Electromagnets, the Electric Bell and Magnetic Shielding

1Why this matters

The bell that ends your lesson, the crane that lifts scrap iron at a recycling yard, the relay that switches on a water pump, the lock on a security door and the MRI scanner in a hospital all use a magnet that can be switched on and off: an electromagnet. And when a sensitive instrument must be protected from stray magnetic fields, engineers wrap it in a magnetic shield. Both ideas are in this lesson.

2An electric current makes a magnetic field

In 1820 the Danish scientist Hans Christian Oersted noticed that a compass needle swung when a nearby wire carried a current. A current produces a magnetic field:

• Around a straight wire, the field lines are circles centred on the wire.
• In a solenoid (a long coil), the circles from all the turns add up. Outside, the field looks like that of a bar magnet; inside, it is strong and uniform. The right-hand grip rule gives the N end.

3The electromagnet

An electromagnet is a solenoid wound on a core of soft iron. When current flows, the soft iron becomes strongly magnetised and adds its field to the coil's, making the magnet hundreds of times stronger. When the current stops, soft iron loses its magnetism almost at once, so the magnet switches off.

The strength of an electromagnet can be increased by:

1. increasing the current;
2. increasing the number of turns of wire (on the same length);
3. using a soft-iron core (rather than air or a non-magnetic core).

For coils of the same length on the same core, the strength depends on the product number of turns × current, called the ampere-turns.

Worked example 1. Coil A has 200 turns and carries 3.0 A. Coil B has 400 turns and carries 1.0 A. They are the same length, on identical cores. Which is the stronger electromagnet?

A: 200 × 3.0 = 600 ampere-turns. B: 400 × 1.0 = 400 ampere-turns. A is stronger, even though it has fewer turns.

4Extension: calculating the field inside a solenoid

For a long solenoid with an air core, the field inside is

B = μ₀NI / L

where N is the number of turns, I the current in amperes, L the length of the solenoid in metres, and μ₀ = 4π × 10⁻⁷ T·m/A (a constant called the permeability of free space).

That is about the strength of a fridge magnet, and about 126 times the Earth's field (6.28 × 10⁻³ ÷ 5.0 × 10⁻⁵ ≈ 126). A soft-iron core would make it many times stronger again.

5Try it in a simulation

Open Faraday's Electromagnetic Lab and choose the Electromagnet screen. Drag the compass around the coil to see that the field looks like a bar magnet's. Then raise the battery voltage (more current) and add loops (more turns), and watch the field get stronger. Reverse the battery and watch the poles swap.

6The electric bell

Look at the diagram. The bell uses a make-and-break circuit:

1. Pressing the switch (K) completes the circuit. Current flows from the battery (U) through the coils of the electromagnet (E), through the springy strip and armature (A), and through the contact screw (T) back to the battery.
2. The electromagnet is switched on and attracts the soft-iron armature. The hammer on the end of the armature strikes the gong (B).
3. As the armature moves towards the magnet, the springy strip moves away from the contact screw, so the circuit is broken. The current stops.
4. The electromagnet loses its magnetism. The spring pulls the armature back, the strip touches the contact screw again, and the circuit is made once more.
5. Steps 2 to 4 repeat many times a second, so the bell rings continuously for as long as the switch is held down.

The core and the armature must be soft iron. If they were steel, they would stay magnetised when the current stopped, the armature would stay stuck to the magnet, and the bell would strike once and then fall silent.

7More uses of electromagnets

• Lifting magnets: cranes at scrap yards pick up iron and steel and separate them from aluminium and copper; switch off and the load drops.
• Relays: a small current in an electromagnet pulls a switch that controls a large current, such as the motor of a water pump.
• Circuit breakers: a large fault current makes an electromagnet strong enough to pull a switch open and cut off the supply.
• Loudspeakers, electric door locks and MRI scanners.

8Magnetic shielding

Field lines pass far more easily through soft iron than through air. Place a hollow ring or box of soft iron in a magnetic field and the field lines crowd into the iron walls and travel round inside them, instead of crossing the space in the middle. The space inside the box is left almost free of magnetic field. This is magnetic shielding.

Shields protect sensitive instruments (such as meters and navigation compasses on ships), anti-magnetic watches (which have a soft-iron inner case), and the rooms around MRI scanners. The shield must be made of a magnetic material: a box of copper, aluminium, plastic or wood does not stop a steady magnetic field at all, because the field passes straight through non-magnetic materials.

Practice