CBSE Class 10 Science Magnetic Effects of Electric Current Notes Set 02

Magnets and magnetic field

A magnet attracts objects of iron, cobalt and nickel. A freely suspended bar magnet comes to rest along the north-south direction, and like poles repel while unlike poles attract. The region around a magnet in which its force can be felt is the magnetic field. It has both magnitude and direction, so it is a vector quantity.

Magnetic field lines

Field lines are imaginary curves that show the path a free north pole would follow. Their key properties are:

1. Outside the magnet they run from the north pole to the south pole, and inside the magnet from south to north, so they form closed loops.
2. Two field lines never cross, because the field at a point can have only one direction.
3. Where the lines are crowded the field is strong, and where they are far apart it is weak.

Field due to a straight current-carrying wire

Oersted found that a compass needle deflects near a wire carrying current, so electricity and magnetism are linked. The field around a straight wire forms concentric circles centred on the wire. Their direction follows the right-hand thumb rule: imagine holding the wire in your right hand with the thumb along the current, and the curled fingers then show the direction of the field lines. The field gets weaker as you move away from the wire and stronger as the current increases.

Field due to a circular loop and a solenoid

Every small part of a circular loop produces a field in the same sense inside the loop, so the field is strongest at the centre. For a coil, the field at the centre grows when the current rises, when the number of turns rises, and when the radius falls.

A solenoid is a long coil of closely wound insulated copper wire shaped like a cylinder. Its field resembles that of a bar magnet, with one end acting as a north pole and the other as a south pole. Inside the solenoid the field lines are parallel straight lines, which means the field is uniform there. A soft iron core placed inside a solenoid turns it into an electromagnet, because the iron becomes strongly magnetised while current flows.

Force on a current-carrying conductor

A conductor placed in a magnetic field feels a force when current flows through it. The force is largest when the current is at right angles to the field. Fleming's left-hand rule gives its direction: stretch the thumb, forefinger and middle finger of the left hand at right angles to one another. If the forefinger shows the field and the middle finger shows the current, the thumb shows the direction of the force (motion).

Electromagnetic induction

When the magnetic field linked with a coil changes, for example by moving a magnet in or out of it, a potential difference and hence a current is induced in the coil. This is electromagnetic induction. Fleming's right-hand rule gives the direction of the induced current: with the forefinger along the field and the thumb along the motion of the conductor, the middle finger points along the induced current.

Direct and alternating current

Direct current (DC) flows in one direction only and comes from cells and batteries. Alternating current (AC) reverses its direction periodically. In India the AC supply has a frequency of 50 Hz, so it reverses direction every 1/100 second. AC is preferred for supply because it can be sent over long distances with much less loss of energy.

Domestic electric circuits

Homes get AC at 220 V and 50 Hz through three wires. The live wire carries the current (red or brown insulation), the neutral wire completes the circuit (black or blue), and the earth wire (green) is joined to a metal plate buried deep in the ground. The earth wire carries any leaking current away so that a person touching the metal body of an appliance does not get a shock. A short circuit occurs when live and neutral wires touch, and overloading occurs when too many appliances draw current from one socket. In both cases the current becomes very large. A fuse is a thin wire of low melting point connected in series, and it melts to break the circuit when the current exceeds its rating.

Worked examples

Example 1. A long straight wire carries current towards the north. Find the direction of the magnetic field just above the wire.
Hold the wire in the right hand with the thumb pointing north. Above the wire the fingers curl towards the east, so the field there points east (and below the wire it points west).

Example 2. A current flows towards the east in a conductor placed in a field directed towards the north. Find the direction of the force.
Use the left hand: forefinger north (field), middle finger east (current). The thumb then points vertically upward, so the force on the conductor is upward.

Example 3. What fuse rating is suitable for an electric heater of 1 kW used on a 220 V supply?
Current I = P/V = 1000/220 ≈ 4.5 A. A fuse of slightly higher rating, 5 A, is chosen so that it carries the normal current but melts if the current rises well above it.

Quick Check Questions