# Loop released from rest and allowing to fall.

1. Jun 7, 2007

### ANON

1. The problem statement, all variables and given/known data
The plane of a 20cm x 20cm metal loop with a mass of 10g and a resistance of 0.010 ohms is oriented vertically. A 1.0 T horizontal magnetic field, perpendicular to the loop, fills the top half of the loop. There is no magnetic field through the bottom half of the loop. The loop is released from rest and allowed to fall.
a) Show that the loop reaches a terminal velocity and find a value for the terminal velocity.
b) How long will it take the loop to leave the field? Assume that the time needed to reach the terminal velocity is negligible. How does this compare to the time it would take the loop to fall the same distance in the absence of a field?

2. Relevant equations
terminal velocity equation = sqrt. (4mg/A)
magnetic flux = A B cos(theta)
I = V/R
I'm not sure if I need other equations....

3. The attempt at a solution

I tried finding the magnetic flux, and I got 0.02 Wb. I know that the initial velocity of the loop is 0 m/s. I am also wondering if there would be some sort of force upward to resist the movement of the loop downward, so that maybe the speed of the loop is slowed somewhat during falling.

Any help would be greatly appreciated, as I am really stuck on how to answer this questions.
Thanks!

2. Jun 9, 2007

### Staff: Mentor

Consider the induced EMF in the loop as it cuts through the magnetic field and the resulting current. Then consider that you have a current-carrying wire moving through a magnetic field--what's the force on it?

Terminal velocity will be reached when the net force on the loop is zero.

3. Jun 9, 2007

### esalihm

use
E=B*length*v
then divide E by R to get I
then use
F=B*I*length=mg
solve for speed, then distance/time will give you time needed

for the second part consider the direction of the force applied

4. Apr 14, 2008

### vertabatt

First, I know this is an old thread... But can you expand on this?

How can you solve for E = B*L*v if you don't know v? And then, how could you solve for I?

I think the logic here makes sense to me, but I am not making all the connections.

5. Apr 14, 2008

### vertabatt

Nevermind, I see that you are plugging these formulas into each other. Got it!