10.2.2Magnetism and the Motor Effect

Magnetic Fields in Solenoids

5 min read 6 parts Lesson 7 of 11 1 quiz

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Section 1 of 6From a loop to a solenoid

1One loop

One loop A single circle of current-carrying wire has field lines looping round the wire and passing through its centre.

2Many loops

Many loops A solenoid is a long coil of many turns of wire.

3Fields combine

Fields combine Inside the coil, the fields of individual turns add together to give a strong, nearly uniform field along the axis.

4Outside

Outside Outside, the fields tend to cancel, so the field is weaker and spreads out like that of a bar magnet.

Section 2 of 6Poles of a solenoid

Like a bar magnet

Like a bar magnet One end acts as a north pole and the other as a south pole.

Reading the pole

Reading the pole Look at the end of the coil: if the current goes round anticlockwise, that end is a north pole;
- if it goes clockwise, it is a south pole.

Reversing the current

Reversing the current Swapping the direction of the current swaps the poles.

No current

No current With no current, there is no field at all, which makes an electromagnet easy to switch off.

Section 3 of 6Making a stronger electromagnet

Change	Why it strengthens the field
Larger current	Each turn creates a stronger field.
More turns in the same length	More turns contribute to the field inside the coil.
Shorter coil with the same turns	Turns are packed closer together, so the field is more concentrated.
Soft iron core	The core becomes magnetised by the coil and adds its own field, giving a much stronger magnet overall.

Section 4 of 6Field features to describe in an exam

Solenoid field

Uniform field

Uniform field A uniform field has the same strength and direction at every point, shown by parallel, equally spaced lines.

Inside

Inside Strong and uniform along the centre.

Outside

Outside Weaker, with lines curving from one end to the other.

Section 5 of 6Key terms

Term	Meaning
Solenoid	A long coil of wire that produces a magnetic field when a current flows.
Electromagnet	A magnet produced by an electric current, often with an iron core.
Uniform field	A field with the same strength and direction everywhere.
Turn	One complete loop of wire in a coil.
Core	A piece of magnetic material, usually soft iron, placed inside a coil.

Section 6 of 6Exam technique

Writing 'add more coils' - the correct phrase is 'add more turns to the coil'.

Using a steel core and expecting it to switch off with the current - soft iron is used because it demagnetises easily.

Getting the poles the wrong way round when reading clockwise and anticlockwise currents.

Drawing the field lines inside the solenoid as curved rather than straight and parallel.

Transformer Golden Equations Physics Standard

Transformer Turns Ratio & Efficiency

Ratio: V_p / V_s = N_p / N_s = I_s / I_p

Step-up reduces grid thermal loss (P = I²R).
Laminated core prevents eddy currents.