Electromagnetism: the field round a wire and a solenoid

A current makes a magnetic field

A magnetic field isn't only made by magnets. Whenever a current flows in a wire, there is a magnetic field around that wire. No current, no field — which is why the compasses in the animation sit pointing north until you close the switch.

The field is real, but it is usually weak. A single wire carrying a small current will only nudge a plotting compass sitting right next to it.

The field round a straight wire

The field lines are circles around the wire, at right angles to it. That's why the card in the animation is pushed onto the wire at right angles: the circles lie flat on the card.

• The circles are closest together near the wire — that's where the field is strongest.
• Further out, the circles are further apart, so the field is weaker.
• Every field line has an arrow on it showing the direction the field points.
• Reverse the current and the field reverses too — all the arrows turn round.

The ⊙ and ⊗ symbols come up constantly in exams. Think of an arrow: ⊙ is the point of the arrow coming towards you, ⊗ is the feathers going away from you.

Right-hand grip rule: thumb = current, fingers = field

A solenoid with an iron core: like a bar magnet outside, uniform inside

The field round a solenoid

A solenoid is a long coil of wire. Each turn makes its own circular field, and together they add up to something much stronger and much more useful.

• Outside, the pattern looks exactly like a bar magnet's: the lines leave the north pole, curve right round, and go back in at the south pole.
• Inside, the lines carry straight on through the middle, from S to N, so every line makes a complete loop. Unlike a bar magnet, you can see inside.
• Inside the solenoid the field is strong and uniform — the lines are parallel and evenly spaced, all pointing the same way.
• The solenoid has a north end and a south end, and reversing the current swaps them over.

Making an electromagnet

Put a soft iron core inside the solenoid and you have an electromagnet. The iron becomes magnetised by the coil's field, and the field becomes much stronger.

The great advantage over a bar magnet: you can switch it on and off, and you can change how strong it is.

Describe the construction of an electromagnet [3]: a coil of wire (a solenoid), wrapped around a soft iron core, with a current flowing through the coil.

Electromagnets are used anywhere you need a magnet you can control: scrapyard cranes lifting and dropping cars, relays and circuit breakers, electric bells, and the magnetic locks on doors.

Electromagnetic devices AQA triple only

Each of these uses an electromagnet that is switched on and off by the circuit itself. Try them in tab 3 of the animation, then learn the steps in this order.

1. Close the switch: current flows through the coils.
2. The solenoid and iron core become an electromagnet: a magnetic field is created.
3. The electromagnet attracts the iron arm downwards, so the hammer hits the gong.
4. The arm moves away from the contact, which breaks the circuit.
5. No current, so the magnetic field stops: the iron arm is no longer attracted downwards.
6. The springy arm moves back up and completes the circuit again, so the cycle repeats and the bell keeps ringing.

A relay lets a small current in one circuit switch on a separate circuit: one that carries a big current, or is a long way from the person working the switch.

1. Close switch S: current flows in the input circuit, through the coil.
2. The coil and iron core become an electromagnet: a magnetic field is created.
3. The magnetic field attracts the iron armature, so the contacts at C move and close.
4. The contacts complete the output circuit.
5. Current flows in the output circuit, so the electric motor turns.

A circuit breaker breaks the circuit if the current gets too high. Normally the current is small and the electromagnet's field is too weak to move the bolt.

1. Current flows through the electromagnet. If the current increases too much…
2. …the electromagnet gets stronger until it attracts the iron bolt downwards, out of the plunger.
3. The spring pushes the plunger, and the reset button is released.
4. The metal end moves away from the switch contacts, which breaks the circuit.
5. No current flows, so the circuit is safe.

Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.

Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.