Magnetic Fields

A magnetic field is a region around a magnet where magnetic forces act. Magnetic field lines show the direction and strength of the field. They always run from north to south outside the magnet. The closer the field lines are together, the stronger the field.

Bar Magnet Field Lines

N

S

Field lines exit from the North pole

Field lines enter at the South pole

Lines never cross each other

Closer lines = stronger field

Like Poles Repel

N-N or S-S: field lines push away from each other. The magnets experience a repulsive force.

Unlike Poles Attract

N-S: field lines connect from one magnet to the other. The magnets experience an attractive force.

Earth's magnetic field: The Earth behaves as a giant magnet. Its magnetic south pole is near the geographic North Pole, which is why a compass needle's north end points north.

Electromagnetism and Solenoids

When an electric current flows through a wire, it creates a magnetic field around the wire. A solenoid is a coil of wire that produces a uniform magnetic field inside it when current flows -- essentially an electromagnet. The field inside a solenoid resembles that of a bar magnet.

Solenoid Magnetic Field

N

S

Coils of wire (solenoid) with current flowing

Uniform field inside the solenoid (parallel lines)

Field pattern outside resembles a bar magnet

Strengthening an Electromagnet

Increase Current

More current = stronger field

More Coils

More turns = stronger field

Iron Core

Soft iron core greatly amplifies field

The Right-Hand Rule

The right-hand rule is a tool for determining the direction of the magnetic field around a current-carrying wire or the polarity of a solenoid.

For a Straight Wire

Point your right thumb in the direction of conventional current (positive to negative). Your fingers curl in the direction of the magnetic field lines around the wire.

Thumb = Current direction

Curled fingers = Field direction

For a Solenoid

Curl your right hand fingers in the direction of conventional current through the coils. Your thumb points to the north pole of the solenoid.

Curled fingers = Current direction

Thumb = North pole

Applications of Electromagnetism

Electric motors: Use the force on a current-carrying conductor in a magnetic field to produce rotation.

Speakers: An electromagnet vibrates a cone in response to varying current, producing sound waves.

MRI machines: Use powerful superconducting electromagnets to create detailed images of the body.

Maglev trains: Use electromagnetic levitation to float above tracks, reducing friction for high-speed travel.

Key Vocabulary

Magnetic Field

A region around a magnet or current-carrying wire where magnetic forces act on other magnetic materials or moving charges.

Solenoid

A coil of wire that produces a uniform magnetic field inside when electric current flows through it. It acts like a bar magnet.

Electromagnet

A solenoid with a soft iron core that produces a strong magnetic field when current flows, and can be switched on and off.

Right-Hand Rule

A technique using your right hand to determine the direction of the magnetic field around a current or the polarity of a solenoid.

Worked Examples

1

A solenoid has current flowing anticlockwise when viewed from the left end. Which end is the north pole?

Step 1: Use the right-hand rule for solenoids: curl your fingers in the direction of current flow.

Step 2: When viewed from the left and current is anticlockwise, curling fingers anticlockwise means your thumb points to the left.

Answer: The left end is the north pole of the solenoid.

2

List three ways to increase the strength of an electromagnet.

1. Increase the current flowing through the coil.

2. Increase the number of turns (coils) in the solenoid.

3. Insert a soft iron core inside the solenoid to concentrate and amplify the magnetic field.

3

Explain the advantage of electromagnets over permanent magnets.

Switchability: Electromagnets can be turned on and off by controlling the current. Permanent magnets cannot be switched off.

Adjustability: The strength of an electromagnet can be varied by changing the current.

Application: This makes electromagnets ideal for scrapyard cranes (picking up and releasing metal), MRI machines, and electric motors where controllable magnetism is needed.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Magnetic field lines around a bar magnet travel from:

North to south outside the magnet South to north outside the magnet Only inside the magnet In random directions

Question 2

Which of the following will NOT increase the strength of an electromagnet?

Increasing the current Using a wooden core instead of iron Adding more coils Using a soft iron core

Question 3

A current-carrying wire produces a magnetic field. Using the right-hand rule, if your thumb points in the direction of current, your fingers show:

The direction of the force on the wire The direction of electron flow The direction of the magnetic field around the wire The north pole direction

Question 4

What happens to a compass needle placed near a current-carrying wire?

Nothing -- compasses only respond to permanent magnets It demagnetises It spins continuously It deflects, aligning with the magnetic field from the current

Question 5

The magnetic field inside a solenoid is:

Strong and uniform (parallel field lines) Strongest at the edges and weakest in the centre Zero Circular around each coil

Magnetism and Electromagnetism