Electrodynamics: direction rules — Grade 12 Electricity and magnetism

Magnetic field around a current

Straight wire — right-hand grip rule: point the thumb of your right hand in the direction of the conventional current; your curled fingers show the direction of the magnetic field lines (circles around the wire).
Solenoid (coil): curl the fingers of your right hand in the direction of the current in the loops; your thumb points to the north pole of the coil.

Field lines run from north to south outside a magnet. Current means conventional current, from the positive to the negative terminal.

Motor effect: force on a conductor

A current-carrying conductor in a magnetic field experiences a force (the motor effect).

📖 Fleming’s left-hand rule (motors)

Hold the thumb, first finger and second finger of your left hand at right angles: First finger = Field (N to S), seCond finger = Current, thuMb = Motion (force).

✏️ Worked example 1

1The magnetic field points into the page and the current in a wire flows up the page.

2First finger into the page, second finger up the page: the thumb points to the left.

3The force on the wire is to the left. Reversing either the current or the field reverses the force.

Some textbooks use an equivalent right-hand palm rule; any correct rule earns the marks if the direction is right.

Generator effect: induced current

When a conductor moves through a magnetic field (or the flux through a coil changes), an emf is induced.

📖 Fleming’s right-hand rule (generators)

Use your right hand: first finger = field, thumb = motion of the conductor, second finger = induced current.

✏️ Worked example 2

1A rod moves to the right through a magnetic field that points into the page.

2First finger into the page, thumb to the right: the second finger points up the page.

3The induced current flows up the rod.

Lenz’s law check, and the generator coil

Lenz’s law: the induced current flows in a direction that opposes the change that caused it. In worked example 2, the current in the rod experiences a force (motor effect) to the left — opposing the motion — which confirms the direction.

In an AC generator, the emf is maximum when the plane of the coil is parallel to the field (the sides cut field lines fastest, flux changing fastest) and zero when the plane is perpendicular to the field (flux maximum, not changing). Slip rings give AC; a split-ring commutator gives DC.

Answer in your exercise book, then open each memo to mark your work.

Question 1 · (2 marks)

State Lenz’s law.

Show memo

The induced current flows in a direction such that its magnetic field opposes ✓ the change in magnetic flux that produced it ✓.

Question 2 · (3 marks)

A horizontal wire carries current from west to east in a magnetic field that points north. Use a hand rule to find the direction of the force on the wire. Name the rule.

Show memo

Fleming’s left-hand rule ✓ (or a correct equivalent). First finger north, second finger east → thumb points upwards ✓. The force on the wire is vertically upwards ✓.

Question 3 · (3 marks)

A conductor is pushed downwards between the poles of a magnet; the field points from left to right. In which direction is the current induced? Show which rule you used.

Show memo

Fleming’s right-hand rule ✓: first finger to the right (field), thumb downwards (motion) ✓ → second finger points out of the page (towards you) ✓.

Question 4 · (4 marks)

Explain why the emf of an AC generator is zero when the plane of the coil is perpendicular to the magnetic field, and maximum when it is parallel.

Show memo

Perpendicular: flux through the coil is maximum ✓ but its rate of change is zero, so no emf is induced ✓.
Parallel: flux is zero ✓ but it changes fastest (the sides cut the field lines at the greatest rate), so emf is maximum ✓.