Current and the Magnetic Field

Electric current creates a magnetic field around it. This field can push or pull on magnets, compasses, and other currents. Understanding its pattern and direction helps explain devices like relays, loudspeakers, and motors.

Magnetic field around a straight wire

• The field forms concentric circles around the wire.
• Closer to the wire the field is stronger; it gets weaker with distance.
• Increasing the current makes the field stronger.
• Direction: use the right-hand grip rule—thumb points in the direction of the current, curled fingers show the circular field direction.
• Reversing the current reverses the field direction.

Seeing the field (simple experiments)

• Iron filings: Place a card over a vertical wire. Switch on the current; filings line up in circles. Switch off and the pattern disappears (the field exists only when current flows).
• Plotting compass: Move a small compass around the wire or coil. Draw tiny arrows to map the circular pattern and direction.

Solenoids (coils of wire)

• A solenoid’s field is like a bar magnet: one end acts as a North pole, the other as a South pole. Inside the coil, field lines are strong and nearly parallel.
• Stronger field with more turns, bigger current, or a soft iron core.
• Direction: curl your right-hand fingers in the direction of the coil’s current; your thumb points to the North pole. Looking at an end, clockwise current makes a South pole; anticlockwise makes a North pole.

Force on a current in a magnetic field (motor effect)

• A current-carrying wire in a magnetic field feels a push (force), unless it is exactly parallel to the field.
• Direction of force depends on both current and field. Reverse the current or the field, and the force reverses. Reverse both, and the force stays the same direction.
• The force is bigger with stronger field, bigger current, or a longer wire in the field: $F \propto B I L$.

Charged particle beams

• Positive charges are pushed like conventional current. Electrons are pushed the opposite way because their charge is negative.

Common misconceptions

• The field around a current is not straight lines; it forms circles around a straight wire.
• Turning off the current removes the electromagnet’s field.
• Field line density shows strength: closer lines mean a stronger field.