Circuits: Power, Voltage, Resistance

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Homework Help Overview

The discussion revolves around a circuit problem involving a 12V, 24W lamp, focusing on calculating the resistance of the lamp and determining a resistor value that allows for varying voltage across the lamp. The subject area is electrical circuits, specifically dealing with power, voltage, and resistance.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the calculation of the lamp's resistance and express uncertainty about how to approach the problem of varying the voltage across the lamp. Questions arise regarding the role of an unspecified component in the circuit and the configuration of resistors when using a potentiometer.

Discussion Status

Some participants have provided insights into the nature of the potentiometer and its function as a variable voltage divider. Others are exploring the implications of combining resistances in parallel and the resulting calculations, although there is no clear consensus on the correct approach yet.

Contextual Notes

There is a lack of information regarding the unspecified component in the circuit, which is causing confusion in determining the correct setup for the voltage divider. Participants are working within the constraints of the problem as presented, without additional context or details.

barnaby
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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.
 
Last edited by a moderator:
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barnaby said:

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.
 
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.
 
Last edited:
barnaby said:
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
 

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