Describe how to show that a current can create a magnetic effect around a…

The idea

A long straight conductor carrying a current is surrounded by a magnetic field. To show this, pass the wire vertically through a horizontal card, connect it to a cell through a switch, and sprinkle iron filings on the card or place small plotting compasses around the wire. Close the switch and tap the card gently: the filings form concentric circles centred on the wire, and the compass needles align tangentially, showing the field is circular in a plane perpendicular to the wire. Reverse the current by swapping the cell connections and the compass needles reverse direction, so the field direction depends on current direction. The field is strongest close to the wire and weakens with distance.

What earns the marks

• States that iron filings or plotting compasses placed around a current-carrying straight wire reveal a magnetic field.
• Describes the field as concentric circles centred on the wire in a plane perpendicular to the conductor.
• Explains that reversing the current reverses the direction of the magnetic field, linking field direction to current direction.
• Identifies that the field is strongest near the wire and becomes weaker further away.

Common mistakes and how to put them right

• Draws the field lines as radial spokes pointing away from the wire.

  Put it right: Draw them as concentric circles around the wire instead.

• Claims the field direction is fixed regardless of current.

  Put it right: Reverse the current and the field direction reverses too.

• Places the circular field lines in the same plane as the wire.

  Put it right: Orient the circles in a plane perpendicular to the conductor.

Exam tips

• Name the apparatus explicitly: iron filings or plotting compasses plus a straight wire, cell and switch.
• State the shape in words: concentric circles centred on the wire.
• Link direction to current by describing what happens when the cell connections are swapped.