The Motor Effect: Force on a Current
Watch a current-carrying conductor feel a push in a magnetic field. In one view a thick wire hangs between a north and a south pole; switch on the current and it swings out, with arrows for the field, the current and the force and the force worked out as F = B × I × L (for instance 0.4 T × 3 A × 0.1 m = 0.12 N). In the other view a coil sits on an axle between the poles with a split-ring commutator that reverses the current every half turn, so the coil spins up to a steady speed in rpm while the voltage it pushes back with cuts the current. Change the field strength, the voltage, the length of wire, the number of turns, and reverse the current or the field to see the force and the turning direction flip.
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What you can adjust
- What to show
- 0 – 1
- Field strength
- 0.05 – 1 T
- Current in the wire
- 0.5 – 10 A
- Supply voltage
- 1 – 12 V
- Wire or coil side length
- 2 – 20 cm
- Turns in the coil
- 1 – 200
- Current direction
- 0 – 1
- Field direction
- 0 – 1
A single wire that is pushed sideways when a current flows through it, or a simple motor: a coil on an axle that is kept turning by a split ring that reverses the current every half turn.
How strong the magnetic field between the poles is, in tesla. A fridge magnet is about 0.005 T; the strong magnets in a small motor give 0.2 to 0.5 T.
The current through the single wire. Used by Wire in a field only: the force is the field strength times this current times the length of wire in the field.
The voltage across the motor. Used by Simple motor only. The current is set by this voltage, the resistance of the coil and the voltage the spinning coil pushes back with, so it drops as the motor speeds up.
The length of wire inside the field. For the motor it is the side of the square coil, so a longer side gives a bigger coil area.
How many times the wire goes round the coil. Used by Simple motor only. More turns give more force, but also more resistance and a bigger voltage pushing back as the coil spins.
Reverse the current and the force, or the turning direction, reverses. Reverse both the current and the field and nothing changes.
Reverse the field (swap the north and south poles) and the force, or the turning direction, reverses. Reverse both the current and the field and nothing changes.
About this simulation
What it shows
A wire or a coil sits between a north and a south pole, seen end-on, with arrows for the field, the current and the force. A graph beside it follows the swing of the wire or the speed of the motor over time.
The key idea
A current in a magnetic field feels a force at right angles to both. For a wire it is F = B × I × L, with its direction from the left-hand rule: first finger field, second finger current, thumb force. A split ring reverses a motor coil's current every half turn so it keeps turning one way.
Try this
Run the defaults: the motor settles at about 390 rpm. Set Current direction to Reversed and it turns the other way. Then set What to show to Wire in a field: 0.4 T × 3 A × 0.1 m gives 0.12 N.
A simple model: a steady field and a square coil. The rpm are the real speed, but the drawn coil is slowed down so you can follow it, and the picture says by how much. The voltage the spinning coil pushes back with is averaged over a turn, the friction is a small fixed amount, and the hanging rod is heavy enough that its swing angle depends on the field and the current but not on its length. In the results, a force direction of 1 means to the right and −1 to the left; a rotation direction of 1 means anticlockwise, −1 clockwise and 0 means the coil does not turn.