← Electricity and electromagnetism

Magnetic field patterns and the hand rules

Three separate rules cover three separate questions — field DIRECTION around a current, field direction inside a solenoid, and FORCE direction on a current in a field. Mixing up which rule answers which question is the commonest slip in this whole area.

The right-hand grip rule (field direction)

  • Around a STRAIGHT wire: point your right thumb in the direction of conventional current, and your curled fingers show the direction of the circular field lines around the wire.
  • Inside a SOLENOID (coil): curl your right-hand fingers in the direction of current flow around the loops, and your thumb points toward the solenoid's NORTH pole.
  • Reversing the current direction reverses the field direction in both cases.

The left-hand motor rule (force direction)

  • A completely DIFFERENT rule, for a completely different question: the FORCE on a current-carrying wire sitting IN an existing magnetic field.
  • Thumb = Force (motion), First finger = Field (direction of B, from external N to S), seCond finger = Current — "FBI" arranged as thuMb-Force, First-Field, seCond-Current.
  • All three (force, field, current) must be mutually perpendicular for the rule to apply directly — reorient your hand to match the given directions before reading off the answer.
Worked example Finding the missing direction with the left-hand rule A wire carries current from left to right through a magnetic field pointing into the page. Find the direction of the force on the wire. Set up the left hand: seCond finger (Current) points right, First finger (Field) points into the page. Orient your hand so those two fingers match, keeping them perpendicular. The thuMb (Force) then points upward, out of the plane described by current and field. ✓ The force acts upward. Practising this physically with your own hand, rather than trying to visualise it, is far more reliable under exam pressure. ✕

Using the right-hand rule for a force question, or the left-hand rule for a field question

These two rules answer two DIFFERENT kinds of question and are not interchangeable. The right-hand grip rule finds the FIELD produced by a current (no external field involved). The left-hand motor rule finds the FORCE on a current that is sitting inside an ALREADY-EXISTING external field. A question that gives you a field and a current and asks for a force needs the left hand; a question that gives you only a current and asks about the field it creates needs the right hand.

Practise just this