Tag Archives: motor

Fleming’s Left Hand Rule

Any charged particle moving through a magnetic field will experience a force that will cause it to move in a particular direction. An easy way to remember the direction of this force is Fleming’s Left Hand Rule (where the direction of current is the direction in which positive charges move).

Illustration of Fleming's Left Hand Rule

Illustration of Fleming’s Left Hand Rule

So for the example above, a positive particle moving into a uniform magnetic field experiences a force that pushes it up away from the magnetic field. This “motion” of the charged particle is due to the magnetic field that the moving charge makes, interacting with the magnetic field it is moving through (just like two magnets can repel each other).

We can use this rule to figure out the direction in which the rotating arm of a motor will move.


This effect is used to define the standard international (S.I.) unit of magnetic field – the Tesla.

1 Tesla = the value of magnetic field (B) that causes a force of 1 Newton to act on a 1 meter length of conductor (i.e. copper) carrying a current of 1 Ampere at right angles to the magnetic field.

Homopolar motors

A simple motor that doesn’t need commutators or brushes, only:
– An AA battery
– Some disc-shaped strong magnets
– Some wire and wire cutters and/or pliers
Homopolar motor

The aim is to bend the wire in a roughly heart shaped design so that the top just touches the + end of the battery and the bottom curls around the base (i.e. magnet). It needs to be close enough so that a current will flow but not so close that friction stops it from moving.

When a current flows through the wire a force is exerted on it due to the magnetic field of the magnet at the bottom of the battery. (The direction of this can be determined using Fleming’s left hand rule.) This produces a torque on the wire that results in it rotating about the battery: in the example above the wire would rotate clockwise.