If you have learned this as being due to lines of flux being cut, the rule is that the emf is equal to the flux cut per second. (Not the rate of change of the flux cut)
You calculate the area A swept out by a spoke in one second and multiply it by B to get the flux cut.
As the wheel is spinning uniformly, the spokes cut a constant amount of flux every second. They sweep out a constant area per second.
I find students often get confused by this. You can consider an emf being generated when there is a change of flux linking an area. The emf then depends on the rate of change of flux linkage.
You can also think of the emf as being due to a wire cutting through lines of flux. In this case the emf depends on the rate at which the flux is cut.
You should have been shown at some point that the two ways of thinking of this are equivalent.
In this case, it's easier to think in terms of the amount of flux cut per second.
The direction of the induced emf is given by the Right Hand Rule. The rule gives the direction of the induced emf (centre finger) by considering the field (into the page, pointer finger) with the direction of motion of the spoke (thumb).
Last edited by Stonebridge; 26-05-2012 at 17:49.
In the LHR the centre finger points in the direction of flow of conventional (+ive) current. This rule is nearly always then used to find the direction of the force (thumb) on that current when in a field (pointer finger). That's why it is often referred to as the LH (Motor) Rule.
(Original post by Antimony)
Thanks, doesn't the centre finger indicate the direction of conventional current? Or is that just for FLHR and for FRHR, the centre finger indicate the +ve end of emf and so flow of electrons?
In this rule the current is already flowing and we want the force acting on it.
In the RH Rule the centre finger gives the direction of the conventional (+ive) induced emf, and the direction of flow of any positive current if there is a circuit for it to move around. The thumb is the direction of motion of the wire that is being moved across the field (pointer finger).
This rule is used to find the direction of induced (+ive) emfs when you move a wire through a magnetic field. That's why it's often called the RH (Dynamo) Rule.
Last edited by Stonebridge; 30-05-2012 at 22:36.
The line - if there is no battery, the rod IS the battery - helped everything.
(Original post by Stonebridge)
This does seem to cause a problem.
Rather than try to explain this here now, it will probably be more useful if you take a look at this current (just finished!) thread where the same question has come up and there has been plenty of discussion, with, I hope, a satisfactory explanation.
If it still doesn't make sense come back here and ask again.
Using right hand rule, induced/conventional current is from P to Q. So electrons must move in opposite direction from Q to P.
So if electrons are moving from Q to P, P would become negative and hence have lower potential.
Oooooh now I get this one!!!
Since induced current is from hub to rim, electrons must move in the opposite direction from rim to hub. Considering the spoke as the battery itself - the hub becomes negative due to the collection of electrons.
Many many many many many thanks!!! God know how many marks I might have lost due to this!!! I think I might love you :') Thanks again!
PS: I think I faced the problems because I considered the conventional current direction as flowing outside the spoke/rod. In a real battery, the electrons are prevented from flowing directly between the terminals and so electrons have to flow externally . Whereas, the rod/spoke is conducting and so electrons can flow internally - and hence all the confusion
Last edited by Parthenon93; 08-06-2012 at 20:23.