A steady field inside a solenoid

The idea

The field lines inside a current-carrying solenoid are parallel straight lines, showing that the magnetic field is the same at all points inside, that is, uniform.

A compass in a coil

A compass is moved from place to place inside a long current-carrying coil.

What does the needle do?

• It keeps pointing the same way
• It points a different way at each place
• It spins round and round

Inside and outside

Press Next step to look inside the solenoid and then outside it.

A uniform field

Inside a solenoid the field lines are parallel straight lines, all pointing the same way and equally spaced. That means the magnetic field is the same at all points inside. We say the field is uniform. Outside, it is different.

The field lines inside a current-carrying solenoid are parallel straight lines, showing that the magnetic field is the same at all points inside, that is, uniform.

Notes

The field inside a solenoid is uniform: the lines are parallel straight lines and the field is the same at all points inside.

Check yourself

Inside a current-carrying solenoid the magnetic field lines are…

Parallel straight field lines inside a solenoid show that the magnetic field inside is…

A small compass is moved from point to point inside a current-carrying solenoid. The needle…

A field that is the same at all points inside a region is called…

• concentric circles. Concentric circles are the field lines around a straight wire. Inside a solenoid the lines are parallel straight lines.
• curved lines that spread apart in the middle. Inside the solenoid the field lines are parallel straight lines.
• parallel straight lines — correct. Yes!
• the same at all points — correct. Yes!
• zero. The field lines inside show that there is a field. It is the same at all points inside.
• different at different points. Parallel straight lines show that the field is the same at all points inside.
• points in a different direction at each point. The field is the same at all points inside, so the needle points the same way at each point.
• keeps pointing in the same direction — correct. Yes!
• points along circles round the solenoid. The lines inside are parallel straight lines, so the needle keeps pointing along them.
• concentric. Concentric describes circles with the same centre. A field that is the same at all points is called uniform.
• closed. Closed describes field lines that join up into curves. A field that is the same at all points is called uniform.
• uniform — correct. Yes!