physics question

[moto_za]

i have a quick physics question i do not understand: Two solenoids are coaxial and fairly close to each other. While the resistance of the variable resistor in the left-hand solenoid is increased at a constant rate, the induced current through the resistor R will:
A. flow from a to b
B. flow from b to a
C. be zero because the rate is constant

the answer is b but i am not sure why? thanks for any help!

---

Members don't see this ad.

 

[jochi1543]

Which solenoid is a, which is b, and which is the resistor R? It's not very clear from the description.

 

[moto_za]

sorry forgot to attach the image!

 

[moto_za]

i forgot to post another question i had so if anyone has the time, i would appreciate your help.

A metal loop is held above the S pole of a bar magnet, as shown in the figure , when the magnet is suddenly dropped from rest. Just after the magnet is dropped, the induced current in the loop, as viewed from above it, is directed
a) clockwise
b)counterclockwise

 

[jochi1543]

i forgot to post another question i had so if anyone has the time, i would appreciate your help.

A metal loop is held above the S pole of a bar magnet, as shown in the figure , when the magnet is suddenly dropped from rest. Just after the magnet is dropped, the induced current in the loop, as viewed from above it, is directed
a) clockwise
b)counterclockwise

I'm no physics expert, but here's my train of thought - the magnetic field lines point from the North to the South pole of the magnet, so upwards into the ring. When the magnet is dropped, the distance between it and the ring increases, leading to a decrease in the applied magnetic field on the ring. Therefore, the induced current will flow in such a direction that it complements the original direction of the magnetic field, trying to counteract the decrease. So the induced field will be upwards inside the ring, as well. For the induced field to be upwards inside the ring, the current will have to flow counter-clockwise.

That's what I think, anyway. I have a midterm on this crap in a week, so I hope I'm right.

 

[jochi1543]

sorry forgot to attach the image!

Sorry, this is beyond my physics abilities.

 

[moto_za]

I'm no physics expert, but here's my train of thought - the magnetic field lines point from the North to the South pole of the magnet, so upwards into the ring. When the magnet is dropped, the distance between it and the ring increases, leading to a decrease in the applied magnetic field on the ring. Therefore, the induced current will flow in such a direction that it complements the original direction of the magnetic field, trying to counteract the decrease. So the induced field will be upwards inside the ring, as well. For the induced field to be upwards inside the ring, the current will have to flow counter-clockwise.

That's what I think, anyway. I have a midterm on this crap in a week, so I hope I'm right.

thanks for the explanation, but the answer is actually clockwise...kinda confused.

 

[jochi1543]

thanks for the explanation, but the answer is actually clockwise...kinda confused.

Story of my life!


Can't wait to be done with Physics FOREVER this August.

 

[jochi1543]

OK, I looked and I have an identical problem in one of the chapters I've read recently, except for there the magnet is with its North pole upwards....and they say that when it's pulled down, the current is counterclockwise, which means that if the South pole is facing up (as in your example problem), the induced current will indeed be clockwise, as your answer says. This is because I'm an idiot and confused the direction of the magnetic field....the applied field is actually DOWNWARD, not upward, so the induced field will be downward, too, to compensate for the decrease. For it to be downward, the current will have to be clockwise.

 

[jochi1543]

Here:

http://s10.photobucket.com/albums/a141/Jochi1543/?action=view&current=magnet.jpg

 

[Psychotropic]

i have a quick physics question i do not understand: Two solenoids are coaxial and fairly close to each other. While the resistance of the variable resistor in the left-hand solenoid is increased at a constant rate, the induced current through the resistor R will:
A. flow from a to b
B. flow from b to a
C. be zero because the rate is constant

the answer is b but i am not sure why? thanks for any help!

As Resistance is increased, current decreases. That means the induced magnetic field, in the solenoid on the left which points to the right, decreases as well. In order to compensate for the decreased field towards the right, current will flow from b to a to create a field to the right, in the second solenoid. I hope that made some sense.