Explain how inside a solenoid (an example of an electromagnet) the fields…

The idea

A solenoid is a long coil of wire. When current flows, each turn behaves like a small electromagnet with its own field. Inside the solenoid the field from every turn points the same way along the axis, so the contributions add together to give a very strong field that is almost uniform along the centre. Outside, the fields from turns on opposite sides of the coil point in opposing directions and largely cancel, so the external field is much weaker. The field pattern resembles that of a bar magnet, with the field leaving one end and entering the other. To answer, decide whether a described region is inside or outside the coil, then judge whether the individual coil fields reinforce or oppose there.

What earns the marks

• States that inside the solenoid the fields from the individual coils point in the same direction and add together.
• Explains that the combined internal field is very strong and almost uniform along the centre of the solenoid.
• States that outside the solenoid the fields from the coils oppose one another and largely cancel.
• Concludes that the field outside the solenoid is weaker than the field inside it.
• Identifies the solenoid as an example of an electromagnet whose field depends on the current.

Common mistakes and how to put them right

• Claims that the field is strongest outside the solenoid.

  Put it right by noting that cancellation outside leaves a weaker field.

• Asserts that the individual coil fields cancel inside the solenoid.

  Put it right by stating that they add along the axis.

• Describes the internal field as varying wildly from point to point.

  Put it right by stating it is almost uniform along the centre.

Exam tips

• Separate your reasoning into an inside part and an outside part so each clause of the statement is addressed.
• Use the words add and cancel precisely, linking each to the correct region.
• Check the direction of each coil's field before deciding whether contributions reinforce or oppose.