# What am I suppose to do?

1. May 1, 2005

### shikagami

I don't quite understand what i'm suppose to do in this question:

--> Compare the rate at which mechanical work is done by the force (Fv) with the rate of development of heat in the circuit (i^2 R).

2. May 2, 2005

### SpaceTiger

Staff Emeritus
Okay, to start, where is the "mechanical work" coming from? What is the force involved? What is it acting upon?

Hint: There's more than one formula for the power of a circuit.

3. May 2, 2005

### shikagami

Ok, the other formulas for power are P=V^2/R and P=VI. I don't know anything about the force.

4. May 2, 2005

### SpaceTiger

Staff Emeritus
Ok, good start. It would help, however, if you could give a little more info on the circuit. Other than a resistor, what is it composed of?

5. May 2, 2005

### shikagami

Ummm.... here's the entire problem:

A conducting rod AB makes contact with metal rails CA and DB. The apparatus is in a uniform magnetic field 0.5 T, perpendicular to the plane of the diagram. The rod is 50 cm long.

a)find the magnitude and direction of the emf induced in the rod when it is moving toward the right with a speed 4 m/s.

b)If the resistance of circuit ABDC is 0.2 ohms (assumed constant), find the force required to maintain the rod in motion. Neglect friction.

c)Compare the rate at which mechanical work is done by force (Fv) with the rate of development of heat in the circuit (i^2 R).

6. May 2, 2005

### SpaceTiger

Staff Emeritus
Ok, then it's referring to the force that's pushing the rod. If you did part b, you should already have that. Did you do part b?

7. May 2, 2005

### shikagami

yup. I got 0.0125 N.

8. May 2, 2005

### SpaceTiger

Staff Emeritus
So then calculate the rate of work done by that force (0.0125 N * v) and compare it to the power dissipated in the circuit (i^2R). Did you calculate the current in the circuit?

9. May 2, 2005

### shikagami

Ok, I got 0.0500 j for the work. And I got .500 Amps for the current. Now what?

10. May 2, 2005

### SpaceTiger

Staff Emeritus
Can you show your work on the first two parts? I'm getting a number 100 times larger than yours.

11. May 2, 2005

### shikagami

Ok.
a) emf= Bvl --> (4.00m/s)(0.500T)(0.0500m) = .100 V

b) I=emf/R --> (0.100V)/(0.200 ohms) = 0.500 A

Fmag= IBl --> (.500Amps)(0.500T)(0.0500m) = 0.0125N

12. May 2, 2005

### SpaceTiger

Staff Emeritus
How many meters is 50 centimeters?

13. May 2, 2005

### shikagami

OOps... sry.. its only 0.500 m = to 50 cm. Sorry bout that. need to change all my data now...

14. May 2, 2005

### shikagami

Everything moves 1 decimal point to the right.

15. May 2, 2005

### SpaceTiger

Staff Emeritus
Everything?

16. May 2, 2005

### shikagami

lol.. ur funny. Well... here's what I have now:

a) emf =1.00 V
b) I=5.00 Amps, Fmag=1.25N
c) Fv=5.00j

wait.. u got me.. crap.. I keep messing up. Thnx for dealing with my overwhelming stupidity.

17. May 2, 2005

### SpaceTiger

Staff Emeritus
You're not being stupid, it's alright. Two things:

1) Look at the units in the last part. Are you sure they should be Joules?
2) What do you get when you calculate the power dissipated in the resistor (i^2R)?

18. May 2, 2005

### shikagami

Wahh...
1) it's watts not joules ... sorry.
2) wow.. I^2 R --> (5.00Amps)^2 (0.200 ohms) = 5.00 watts

19. May 2, 2005

### SpaceTiger

Staff Emeritus
The point of all this is that the work you put into the system by pulling the rod along the tracks is dissipated as thermal energy in the resistor. Note that none of the work is going into accelerating the rod because it is moving at a constant velocity!

20. May 2, 2005

### shikagami

WOW ur a genius!!! Will u be my homework buddy? LOL Thanks a lot for the help even though it's like uber late there in NJ. I might be bugging you for more physics stuff. Hope you don't mind.