Chapter 12: Magnetic Effects of Electric Current (Physics)

See the diagram: closed curves from the N pole to the S pole outside the magnet, crowded near the poles.

• Outside a magnet they go from the north pole to the south pole; inside the magnet, from south to north. So they are closed curves.
• The direction of the field at a point is along the tangent to the field line there.
• They are closer together where the field is stronger (near the poles).
• No two field lines cross each other.

They are closed curves from N to S outside the magnet, crowded where the field is strong, and never intersect.

The direction of the magnetic field at a point is given by the tangent to the field line through it. If two lines crossed, there would be two tangents at the crossing point, i.e. a compass needle there would point in two directions at once, which is impossible.

Because the field at any point has only one direction; crossing lines would give two directions there.

Apply the right-hand thumb rule to each part of the loop, with the thumb along the clockwise current. The fingers curl so that the field lines go into the table inside the loop (vertically downwards) and come out of the table outside the loop (vertically upwards).

Inside the loop: perpendicular to the table, into it (downwards). Outside the loop: out of the table (upwards).

Parallel, equally spaced straight lines, all pointing in the same direction.

Inside a long solenoid the field lines are parallel straight lines, so the field is uniform.

(d) is the same at all points.

The magnetic force on a moving charge is perpendicular to its motion, so it changes the direction of motion but not the speed. Velocity and momentum depend on direction, so they change; mass and speed do not.

(c) velocity and (d) momentum

• An electric fuse (or MCB) in series with the circuit, which melts and breaks the circuit when the current becomes too large.
• Earthing of metal appliances, so that any leakage current flows to the earth and the user does not get a shock.

An electric fuse (or MCB) and earthing of appliances.

Current drawn: $I=\frac{P}{V}=\frac{2000}{220}\approx 9.1$ A.

This is much more than the 5 A rating, so the circuit is overloaded. The wires get overheated and the fuse will blow (or the MCB will trip), cutting off the supply. Without a fuse, the overheated wires could start a fire.

The oven draws about 9.1 A, more than 5 A, so the fuse blows (the circuit is overloaded).

• Do not connect too many appliances to a single socket.
• Do not use high-power appliances (heaters, ovens, geysers) together on a low-rating (5 A) circuit; use separate 15 A power circuits for them.
• Use a fuse or MCB of the correct rating, and good-quality wires of proper thickness.

Avoid connecting too many appliances at once, use separate power circuits for heavy appliances, and use fuses of the correct rating.

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

In a short circuit the resistance becomes almost zero, so the current becomes very large.

(c) increases heavily.

1. True. Near the centre of the coil, the field lines are nearly straight and parallel.
2. False. Green (insulation) is the earth wire; the live wire has red (or brown) insulation and the neutral black (or blue).

(a) True (b) False

• Using a permanent magnet, such as a bar magnet or a horseshoe magnet.
• Passing an electric current through a conductor, such as a straight wire, a circular coil or a solenoid (an electromagnet).

With a permanent magnet, and with a current-carrying conductor (e.g. a solenoid).

When the direction of the current is perpendicular to the direction of the magnetic field. (It is zero when they are parallel.)

When the current is at right angles to the magnetic field.

Electrons carry negative charge, so the current is opposite to their motion: from the front wall towards the back wall. The force is towards the right.

By Fleming's left-hand rule, with the middle finger pointing backwards (current) and the thumb to the right (force), the forefinger points vertically downwards.

Vertically downwards.

1. Right-hand thumb rule: hold the conductor in the right hand with the thumb along the current; the curled fingers show the direction of the field lines.
2. Fleming's left-hand rule: with the forefinger along the field and the middle finger along the current (both perpendicular), the thumb gives the direction of the force (motion).
3. Fleming's right-hand rule: stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular; with the forefinger along the field and the thumb along the motion of the conductor, the middle finger gives the direction of the induced current.

(i) Right-hand thumb rule (ii) Fleming's left-hand rule (iii) Fleming's right-hand rule

A short circuit occurs when the live wire and the neutral wire come into direct contact, for example when the insulation is damaged or an appliance is faulty. The resistance of the circuit becomes nearly zero, so a very large current flows, which can overheat the wires and cause a fire.

When the live and neutral wires touch directly, so the resistance drops to almost zero and the current becomes very large.

The earth wire (green insulation) connects the metal body of an appliance to a metal plate buried deep in the earth. It provides a low-resistance path for current.

If the live wire accidentally touches the metal body of an appliance (through leakage or a fault), the current flows to the earth through the earth wire instead of through the person touching it. This keeps the potential of the body at that of the earth and prevents a severe electric shock. So metallic appliances such as irons, toasters and refrigerators must be earthed.

It gives leakage current a low-resistance path to the earth, so a person touching a faulty metal appliance does not get a severe shock.