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# Hi A2 Physicists!!(or tutors) Electromagnetic Induction AQA PHYS A Watch

1. Summary question 3 page 122,
"A horizontal rod aligned along a line from east to west is dropped through a horizontal magnetic field which is directed from south to north.
1, What is the direction of the velocity of the rod?
2, Determine which end of the rod is positive. Explain your answer."

The ans to 1 is obviously downwards right? (as it's "dropped")
I'm really confused about the 2nd question, for example, is the direction of velocity = direction of force(motion) or = electron direction? or something else? And when you try to find the force, do you use the left hand or right hand rule?? (I sometimes really dont get when you use which, even though I do remember left for motor and riGht for Generator. But the diagram in the previous page P121 and the explanation below it just seems to be really confusing, as it still seems to be using left hand rule for generator)
2. You use the right hand rule because the motion of the rod through the magnetic field (the dynamo principle) is generating an emf between its ends.

The left hand rule is used to find the force acting on a wire (the motor principle) when there is already a current in it, and you place it in a field.

In this case
The magnetic field is S to N (first finger/pointer finger0
The motion is downwards (thumb)
The direction of the induced emf is in the direction of the middle finger.
The direction of the emf is always "conventional", that is to say, the direction of the positive terminal. (And the direction a positive current would flow if there was a circuit.)
3. (Original post by Stonebridge)
You use the right hand rule because the motion of the rod through the magnetic field (the dynamo principle) is generating an emf between its ends.

The left hand rule is used to find the force acting on a wire (the motor principle) when there is already a current in it, and you place it in a field.

In this case
The magnetic field is S to N (first finger/pointer finger0
The motion is downwards (thumb)
The direction of the induced emf is in the direction of the middle finger.
The direction of the emf is always "conventional", that is to say, the direction of the positive terminal. (And the direction a positive current would flow if there was a circuit.)
Thanks I'm still a bit confused with the direction of motion, in the diagram on page 121, it showed the direction of force different from the direction of velocity, wouldn't this force be the direction of motion?

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Updated: December 8, 2010
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