# Problems involving magnetic flux and induced voltage

Discussion in 'Homework Help' started by tjohnson, Mar 23, 2015.

1. ### tjohnson Thread Starter Active Member

Dec 23, 2014
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Several times in my physics lessons, I have encountered a problem similar to the following:

This coil has been moving in such a way as to decrease flux. After the switch is closed, will the induced voltage cause electrons in the coil to move clockwise or counterclockwise?

and I always seem to have the wrong answer for this type of problem. I understand that the Xs mean that the magnetic field is pointed into the drawing, rather than out of it, but I'm not sure how to solve for the right direction.

2. ### praondevou AAC Fanatic!

Jul 9, 2011
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Look here: http://en.wikipedia.org/wiki/Right-hand_rule

In the first picture to the right on Wiki the magnetic field lines come out of the picture, in your drawing the lines go into the picture. Your (conventional) current flows clock-wise. Electron flow is then counter-clockwise.

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3. ### tjohnson Thread Starter Active Member

Dec 23, 2014
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122
Thanks. I'm familiar with the right-hand rule, and know how to use it correctly for some other magnetic field problems. But in a problem like the one I posted, how do I know which direction to point my hand in if I don't know which direction the current is flowing in? That's what I can't quite figure out.

4. ### praondevou AAC Fanatic!

Jul 9, 2011
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Bend your hand around the wire as shown on Wiki. Your thumb will show current direction. The other fingers point in the direction of the magnetic field. The pic is the inverse of your drawing therefore the current flows "down".

See this picture, turn the loop over and you'll see it.

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5. ### tjohnson Thread Starter Active Member

Dec 23, 2014
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So am I correct that the direction of current flow in my drawing is to the left? How do I know from that the direction in which the coil will rotate?

6. ### praondevou AAC Fanatic!

Jul 9, 2011
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Oops, I didn't see we are talking about a rotating coil.

This involves a third vector, the force vector.

If you google images "single loop motor force" you will tons of drawings exactly what you need to know.

It depends a little on what kind of motor your are talking here.

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7. ### tjohnson Thread Starter Active Member

Dec 23, 2014
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122
That makes perfect sense now. When I said in my previous post about current flowing to the left, I wasn't thinking about the ambiguity of such a statement (in more than one way). After googling "single loop motor force", I now also understand that the side of the coil through which the current flows first is the one that will move upwards. Assuming that the reference frame for measuring the direction is viewing the motor from the front, I can see that the correct answer to the problem that I posted is counterclockwise.

@praondevou: All of your posts helped me figure this out. Thank you!