Investigating Current and the Magnetic Field

Electric current creates a magnetic field. We can observe its pattern, find its direction, and see how its strength changes. This helps explain devices like electromagnets, relays, and loudspeakers.

Magnetic fields made by currents

A current in a wire produces circular magnetic field lines around the wire. The field is strongest close to the wire and gets weaker further away. Increasing the current $I$ makes the magnetic field $B$ stronger.

Investigating a straight wire

• Set-up: Vertical straight wire through a hole in a card, connected to a battery and switch. Place iron filings on the card or use a small plotting compass.
• Observation with iron filings: Gently tap the card. Filings form concentric circles around the wire. Closer circles = stronger field.
• Observation with a compass: Move the compass around the wire. The needle is always tangent to the circles, showing direction of the field.
• Change the current: Increase $I$ → stronger alignment of filings/compass; reverse the battery → field direction reverses.

Investigating a solenoid (a coil of wire)

• Set-up: Long coil connected to a battery. Use iron filings on a card or several compasses around the coil.
• Pattern: Outside, lines look like a bar magnet. Inside the coil, lines are nearly parallel and close together (a strong, almost uniform field).
• Poles and changes: One end behaves as a North pole, the other as South. Reversing the current swaps the poles. More turns, larger current, or an iron core make the field stronger.

Finding the direction quickly

• Right-hand grip rule (wire): Thumb = current direction; curled fingers = magnetic field direction around the wire.
• Right-hand grip rule (solenoid): Curl fingers in the direction of current around the coils; thumb points to the solenoid’s North pole.

Real-world links

• Electromagnets: lifting scrap metal; doorbells and relays use current-made fields to pull a switch.
• Loudspeakers: a coil’s field interacts with a permanent magnet; changing current moves the cone to make sound.

Common mistakes

• Field lines are models, not physical objects. A compass shows the local direction (tangent to a line).
• The Earth’s magnetic field can affect compasses; keep the current strong enough and test away from large magnets.
• Conventional current direction is from positive to negative. Reversing it reverses the field/poles.
• Iron filings can clump and slightly disturb the pattern; use light, even sprinkling and gentle taps.