12. Magnetic Effects of Electric Current

• Field lines emerge from the north pole and merge at the south pole outside the magnet.
• Inside the magnet, they go from the south pole to the north pole. So they are closed curves.
• Field lines are closer where the magnetic field is stronger, like near the poles.
• No two field lines ever intersect each other.

If two field lines crossed, there would be two directions of the magnetic field at that point. A compass needle placed there would point in two directions at once. This is not possible. So two field lines never intersect.

Hold the wire in the right hand with the thumb along the current. The curled fingers show the direction of the field.

For a clockwise current, the magnetic field is into the table (downward) inside the loop. It is out of the table (upward) outside the loop.

Answer: (d) is the same at all points.

The field inside a long solenoid is uniform.

Answer: (c) velocity and (d) momentum

The magnetic force changes the direction of motion of the proton. So its velocity and momentum change. Its mass and speed stay the same.

The force on the rod depends on the current, the strength of the magnetic field and the length of the rod. So in all three cases the force increases.

(i) The displacement increases. (ii) The displacement increases. (iii) The displacement increases.

Answer: (d) upward

By Fleming’s left-hand rule, current is towards west and force is towards north. So the field must be upward.

• An electric fuse in the circuit.
• Earthing of metallic electric appliances.

The oven needs about 9 A, but the circuit is rated for only 5 A. So the circuit gets overloaded. The fuse will melt and break the circuit. If there is no fuse, the wires may get very hot and cause a fire.

• Do not connect too many appliances to a single socket.
• Do not use high-power appliances together on the same circuit.
• Use a fuse of proper rating in the circuit.

Answer: (d) The field consists of concentric circles centred on the wire.

Answer: (c) increases heavily.

(a) True

(b) False – the wire with green insulation is the earth wire. The live wire usually has red insulation.

• By using a permanent magnet, such as a bar magnet.
• By passing an electric current through a conductor, such as a straight wire, a circular loop or a solenoid (electromagnet).

The force is largest when the direction of the current is perpendicular to the direction of the magnetic field.

Electrons move from the back wall to the front wall. So the current is from the front wall to the back wall. The force is towards the right.

By Fleming’s left-hand rule, the magnetic field is vertically downward.

Hold the current-carrying conductor in the right hand with the thumb pointing in the direction of current. The direction in which the fingers wrap around the conductor gives the direction of the magnetic field.

Stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of current, then the thumb gives the direction of the force on the conductor.

Stretch the thumb, forefinger and middle finger of the right hand so that they are 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 gives the direction of the induced current.

A short circuit occurs when the live wire and the neutral wire come in direct contact. This can happen when the insulation of wires is damaged or there is a fault in an appliance. The resistance becomes very small, so the current increases suddenly.

The earth wire connects the metallic body of an appliance to a metal plate deep in the earth. It gives a low-resistance path to the current.

If live wire touches the metallic body due to a fault, the current flows to the earth through the earth wire. So the body of the appliance stays at the potential of the earth. The user does not get a severe electric shock. That is why metallic appliances are earthed.