Homework Help: Emf by a magnetic field

1. Apr 11, 2004

pattiecake

Consider a circuit comprised of a U-shaped perfectly conducting rail. In the circuit there is a voltage source, V, a resistor, R, and a switch.

A perfectly conducting rod of mass, m, is free to slide on the U-shaped rail. Imagine the top end of the U facing open to the right, with the rod on top, parallel to the open end, and free to slide left or right.

There is a uniform magnetic field directed into the page. At time t=0 the switch is closed.

A) How much current initially flows in the circuit?

There's about a dozen equations in my physics book that don't really make sense, but I'm gonna go with: I=E/R^e(-t/time constant), where E=emf.

B) As time goes on, does the current in the circuit: increase, decrease, or stay the same?

Well it should remain constant if the rod is stationary. If the rod moves the circuit will change. It will increase if it moves to the right and decrease if it moves to the left??

C) What is the initial acceleration of the rod?

Whoa- why is the rod moving?

D) After a long time, the rod wil reach a constant terminal velocity. What is the value of this velocity?

Still wondering why it's moving...

E) After the rod has reached constant terminal velocity, what is the value of the current flowing in the circuit?

Ummm, just taking a stab here, equal to part A?

HELP! HELP! HELP!

Last edited: Apr 11, 2004
2. Apr 11, 2004

Staff: Mentor

Two hints: (1) a current-carrying wire will experience a force in a magnetic field--that's why the rod starts to move, and (2) a conductor moving across a magnetic field will experience an induced EMF.

3. Apr 14, 2004

pattiecake

thank you very much for your reply. i apologize i haven't gotten a chance to sit down and work at it again. the pace of my class is so crazy. by the time i start to figure things out, the professor is already three chapters ahead of where i am. but thanks again, when i get the change to digest this, i'm sure i'll be posting additional questions.