Electromagnetism Class 10 ICSE Important Questions with Answers

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).

(a) Right-hand thumb rule: Hold the current-carrying wire in the right hand with the thumb pointing in the direction of the current; the curled fingers then give the direction of the magnetic field lines around the wire.

(b) The field lines are concentric circles centred on the wire, closer together near the wire where the field is stronger. Seen from above, with the current coming up towards you, they run anticlockwise.

(c) The circles stay the same but their direction reverses: they now run clockwise as seen from above.

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)

(b) A d.c. motor has a rectangular coil of insulated copper wire, wound on a soft-iron armature, placed between the poles $N$N and $S$S of a strong magnet. The ends of the coil are joined to the two halves of a split-ring commutator, and two carbon brushes press against these halves and connect the coil to a battery. When current flows, the coil rotates. Energy transfer: electrical energy is changed into mechanical (kinetic) energy.

(c) The split-ring commutator reverses the direction of current in the coil after every half rotation, so that the coil keeps rotating in the same direction.

(d) Fleming's left-hand rule.

(a)

(b) An a.c. generator has a rectangular coil (armature) placed between the poles of a strong magnet. The coil is turned by an engine or a turbine. Its two ends are joined to two separate slip rings, and two carbon brushes pressing on the rings connect the coil to the external circuit. It works on electromagnetic induction. Energy transfer: mechanical energy is changed into electrical energy.

(c) $50$50 Hz.

(d) Fleming's right-hand rule.

(e) The voltage of a.c. can be raised or lowered with a transformer, so power can be sent over long distances at high voltage with much less loss of energy.

(i) At the power station: a step-up transformer. In the charger: a step-down transformer.

(ii)

(iii) In a step-up transformer the secondary coil has more turns than the primary and is made of thinner wire, because it carries a smaller current at a higher voltage. The primary has fewer turns of thicker wire. (In a step-down transformer it is the other way round.)

(iv) A transformer works on electromagnetic induction, which needs a changing magnetic field in the core. A steady d.c. gives a constant field, so no voltage is induced in the secondary coil.