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6.1.5Magnetism and Electromagnetism

Electromagnetism — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 10

📚 Edexcel International GCSE Science Double Award 4SD0 - Physics Hub / Topical Revision
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Part 1 of 7Current produces a magnetic field

In 1820 Oersted found that a compass needle near a wire is deflected when the current is switched on. A moving charge, such as an electric current, produces a magnetic field. This connects electricity and magnetism and is called electromagnetism. The field exists only while the current flows.

Part 2 of 7Field around a straight wire

The field around a long straight wire consists of concentric circles centred on the wire, in a plane at right angles to the wire.

The lines are closer near the wire (stronger) and further apart as the distance increases (weaker).
Right-hand grip rule
The direction is given by the right-hand grip rule: point the thumb of the right hand in the direction of the conventional (positive) current,
  • and the curled fingers show the direction of the field lines.

The strength of the field increases when:

Field around a straight wire
the current is increased;
the distance from the wire is decreased.

Reversing the current reverses the direction of the field.

Part 3 of 7Field of a flat circular coil and a solenoid

If a wire is bent into a loop, the field lines inside the loop are all in the same direction and so they add up, producing a stronger field at the centre.

Solenoid
A solenoid is a long coil of many turns.
Its field pattern outside is just like that of a bar magnet, and inside the solenoid the field is strong, uniform and parallel to the axis.
The end where the field lines emerge behaves as a north pole, and the end where they enter is a south pole.

Part 4 of 7Direction for a solenoid

Use the right-hand grip rule again

  • curl the fingers of the right hand around the coil in the direction of the conventional current.
  • The thumb then points to the north pole end.
  • Another quick method: look at one end of the coil.
  • If the current runs anticlockwise it is a north pole;
    • if clockwise, a south pole.

Part 5 of 7Increasing the strength

Increase the current.
Increase the number of turns per unit length.
Soft iron core
Put a soft iron core in the solenoid. The core becomes magnetised by the coil and greatly increases the field, as in an electromagnet (6.1.8).

A solenoid with an iron core is an electromagnet, which can be switched off by turning off the current.

Part 6 of 7Why a coil strengthens the field

The magnetic field of a single straight wire is weak, because the field falls off with distance.

1
Bending the wire into a loop makes the field lines from different parts of the loop point the same way through the centre,
  • so the individual fields add.
2
A coil of many turns adds many such fields, so the field is N times larger than that of a single loop for N turns carrying the same current.
3
This is why electromagnets use many turns of thin insulated wire.

The magnitude of the field inside a long solenoid is proportional to the current and to the number of turns per metre, so doubling the current or doubling the turns per metre doubles the field. The field outside a long solenoid is much weaker, similar to that of a bar magnet but spread widely.

Link to the rest of the topic

The fact that a current produces a field leads to two important consequences. First, a current-carrying wire in an external field feels a force (the motor effect, 6.1.6). Second, a changing magnetic field produces a voltage in a conductor (electromagnetic induction, 6.2.1). Together they explain motors, generators and transformers.

A worked picture of the field of a coil

Imagine looking at a flat circular coil from the side, with the current going up on the near side of the coil and down on the far side.

1
Inside the coil, the fields from both sides point the same way, and add up, so the field through the centre is strong.
2
Outside the coil, the fields from the near and far sides oppose and are weaker.
3
Drawing this gives a pattern of lines that pass through the centre of the coil and loop round the outside, just like a short fat bar magnet.
4
The face from which the lines emerge is the north face.

Part 7 of 7Worked examples

  1. A straight vertical wire carries a current upwards. Describe the field direction as seen from above.

  2. State three changes that would make the field inside a solenoid stronger.

Key terms

TermMeaning
ElectromagnetismProduction of magnetic effects by electric current.
SolenoidA long coil of wire carrying a current.
Right-hand grip ruleThumb along current; fingers show field direction.
ConcentricCircles with the same centre.

Avoid these mistakes

1
Using the left hand for the grip rule; it is the right hand.
2
Drawing straight field lines around a wire.
3
Forgetting that conventional current is from positive to negative.
4
Saying the field is stronger further from the wire.

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