# Homework Help: Right hand rule #2 doesnt work

1. Apr 27, 2005

### michaelw

Indicate the direction of the electric field between the plates of the parallel plate capacitor shown in the drawing if the magnetic field is decreasing in time. Give your reasoning

if the magnetic field is decreasing, so is the flux, so mr. lenz says the induced magnetic field must be out of the page to counteract the decreasing flux

but with right hand rule #2, it completely depends where you put your hand in determining the direction of current

if you put it at the top of the the wire, curl fingers out of page, current goes to the right (right hand, conventional current).. ie) clockwise

if you put it at the bottom, current still goes to the right, but now it means the direction is counter clockwise.

which is right and why??

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2. Apr 27, 2005

### ZapperZ

Staff Emeritus
Eh?

I'm confused too... but not with the right hand rule, but rather what you described. What do "put it at the top of the wire" and "put it at the bottom" mean?

Since the B field is decreasing, Lenz's law, as you have correctly stated, will try to preserve the original B field, and thus, will induce a current in the LOOP in the same direction as the original B field. Thus, it will produce an induced B field pointing OUT of the page. So far, we have that in agreement.

Now, use your right hand, and point your thumb in that direction. The way the rest of your fingers curl is the direction of the induced current. PERIOD.

So the current in your circuit will flow in THAT direction, causing a build-up of charge in the capacitor. I have no idea what the "top of wire" or "bottom of wire" refers to....

Zz.

3. Apr 27, 2005

### michaelw

suppose that the circles on this page each represent my hand, with my fingers curled out of the page, and thumb pointing in direction of current (that is, to the right)

if my hand were at circle #1 (top), then current is clockwise
if my hand were at circle #2 (bottom), current is counter clockwise

the direction of current as indicated by the right hand rule is to the right. however, depending on where your hand is, that current is either going clockwise or counterclockwise

im terribly stumped and exam is tomorrow

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4. Apr 27, 2005

### ZapperZ

Staff Emeritus
No, you are applying the rule in an incorrect fashion. Your THUMB, in this case, points in the direction of the magnetic field, not your curled fingers. Why? It is because the magnetic field is "LINEAR", and has a straight line geometry. It is the CURRENT that is "curling".

For example, look at the current moving in a circular loop. How do you determine what direction the B field is? You curl your FINGERS in the direction that the current is moving, and then how your thumb points is the direction of the B field.

Now look at another situation. What if you have a straight wire with a current? Here, the current now is the one with a LINEAR geometry, i.e. moving in a straight line. In this case, the THUMB now represents the current (thumb=straight). The magnetic field is the one curling around the wire. So your curled fingers respresents the direction of the magnetic field.

Moral of the story: use your thumb to represent the "thing" that's moving in a straight line.

Zz.

5. Apr 27, 2005

### michaelw

oops it appears ive goofed :)
i was mixing up thumb/fingers (And it looks like you replied already =)

thanks for the tip about the linearity
youre a savior

thanks again!