Magnetic Fields and Electromagnets

Magnetic Fields and Electromagnets

Magnetism is a non-contact force. Understanding magnetic fields, how they are produced, and how electromagnets work is essential for explaining motors, generators, and many everyday devices.

Magnetic Poles and Fields

Every magnet has a north pole and a south pole. The magnetic field is strongest at the poles.

• Like poles repel (N-N or S-S)
• Unlike poles attract (N-S)

A magnetic field is a region around a magnet where other magnets or magnetic materials experience a force. Magnetic field lines are used to represent the field:

• Field lines go from north to south (outside the magnet)
• The closer together the lines, the stronger the field
• Field lines never cross
• The direction of the field at any point is the direction a plotting compass would point

Drawing field lines around a bar magnet:

• Lines emerge from the north pole and curve around to enter the south pole
• They are most concentrated at the poles (strongest field)
• Far from the magnet, the field is weaker and the lines are further apart

Magnetic and Non-Magnetic Materials

Only a few materials are magnetic (attracted to magnets):

• Iron (soft iron)
• Steel (hard iron — an alloy)
• Nickel
• Cobalt

These are sometimes called ferromagnetic materials.

Non-magnetic materials (aluminium, copper, wood, plastic, glass) are NOT attracted to magnets.

Permanent and Induced Magnets

A permanent magnet produces its own magnetic field. It always has a north and south pole. Examples: bar magnet, horseshoe magnet, fridge magnet.

An induced magnet is a magnetic material that becomes a magnet when placed in a magnetic field. When you bring a permanent magnet near an iron nail:

1. The nail becomes magnetised by induction

2. The nearest end becomes the opposite pole to the permanent magnet (so it is attracted)

3. When the permanent magnet is removed, the nail loses most of its magnetism (if it is soft iron)

Steel retains its magnetism after the external field is removed — this is how permanent magnets are made.

The Earth's Magnetic Field

The Earth has a magnetic field, as if there were a giant bar magnet inside it (though the actual cause is convection currents in the liquid iron outer core).

• The Earth's magnetic north pole is near the geographic south pole (and vice versa)
• A compass needle (which is a small bar magnet) aligns with the Earth's field — its north pole points towards magnetic north (geographic north)
• The Earth's magnetic field is relatively weak compared to a bar magnet held nearby

Electromagnets

An electromagnet is a magnet created by passing electric current through a wire, usually wound into a solenoid (a coil).

How it works:

• When current flows through a wire, a magnetic field is produced around the wire
• The field around a straight wire forms concentric circles centred on the wire
• Coiling the wire into a solenoid concentrates and strengthens the field
• The field pattern inside a solenoid is similar to a bar magnet — uniform and parallel lines inside, north and south poles at the ends

Increasing the strength of an electromagnet:

• Increase the current — stronger field
• Increase the number of turns (coils) — more concentrated field
• Add a soft iron core inside the solenoid — iron is easily magnetised and greatly strengthens the field

Advantages of electromagnets over permanent magnets:

• Can be switched on and off by controlling the current
• The strength can be varied by changing the current
• The polarity can be reversed by reversing the current direction

Finding the poles: Use the right-hand grip rule — curl the fingers of your right hand in the direction of current flow around the solenoid; your thumb points to the north pole.

Uses of Electromagnets

Application	How it uses an electromagnet
Scrapyard crane	Large electromagnet lifts steel objects; switching off releases them
Electric bell	Electromagnet attracts an iron armature, which strikes the bell; the circuit breaks, the armature springs back, and the cycle repeats
Circuit breakers	Excess current strengthens the electromagnet, which trips a switch to break the circuit
MRI scanners	Very powerful electromagnets create detailed body images
Maglev trains	Electromagnets levitate and propel the train

Required Practical: Plotting Magnetic Field Lines

Method using a plotting compass:

1. Place a bar magnet on a sheet of paper

2. Place a small plotting compass near one pole of the magnet

3. Mark the position of both ends of the compass needle with dots

4. Move the compass so its tail aligns with the previous head dot

5. Repeat until you reach the other pole (or the edge of the paper)

6. Join the dots to form a smooth field line

7. Add arrows showing direction (north to south)

8. Repeat for several starting positions around the magnet

Exam Tips

• Always draw field lines with arrows from north to south
• The right-hand grip rule is for solenoids — do not confuse it with Fleming's left-hand rule (that is for the motor effect)
• When asked why an electromagnet is preferred, always mention that it can be switched on/off and its strength can be varied