Chapter 12: Magnetic Effects of Electric Current

Q1: (d) concentric circles centred on the wire. Q2: (c) increases heavily.

(a) True — inside a long coil/solenoid the field is nearly uniform (parallel straight lines). (b) False — a green wire is the earth wire, not the live wire (live is usually red/brown).

Q4: (i) By a current-carrying conductor (electromagnet); (ii) by a permanent magnet (or by moving a magnet). Q5: The force is largest when the conductor is perpendicular (at 90°) to the direction of the magnetic field.

The electrons move from the back wall to the front wall, so the conventional current is from front to back. The force is to the right. Applying Fleming's left-hand rule, the magnetic field is directed vertically downward.

(i) Right-hand thumb rule (Maxwell's corkscrew rule) — for the field around a straight current-carrying conductor. (ii) Fleming's left-hand rule — for the force on a current-carrying conductor in a magnetic field. (iii) Fleming's right-hand rule — for the current induced in a coil rotating in a magnetic field.

Q8: A short circuit occurs when the live and neutral wires touch directly (e.g., insulation is damaged), so the resistance drops and the current rises heavily. Q9: The earth wire provides a low-resistance path for current to flow to the earth. Metallic appliances are earthed so that any leakage current passes safely to the ground instead of giving the user an electric shock.

Uniform (parallel straight lines).

Force (direction of motion).

Generator (dynamo).

Fuse.

A temporary magnet made by passing current through a coil wound on a soft iron core.

Because at the crossing point there would be two directions of the field, which is impossible.

Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular: forefinger = field, thumb = motion, middle finger = induced current.

It carries any leakage current from a metallic appliance safely to the ground, protecting the user from electric shock.

AC (alternating current) reverses direction periodically; DC (direct current) flows in one direction only.

A rectangular coil is placed in a magnetic field. When current flows, its two sides experience opposite forces (Fleming's left-hand rule), producing a turning effect (torque) that rotates the coil. A split-ring commutator reverses the current after each half rotation, keeping the coil rotating continuously — converting electrical energy into mechanical energy.

When the magnetic field through a coil changes (e.g., by moving a magnet in and out), a current is induced in the coil. This is electromagnetic induction. Its direction is found by Fleming's right-hand rule. It is the basis of the electric generator.

A coil is rotated in a magnetic field. As it rotates, the magnetic flux through it changes, inducing a current (Fleming's right-hand rule). The current reverses direction every half rotation, giving alternating current, which is taken out through slip rings and brushes.

A short circuit occurs when the live and neutral wires touch directly, making the resistance very low and the current very high. Overloading occurs when too many high-power appliances draw current through the same line, exceeding its safe limit. Both can cause heating and fire; a fuse protects against them.

Field lines emerge from the north pole, curve around, and enter the south pole, forming closed loops. They are crowded near the poles (strong field) and spread out in the middle (weaker field), and never intersect.

So that during a fault it breaks the live wire and disconnects the supply, making the appliance safe to touch and stopping the excess current.