A force on a wire in a magnetic field

The idea

Just as a current's magnetic field exerts a force on a nearby magnet, the magnet exerts an equal and opposite force on the conductor, so a current-carrying conductor in a magnetic field experiences a force.

A rod between magnet poles

A current-carrying aluminium rod lies in the magnetic field of a magnet. A current in a wire can deflect a compass needle, which is a small magnet.

What do you expect for the rod in the magnetic field?

• A force acts on the rod
• No force acts on the rod, since aluminium is not a magnet
• The rod stays at rest

Wire and magnet

Press Next step to see the forces between a current and a magnet.

Push and push back

The magnetic field of a current pushes on a magnet. Ampere said the magnet must push back on the conductor with an equal and opposite force. So a conductor with a current, placed in a magnetic field, feels a force. A rod that is free to move is displaced.

Just as a current's magnetic field exerts a force on a nearby magnet, the magnet exerts an equal and opposite force on the conductor, so a current-carrying conductor in a magnetic field experiences a force.

Notes

A current-carrying conductor in a magnetic field experiences a force: the magnet exerts an equal and opposite force to the one the conductor exerts on it.

Check yourself

A current-carrying conductor is placed in a magnetic field. What happens to it?

A current-carrying wire exerts a force on a magnet. Ampere suggested that the magnet must…

In an example, a current-carrying wire exerts a force of 5 newtons on a magnet. How large is the force that the magnet exerts on the wire?

Answer: 5 newtons

The two forces are equal and opposite, so the magnet exerts 5 newtons on the wire.

A current-carrying aluminium rod is placed in a magnetic field. What is observed?

• It experiences no force, because only magnets feel magnetic forces. The magnet exerts a force on a current-carrying conductor, equal and opposite to the force the conductor exerts on the magnet.
• It experiences a force — correct. Yes!
• It feels a force only when the current is switched off. The force on the conductor comes from the current in the magnetic field. It acts when the conductor carries current.
• exert a force on the wire in the same direction. The force on the wire is opposite to the force on the magnet.
• exert no force on the wire. Ampere suggested that the magnet must also exert an equal and opposite force on the conductor.
• exert an equal and opposite force on the wire — correct. Yes!
• The rod is displaced — correct. Yes!
• The rod does not move, since it is not a magnet. A force is exerted on a current-carrying aluminium rod in a magnetic field, and the rod is displaced.
• The rod becomes a bar magnet with a north and a south pole. The rod is displaced because of the force on the current in the field. That is what is observed.