Which Hand Rule Determines Current Direction in Magnetic Fields?

[jayjay112]

Please see question at . . . . http://www.flickr.com/photos/37144556@N07/3476115300/sizes/o/

(i) I am not sure which hand rule to use??
I think its the right hand rule, so does that mean the answer to question (i) is clockwise.

(ii) is the answer " in a circluar direction perpendicular to the wire?? as seen here http://etc.usf.edu/clipart/36000/36057/mag_field_36057_lg.gif

Thanks.

 

[Doc Al]

(i) I am not sure which hand rule to use??
I think its the right hand rule, so does that mean the answer to question (i) is clockwise.

You would use a right hand rule, but the answer is not clockwise. How is the field through the loop changing? The induced current acts to oppose that change.

(ii) is the answer " in a circluar direction perpendicular to the wire?? as seen here http://etc.usf.edu/clipart/36000/36057/mag_field_36057_lg.gif

No. Realize that these two questions are related. The induced current creates the induced magnetic field.

 

[jayjay112]

If you bring the magnet away from the coil, you are decreasing the flux through the coil. Therefore, the current should be induced in the clockwise direction.

So how is it going anticlockwise, what is the induced current opposing?

 

[Doc Al]

If you bring the magnet away from the coil, you are decreasing the flux through the coil.

Right! So current must be induced in a direction that will increase the flux through the loop. Using the right-hand rule, what direction will that induced current flow?

Therefore, the current should be induced in the clockwise direction.

How did you conclude this?

 

[jayjay112]

How did you conclude this?

I had my hand the wrong way, so my thumb should point up in the direction to compensate for the decrease in mag field therefore inducing a current to flow the direction of my fingers ie anticlockwise!

Which also means the induced mag field should point out of the page upwards.

So is this passage from wikipedia incorrect..."Take the north pole of a permanent magnet and a coil in front of it and put a microscopic camera on top of the magnet. As you bring the magnet closer to the coil, you are increasing the flux through the coil. Then by Lenz's law, the current will be in counterclockwise direction as viewed by the camera.
If you bring the magnet away from the coil, you are decreasing the flux through the coil. Therefore, the current should be induced in the clockwise direction as viewed from the camera."

 

[Doc Al]

I had my hand the wrong way,

D'oh!

so my thumb should point up in the direction to compensate for the decrease in mag field therefore inducing a current to flow the direction of my fingers ie anticlockwise!

Good!

Which also means the induced mag field should point out of the page upwards.

Right!

 

[jayjay112]

D'oh!

Good!


Right!

Thanks for your help, appreciated.

 

[Doc Al]

So is this passage from wikipedia incorrect..."Take the north pole of a permanent magnet and a coil in front of it and put a microscopic camera on top of the magnet. As you bring the magnet closer to the coil, you are increasing the flux through the coil. Then by Lenz's law, the current will be in counterclockwise direction as viewed by the camera.
If you bring the magnet away from the coil, you are decreasing the flux through the coil. Therefore, the current should be induced in the clockwise direction as viewed from the camera."

No, that passage is OK. Note that the camera is on the magnet, so you're looking at the coil from underneath.