Recall that the strength of the field depends on the size of the current…

The idea

The magnetic field around a long straight conductor is not uniform. Its strength increases when the current through the wire increases, so a larger current produces a stronger field at any given point. Its strength decreases as the distance from the wire increases, so the field is strongest close to the conductor and weaker further out. These two dependences can be shown practically: increase the current and plotting compass needles deflect more, or move a compass further from the wire and the deflection becomes smaller. The field strength therefore depends jointly on the current magnitude and on the radial distance from the wire.

What earns the marks

• States that increasing the current increases the strength of the magnetic field around the conductor.
• States that the field strength decreases as the distance from the conductor increases.
• Identifies that the field is strongest close to the wire and weakest far from it.
• Recognises that field strength depends on both the current size and the distance from the wire.

Common mistakes and how to put them right

• Claims the field strength is the same everywhere around the wire.

  Put it right: State that it decreases with distance from the conductor.

• Asserts that a larger current weakens the field.

  Put it right: State that a larger current strengthens the field instead.

• Treats distance from the wire as irrelevant to field strength.

  Put it right: Include distance as a factor alongside current size.

Exam tips

• Use the two variables explicitly: current size and distance from the wire.
• Describe the trend in words: stronger with larger current, weaker with greater distance.
• Support the recall with a practical observation, such as greater compass deflection for a larger current.