## 0877 #18 (induced current)

llamacheez
Posts: 10
Joined: Tue Aug 02, 2011 9:57 pm

### 0877 #18 (induced current)

Hey folks, I'm having trouble with this problem about induced current (0877 #18). I'm hoping someone can help me arrive at the quick, correct solution.

So there's an induced EMF $$V = - \frac{\partial \Phi_B}{\partial t}$$, so all we really need is how the magnetic flux through the loop is changing.

What I'm visualizing is the field lines going into the south pole and coming out of the north pole, along the magnet axis. So there would be an induced current to "cancel out" the change in flux, which I think would be counter-clockwise in the figure. That gives answer (C), which is wrong. The correct answer is (E).

Could someone help? Thanks!

Skullgrid
Posts: 59
Joined: Sat Aug 21, 2010 6:50 pm

### Re: 0877 #18 (induced current)

While you're correct that the current will flow counter clockwise as the magnet enters the loop, you've forgotten that as the magnet leaves the loop on the opposite site, the flux will decrease and the current will then flow clockwise. This gives the answer: (E.) Current flows from b to a as the magnet enters the loop, and from a to b as the magnet leaves the loop.

physicsworks
Posts: 80
Joined: Tue Oct 12, 2010 8:00 am

### Re: 0877 #18 (induced current)

llamacheez wrote:I'm hoping someone can help me arrive at the quick, correct solution.
Here it is: 0877/18. Although Skullgrid already gave you the correct solution.

llamacheez
Posts: 10
Joined: Tue Aug 02, 2011 9:57 pm

### Re: 0877 #18 (induced current)

Thanks, folks!

pinkfishegg
Posts: 5
Joined: Sat Aug 15, 2015 10:41 am

### Re: 0877 #18 (induced current)

Hey, I was just working on this problem and was confused about the effect of North Pole Vs South Pole. If the magnet was put through with the North Pole side in first, would the answer be D? I got D when i worked on the problem, but then saw the south pole.

So Faraday's law uses opposite RHR to determine current, and a south pole facing magnet uses opposite-opposite RHR?