SITEMAP   School Physics Notes: Electricity-magnetism 11.2 Fleming's Left Hand rule

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Motor effect of electric current: 11.2 Predicting direction of force - direction of motion in the motor effect from Fleming's left-hand rule

Doc Brown's Physics exam study revision notes

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 90o 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 90o 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

Keywords, phrases and learning objectives on the motor effect and Fleming's left-hand rule

Be able to describe the motor effect of electric current by predicting the direction of maximum force, and direction of motion in the motor effect from Fleming's left-hand rule.

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