Circuits: Power, Voltage, Resistance

[barnaby]

Homework Statement

The circuit shown is used to produce a current-voltage graph for a 12V, 24W lamp:

http://img338.imageshack.us/img338/9139/circuitlu6.jpg
http://img338.imageshack.us/img338/9139/circuitlu6.jpg
a) Calculate the resistance of the lamp in normal operation.
b) Calculate the value for R which wold enable the voltage across the lamp to be varied between 0V and 12V.

Homework Equations

P = IV
V = IR

The Attempt at a Solution

a) P=IV
24 = 12I
I = 2A

V = IR
12 = 2R
R = 6 Ohms

b) Absolutely no idea how to go about doing this... it seems that the voltage across the resistor R ought to be 8V, but I don't know, since we haven't been told what the component with the arrow pointing into the 24 ohm resistor is... Thanks for your help.

 

[kamerling]

Homework Statement

b) Absolutely no idea how to go about doing this... it seems that the voltage across the resistor R ought to be 8V, but I don't know, since we haven't been told what the component with the arrow pointing into the 24 ohm resistor is...

This is a "potentiometer" A resistor with a sliding contact, forming a variable voltage divider.
you have to determine such that you get 12 V across the lamp if the contact is at the top and 0 V if the contact is at the bottom.

 

[barnaby]

So if the contact is at the top, you get what is in effect two resistors in parallel - one at 24 ohms and the other at 6 ohms? I tried solving it like that... and got nowhere - calculated that the total resistance of the two strands of the circuit was 4.8 ohms... don't know if that's helpful at all, though.

 

[kamerling]

So if the contact is at the top, you get what is in effect two resistors in parallel - one at 24 ohms and the other at 6 ohms? I tried solving it like that... and got nowhere - calculated that the total resistance of the two strands of the circuit was 4.8 ohms... don't know if that's helpful at all, though.

That's certainly helpful. Now you have to find the R that makes the voltage divider formed by R and the 4.8 Ohm resistance produce 12 V