# Effective resistance Question

1. Jul 12, 2006

### dblanche

I cant seem to get this one, I just dont know how to calculate the effective resistance, what is the difference between effective resistance and normal resistance??

Here is the question:

A car battery has a source voltage of 12.00 V and an internal resistance of
0.015 Ω. When the headlights are turned on (while the engine is not running)
the output voltage of the battery drops to 11.75 V. The effective resistance of

If someone could just give me a formula or some hints........that would be great!

2. Jul 12, 2006

### Andrew Mason

Think of the battery as a pure source of 12V in series with a .015 Ω resistor. At the current drawn when the headlight is connected, the voltage drop across the resistor is .25V. Work out that current and determine what the resistance of the headlight is.

The effective resistance is the same as the normal resistance.

AM

3. Nov 1, 2011

### chai940610

hello,may i know what the answer?

4. Nov 1, 2011

### Andrew Mason

Welcome to PF. The short answer is NO. You have to try to work it out first.

AM