# Variation of Induced EMF in a cross bar of a ferris wheel

The question is as follows.

A ferris wheel which consists of two parallel large wooden wheels is joined together with metal cross bars as shown in the figure,is erected so that the planes of wheels are in the north-south direction and the magnetic field B which is horizontal at this location.the ferris wheel rotates around the horizontal axis passing through the centers of the two wheels at a constant period of rotation T in the direction shown.LM is the metal cross bars which is at the highest positions shown when t=0.Sketch the Variation of induced electromotive force(e) at the end L of the cross bar with respect to the end M with time t for one complete cycle.

I think that the answer is 1.when the wheel completes a quarter of a cycle we can apply the right hand rule and it tells us that the current is from M to L.That means end L is at a lower potential.So the EMF e at L should be negative at end L compared to end M.

But my teacher insisted that the answer is 2.I don't understand why it is so?

## 1 Answer

If you have a battery with a positive terminal and a negative terminal and the terminals are connected to a resistor, in which direction does the current flow through the resistor?

Through the resistor the current flows from the node at a higher potential, the positive terminal, to the node at a lower potential, the negative terminal.

This means that inside the battery the current flows from the negative terminal to the positive terminal and the current is made to flow in this direction inside the battery by the electrochemical processes going on inside the battery.

With your Ferris wheel (the equivalent of a battery but with the current flowing due to the Lorentz force) if the current flows from end $$M$$ to end $$L$$ then connecting a (stationary) resistor to the ends will result in a current flowing through the resistor from end $$L$$ to end $$M$$.
This means that end $$L$$ is at a higher potential than end $$M$$.