9 min readConcept sheet2 or 3-mark questions, and a 5-mark answer with the solenoid diagram

Field of a circular loop and a solenoid; electromagnets

A coil adds up the fields of all its turns. Inside a long solenoid the field is uniform, and a soft iron core turns it into an electromagnet.

Circular loopField at the centre increases with current; n turns give n times the field

SolenoidCoil of many close circular turns of insulated copper wire, shaped like a cylinder

Field inside a solenoidUniform: parallel straight lines

ElectromagnetSolenoid with a soft iron core; magnetic only while current flows

Circular loop

Bend a straight wire into a circular loop and pass a current. Near the wire, each part of it still makes small circles of field lines. Towards the centre these circles become very large, so at the centre the field lines look like straight lines. Every part of the loop sends the field in the same direction inside it. The field at the centre increases with the current. A coil of n turns gives a field n times as large as one turn, because the current flows the same way in every turn.

Solenoid

A solenoid is a coil of many circular turns of insulated copper wire, wound closely in the shape of a cylinder. Its field pattern is like that of a bar magnet: one end acts as a north pole and the other as a south pole. Inside the solenoid the field lines are parallel straight lines, so the field is uniform, the same at every point. To find the north end, curl the fingers of your right hand along the current in the turns. Your thumb points to the north pole.

Electromagnet vs permanent magnet

ElectromagnetA solenoid with a soft iron core. It is magnetic only while current flows. It becomes stronger with more turns, more current or a soft iron core. Its poles swap when the current is reversed. Used in electric bells, cranes that lift scrap iron, and electric door locks.

Permanent magnetMade of steel or special alloys. It keeps its magnetism without any current. Its strength is fixed and its poles cannot be swapped. Used in compasses, fridge-door seals and toys.

Soft iron loses its magnetism as soon as the current stops, while steel keeps it. So soft iron is used for electromagnets and steel for permanent magnets.

How to draw the field of a solenoid

Draw a cylinder-shaped coil of 8 to 10 turns. Join its two ends to a battery and a key, and mark the current direction on the turns.
Inside the coil, draw 4 or 5 straight, parallel, evenly spaced lines along its length.
Outside, draw curved lines from one end of the coil round to the other end, like the field of a bar magnet.
Put arrows on the lines: inside the coil from S to N; outside from N round to S.
Label the ends N and S, the battery, the key and 'uniform field inside'.

In real life

The electric bell in a school corridor and the crane at a scrap yard that lifts old iron both use electromagnets, which switch on and off with the current.

Exam angle

3 marks: “What is a solenoid? Draw the pattern of field lines of a current-carrying solenoid and state one feature of the field inside it.” A coil of many close turns of insulated copper wire shaped like a cylinder. Field pattern like a bar magnet with N and S ends. Inside: parallel straight lines, so the field is uniform.

2 marks: “List two ways to increase the strength of the magnetic field at the centre of a circular coil.” Increase the current through the coil; increase the number of turns in the coil.

2 marks: “Why is soft iron used as the core of an electromagnet?” Soft iron becomes strongly magnetised while current flows and loses its magnetism as soon as the current stops.

Remember it

A solenoid is a bar magnet you can switch on. Fingers along the current, right thumb points to North.