11.2 Predicting the direction of maximum force - the direction motion in the motor effect

You can predict the direction of the force-motion effect from Fleming's left-hand rule (illustrated below with doc b's handsome left hand!).

Diagram of Fleming's left-hand rule prediction

Imagine a set of x,y, z axes at 90^o to each other, represented by the thumb, first finger and second finger of your left hand.

The thuMb represents the direction the force acts - direction of motion (phonetically emphasise M).

The First Finger represents the direction of the magnetic field N => S (phonetically emphasise the F).

The SeCond finger represents the direction of the convention current (phonetically emphasise the 'hard' C).

 

Fleming's left-hand rule and the motor effect are 'combined' in the diagram above.

Imagine a current carrying wire at the most favourable angle of 90^o to a magnetic field created by permanent magnets.

If you do this in the lab you will see the wire kick to the right with respect the magnetic N=>S pole alignment.

The direction of force creating the motion can be predicted from Fleming's left-hand rule (top right).

However!, you have to twist your hand around, physically (or in your head) to fit in with a given diagram situation.

The result of twisting your hand around is shown in the bottom right of the diagram - check it out!

Note the 'change in direction' rules ....

(i) if you reverse the direction of the current (e.g. ↑ to ↓), you reverse the direction of the force and the wire kicks the other way,

and (ii)  if you reverse the direction of the magnetic field (e.g. N=>S to S<=N), you also reverse the direction of the force and the wire kicks the other way.

 

The motor effect has all sorts of applications e.g. electric motors, loudspeakers, generators, microphones, so see

11.4 Motor effect of an electric current - d.c. electric motor

12. Generator effect, applications e.g. generators generating electricity and microphone

Using SEARCH some initial results may be ad links you can ignore - look for docbrown

INDEX physics notes: motor effect of electric current and applications