24.3 Content
24.3.1 Magnetic Poles and Fields
Magnets have two poles: north and south.
- Like poles repel; unlike poles attract
- Magnetic poles always come in pairs (dipoles)
- There are no magnetic monopoles—cutting a magnet creates two smaller dipoles
Magnetic field lines: - Form closed loops from north to south outside the magnet - Never cross each other - Line density indicates field strength - Direction is from N to S outside, S to N inside
24.3.2 Interactive: Bar Magnet Field
Visualising the magnetic field around a bar magnet:
24.3.3 The Earth’s Magnetic Field
The Earth behaves like a giant bar magnet:
- Geographic North Pole is near the magnetic south pole
- Compasses align with field lines
- The field protects Earth from solar wind particles
A compass needle points toward geographic north because its north-seeking pole is attracted to Earth’s magnetic south pole (located near geographic north).
24.3.4 Magnetic Fields Around Currents
A current-carrying wire creates circular magnetic field lines around it.
Right-hand grip rule: Thumb points in direction of conventional current; curled fingers show field direction.
For a long straight wire:
\[B = \frac{\mu_0 I}{2\pi r}\]
where: - \(B\) = magnetic field strength (T) - \(\mu_0 = 4\pi \times 10^{-7}\ \text{T·m/A}\) (permeability of free space) - \(I\) = current (A) - \(r\) = distance from wire (m)
24.3.5 Interactive: Magnetic Field Around a Wire
Key observation: Field lines form concentric circles around the wire. Closer to the wire, the field is stronger.
24.3.6 Solenoids and Electromagnets
A solenoid is a coil of wire that creates a strong, nearly uniform magnetic field inside:
\[B = \mu_0 n I\]
where: - \(n = N/L\) = number of turns per metre - \(N\) = total number of turns - \(L\) = length of solenoid (m)
24.3.7 Interactive: Solenoid Field
- Inside: nearly uniform field along the axis
- Outside: weak, similar to a bar magnet
- Polarity determined by right-hand grip rule (fingers follow current, thumb points to N pole)
24.3.8 Electromagnets
An electromagnet is a solenoid with a ferromagnetic core (iron):
- The core becomes magnetised by the field
- This greatly increases the field strength
- The electromagnet can be switched on/off with current
Applications: motors, relays, MRI machines, maglev trains
24.3.9 Magnetic Materials and Domains
Magnetic domains are regions within ferromagnetic materials where atomic magnetic moments are aligned:
- In unmagnetised material: domains point randomly, cancelling out
- In magnetised material: domains align, creating net magnetic field
Magnetism can be reduced by: - Heating above the Curie temperature - Mechanical shock (hammer blows) - Alternating magnetic fields (degaussing)