Electro-magnetism — ICSE Class 10 Physics Important Questions

Answer: (b) Fleming's left-hand rule.

Fleming's left-hand rule gives the direction of force (thumb) from the field (forefinger) and current (middle finger). The right-hand rule is used for induced current in a generator.

Answer: (c) electric motor.

A motor uses the force on a current-carrying coil in a magnetic field to rotate, converting electrical energy into mechanical energy. A generator does the reverse conversion.

Answer: (c) is magnetised and demagnetised easily.

Soft iron has high permeability and low retentivity, so it becomes strongly magnetised when current flows and loses its magnetism when current stops — ideal for a temporary (electro)magnet. Steel retains magnetism and is used for permanent magnets.

Answer: (b) twice in each rotation of the coil.

As the coil rotates, each side moves up then down through the field once per revolution, so the induced e.m.f. changes direction twice per rotation — giving alternating current with the two slip rings.

Answer: (a) A d.c. motor does use a split-ring commutator, and its function is exactly to reverse the current in the coil every half turn so the torque keeps acting in the same rotational sense. R correctly explains A.

The strength of an electromagnet can be increased by:

1. Increasing the current flowing through the coil (solenoid).
2. Increasing the number of turns per unit length of the coil.

(A soft iron core in place of air also greatly increases the strength.)

Fleming's right-hand rule: Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular. If the forefinger points in the direction of the magnetic field and the thumb in the direction of motion of the conductor, then the middle finger points in the direction of the induced current.

It is applied to find the direction of the induced current in a generator (electromagnetic induction).

The field of a solenoid is like that of a bar magnet, with one end acting as North and the other as South.

Resemblance to a bar magnet:

1. It has two poles, a North and a South pole, at its two ends.
2. The field lines outside emerge from the North pole and enter the South pole, and the field outside is like that of a bar magnet.

(The field inside the solenoid is strong and uniform, directed from S to N.)

The force on a current-carrying conductor in a magnetic field depends on:

1. Strength of the magnetic field $B$B — the force increases as $B$B increases.
2. Current $I$I in the conductor — the force increases as $I$I increases.
3. Length $l$l of the conductor in the field — the force increases as $l$l increases.

The force is greatest when the conductor is perpendicular to the field and zero when it is parallel to the field.

Apply Fleming's left-hand rule: forefinger points along the field (vertically downward), middle finger along the current (west to east). The thumb then points towards the north (horizontally).

So the force on the wire is directed towards the north.

(i) Current reversed (east to west): the force reverses direction and now points towards the south; its magnitude is unchanged.

(ii) Current doubled: since force $\propto I$I, the force doubles in magnitude, its direction remaining towards the north.

(a) Construction: A d.c. motor has a rectangular coil ABCD wound on a soft-iron armature placed between the poles of a permanent magnet. The ends of the coil are connected to the two halves of a split-ring commutator, which press against carbon brushes connected to a battery.

(b) Working: When current flows through the coil, side AB and side CD carry current in opposite directions in the magnetic field. By Fleming's left-hand rule, AB experiences an upward force and CD a downward force. This couple rotates the coil. After every half rotation, the commutator reverses the current, so the force on each side keeps the coil turning in the same direction continuously.

(c) Function of split-ring commutator: It reverses the direction of current in the coil after every half rotation, so that the torque always acts in the same rotational sense and the coil rotates continuously in one direction.

(a) Principle: An a.c. generator works on electromagnetic induction — when the magnetic flux linked with a rotating coil changes, an e.m.f. is induced in it.

Construction and working: A coil is rotated in a magnetic field; its ends are connected to two slip rings pressing on carbon brushes. As the coil rotates, the flux through it changes continuously, inducing an alternating e.m.f. When one side moves up and the other down, current flows one way; half a rotation later the motion reverses, so the current reverses — giving alternating current.

(b) Output waveform: The current varies sinusoidally with time, reversing direction twice per rotation.

(c) Differences:

1. An a.c. generator uses two slip rings; a d.c. generator uses a split-ring commutator.
2. The output of an a.c. generator is alternating current (reverses periodically); the output of a d.c. generator is direct (unidirectional) current.

(i) Soft iron is used because it is magnetised strongly when current flows and loses its magnetism instantly when the current stops (low retentivity). Steel would retain magnetism and the armature would not release, so the bell could not work repeatedly.

(ii) The contact screw makes and breaks the circuit: as the armature moves, the circuit is broken at the screw, demagnetising the core; the spring then returns the armature and remakes contact. This make-and-break action makes the hammer strike repeatedly.

(iii) It works on the magnetic effect of electric current (an electric current produces a magnetic field).

(iv) Any one of: electric relay, loudspeaker, telephone receiver, magnetic crane, or a d.c. motor.