(a) True. (b) With either source, current flows through the coil and the soft iron becomes an electromagnet producing a magnetic field. An iron pin is attracted by a magnet whatever its polarity, so even though the a.c. source keeps reversing the poles, the pin is still attracted.

ICSE 2026 sample paper · 1 mark

For an ideal step up transformer:

(Voltage_primary)/(Voltage_secondary) > 1
(Current_primary)/(Current_secondary) < 1
(number of turns_primary)/(number of turns_secondary) = 1
(power_primary)/(power_secondary) = 1

Show answer

(power_primary)/(power_secondary) = 1

ICSE 2026 board paper · 1 mark

The top view of a straight Wire A, shown by a cross (X), carrying current into the plane of the paper, is described below. Two compasses are placed on a circle around the wire: compass 1 directly above the wire, with its needle pointing to the right; compass 2 directly to the right of the wire, with its needle pointing upwards (towards the top of the page). Which of the compasses is correctly aligned with the magnetic field, produced by the current carrying wire?

Only 1 is aligned
Only 2 is aligned
Both 1 and 2 are aligned
Both 1 and 2 are not aligned

Show answer

Only 1 is aligned

ICSE 2026 board paper · 2 marks

In an AC generator, name the part which has the following functions: (a) intensifies the magnetic field. (b) maintains electrical contact between the rotating parts and the external circuit.

Show answer

(a) The soft iron core (armature) on which the coil is wound. (b) The carbon brushes (pressing against the slip rings).

ICSE 2026 board paper · 3 marks

A coil is connected to a galvanometer G. A bar magnet, held with its N pole facing the coil, is moved towards the coil at a speed of 5 ms⁻¹, and the galvanometer shows a certain deflection to the right. How will the direction and magnitude of deflection change when the coil also moves with a speed of 5 ms⁻¹: (a) in the direction of the motion of the magnet? (b) in the opposite direction of the motion of the magnet?

Show answer

(a) There is no relative motion between the magnet and the coil, so the magnetic flux through the coil does not change: no current is induced and the galvanometer shows no deflection (zero). (b) The relative speed becomes 5 + 5 = 10 m s⁻¹ with the magnet still approaching the coil, so the deflection stays to the right but its magnitude increases (about double), as the flux changes faster.

ICSE 2025 compartment paper · 1 mark

A current carrying circular loop is lying in a horizontal plane as described below. Which of the following is the correct statement with respect to the direction of magnetic lines of force. (Viewed from above, the current in the loop flows clockwise. Point X lies on the plane outside the loop and point Y lies inside the loop, near its centre.)

upward at X and downward at Y
downward at X and upward at Y
upward at both X and Y
downward at both X and Y

Show answer

upward at X and downward at Y

ICSE 2025 sample paper · 2 marks

(a) Name the principle of AC generator. (b) State its one use.

Show answer

(a) Electromagnetic induction (Faraday's laws of electromagnetic induction). (b) It is used to generate electricity at power stations / as a back-up power supply (e.g. for lifts in tall buildings) during electricity failure.

ICSE 2025 compartment paper · 2 marks

State two properties of magnetic lines of force around a straight conductor carrying current.

Show answer

1. They are concentric circles with the conductor at their centre, lying in a plane perpendicular to the conductor. 2. Their direction is given by the right-hand thumb rule (reverses when the current reverses); they are closer together near the conductor, showing a stronger field there, and they never intersect.