Electromagnetism notes: ICSE Class 10 Physics (Science Paper 1)

1. Magnetic field of a current

Oersted’s experiment: a compass needle below a north–south wire is deflected when a current flows; the deflection reverses with the current. A current produces a magnetic field.
Straight wire: field lines are concentric circles in a plane perpendicular to the wire; closer together (stronger field) near the wire and for a larger current.
Right-hand thumb rule: grip the wire in the right hand with the thumb along the current; the fingers curl in the direction of the field.
Circular loop: circles round each part of the wire; near the centre the lines are nearly straight along the axis. One face acts as N, the other as S. More turns or more current give a stronger field.

Table of field-line shapes for three current-carrying conductors
Conductor	Shape of field lines
Straight wire	Concentric circles
Circular loop (near centre)	Nearly straight, along the axis
Solenoid (inside)	Straight and parallel

Right-hand thumb rule: If a current-carrying wire is held in the right hand with the thumb along the current, the curled fingers point along the magnetic field lines.
Magnetic field line: A line along which a free north pole would tend to move; its tangent gives the field direction at a point.

Solenoid: a long coil of insulated wire; its field is like a bar magnet’s: straight and parallel inside, looping from N to S outside.
Clock rule: viewed from one end, anticlockwise current — that end is N; clockwise — S. Reversing the current reverses the poles.
Electromagnet: a solenoid on a soft iron core; a temporary magnet while current flows.
Stronger electromagnet: more turns, more current, soft iron core (U-shaped to bring the poles close).
Soft iron, not steel: soft iron is magnetised strongly and loses its magnetism when the current stops; steel keeps some magnetism.
Uses: electric bell, cranes for scrap iron, relays, sorting iron from waste, loudspeakers, motors.

Solenoid: A long cylindrical coil of many turns of insulated wire whose magnetic field, when a current flows, resembles that of a bar magnet.
Electromagnet: A temporary magnet made by passing a current through a coil wound on a soft iron core.

A current-carrying conductor in a magnetic field experiences a force: greatest when it is perpendicular to the field, zero when parallel; reversing the current or the field reverses the force.
Fleming’s left-hand rule: thumb, forefinger and middle finger of the left hand mutually perpendicular — forefinger: field, middle finger: current, thumb: force (motion).
DC motor (parts and energy only): field magnet; armature coil on an axle; split ring commutator; carbon brushes; DC source. Energy: electrical → mechanical.
Uses: fans, mixers, washing machines, pumps, electric vehicles.

Table of the main parts of a DC motor
Part of DC motor	What it is
Field magnet	Provides the magnetic field
Armature	Coil that turns on an axle
Split rings	Commutator joined to the coil ends
Brushes	Carbon contacts to the battery

Fleming’s left-hand rule: With the left hand’s thumb, forefinger and middle finger at right angles, forefinger along the field and middle finger along the current, the thumb gives the direction of the force.
Commutator: The split ring joined to the ends of the coil of a DC motor, on which the carbon brushes press.

Electromagnetic induction: a current is induced in a closed coil whenever the magnetic field linked with it changes (magnet or coil moving, or a nearby current changing). No change, no induced current.
Larger induced current: faster motion, more turns, stronger magnet, soft iron core. Reversed motion or pole reverses it.
Fleming’s right-hand rule: thumb — motion, forefinger — field, middle finger — induced current.
AC generator (parts and energy only): field magnet; armature coil turned on an axle; two slip rings; carbon brushes; external circuit. Energy: mechanical → electrical.
Household AC in India: 50 Hz; period $T = 1/f = 0.02$ s; the direction reverses twice per cycle.

Table comparing the DC motor and the AC generator
Machine	Energy change	Rings
DC motor	Electrical → mechanical	Split rings
AC generator	Mechanical → electrical	Slip rings

Electromagnetic induction: The production of an electric current in a closed circuit by changing the magnetic field linked with it.
Frequency of AC: The number of complete cycles of an alternating current in one second, measured in hertz.

AC advantages: voltage easily changed by transformers (high-voltage, low-current transmission with little loss); generators simpler for large power; easily converted to DC. DC needed for charging batteries and electroplating.
Transformer: primary and secondary coils on a laminated soft iron core; works only on AC.
$\dfrac{V_{s}}{V_{p}} = \dfrac{N_{s}}{N_{p}}$ . Step-up: $N_{s} > N_{p}$ (power stations). Step-down: $N_{s} < N_{p}$ (chargers, doorbells, the pole transformer).
For an ideal transformer, power in = power out: $V_{p}I_{p} = V_{s}I_{s}$ , so stepping the voltage up steps the current down.

Table comparing step-up and step-down transformers
Type	Turns	Voltage	Example
Step-up	Ns > Np	Raised	At a power station
Step-down	Ns < Np	Lowered	Phone charger

Step-up transformer: A transformer with more turns on the secondary coil than on the primary, so that the output voltage is greater than the input voltage.
Step-down transformer: A transformer with fewer turns on the secondary coil than on the primary, so that the output voltage is less than the input voltage.

Common mistakes

Using the left hand for the field of a wire, or the right hand for the force on a wire.
Saying a current is induced by a magnet at rest inside a coil.
Mixing up split rings (motor) and slip rings (generator).
Saying a transformer works on DC, or that a step-up transformer increases the current too.

How marks are usually awarded

Rules: the hand, what each finger represents, and that they are at right angles.
Field diagrams: correct shape, arrows, spacing.
Transformer sums: the ratio, substitution, answer with unit, and the type (step-up or step-down).