Recall that a current carrying conductor placed near a magnet experiences…

The idea

When a wire carrying a current sits in a magnetic field, the interaction between the field of the current and the field of the magnet produces a force on the conductor. The same interaction produces a force on the magnet itself. These two forces are equal in size and opposite in direction, in line with Newton's third law: the conductor pushes on the magnet exactly as hard as the magnet pushes on the conductor, but the other way. The force on the wire is the basis of the motor effect. To answer, check that a response names both forces and states that they are equal and opposite, and that it does not treat the force as acting on only one object.

What earns the marks

• States that a current-carrying conductor placed near a magnet experiences a force.
• States that an equal and opposite force acts on the magnet.
• Recognises that the two forces act on different objects, the conductor and the magnet.
• Links the pair of forces to the interaction between the magnetic field of the current and the field of the magnet.

Common mistakes and how to put them right

• States that only the conductor feels a force.

  Put it right by adding that an equal and opposite force acts on the magnet.

• Describes the two forces as equal and in the same direction.

  Put it right by stating they are opposite in direction.

• Treats the two forces as acting on the same object.

  Put it right by naming the conductor and the magnet as separate objects.

Exam tips

• Name both objects that experience a force so the pairing is explicit.
• Use the phrase equal and opposite to capture both magnitude and direction.
• Check that your chosen option does not imply the forces cancel on a single object.