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Edexcel GCSE Physics · 1PH0
Edexcel 1PH0 · 12.9 Check the specification (PDF) (opens in a new tab)
An electric current produces a magnetic field around a wire. If the wire is bent into a loop, its field passes through the centre of that loop. A consists of many such loops, or turns, arranged closely together in a long coil. It is an example of an because its magnetic field is produced by a current.
Each turn produces its own magnetic field. The solenoid's overall field is the combined effect of all these individual fields.
Along the centre of the solenoid, the fields produced by the different turns point in the same direction. They therefore add together, producing a very strong magnetic field.
Away from the ends, each turn has a similar effect on points along the central region. The resultant field there has nearly the same strength and direction from place to place. It is therefore described as almost uniform and is represented by straight, parallel, approximately evenly spaced field lines.
Outside the solenoid, the fields from different turns point in opposing directions in many places. They therefore partly cancel one another. The resultant external field is weaker than the field along the centre; it is not generally zero.
This contrast comes from how the many fields overlap: reinforcement dominates inside the coil, while cancellation dominates outside it. The complete field pattern resembles that of a bar magnet, with field lines curving around from one end of the solenoid to the other.
Fields from successive turns reinforce one another along the centre of a solenoid but partly cancel outside, producing a strong, almost uniform central field and a weaker external field.
A field-line diagram is a model of the combined magnetic field, not a set of physical lines. Parallel, closely spaced lines through the centre show the strong, almost uniform central field. More widely spaced curved lines outside show the weaker external field. If the current is switched off, the wire no longer produces this magnetic field.
Use the specification wording: the fields from individual turns add together inside the solenoid but largely cancel outside it.
Describe the central field as almost uniform, not perfectly uniform. It is represented by straight, parallel, evenly spaced field lines.
Do not say that the field outside is zero: cancellation makes it weaker than the field along the centre.
It can see this note and what your specification asks for.
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Last updated
Solenoid
A long coil made from many turns of insulated current-carrying wire.
Magnetic field
A region in which a magnetic material or a current-carrying conductor experiences a magnetic force.
Uniform magnetic field
A magnetic field that has the same strength and direction at every point in the region considered.
Electromagnet
A magnet produced by an electric current, so its magnetic effect can be switched on and off.
Put your knowledge into practice — try past paper questions for Physics
Solenoid
A long coil made from many turns of insulated current-carrying wire.
Magnetic field
A region in which a magnetic material or a current-carrying conductor experiences a magnetic force.
Uniform magnetic field
A magnetic field that has the same strength and direction at every point in the region considered.
Electromagnet
A magnet produced by an electric current, so its magnetic effect can be switched on and off.