# Homework Help: Internal resistance

1. Apr 7, 2005

### StaceyParker

I'm doing an experiment to measure the internal resistance of a zinc carbon cell (battery) and have slightly confused myself! Do I use the ε= IR + Ir formula? And if so how in the world do I use it! I know that I can work out R by using V and I values but how do I find the EMF of the battery? And what value do I use for I cos I have hundreds cos current isn't a constant!
I've royally confused myself!

2. Apr 7, 2005

### GCT

The emf of the battery should be listed on the battery, the internal resistance can be found either through research or it may even be indicated on the battery

$$\Delta V=EMF-IR$$

The internal resistance can be experimentally determined by hooking the battery up to the resistor,

the total V of the circuit will be $$\Delta V_{circuit}=I(R)=I(R_{eq}+R_{internal})$$

3. Apr 7, 2005

### Chi Meson

The emf will be the terminal voltage when there is no current flowing. THat is, test the voltage of the battery when it is not attached to the circuit.

4. Apr 7, 2005

### Delta

You could do a plot of various values of V (=IR) against the current I. As shown by the formula IR = EMF - Ir, you will see that the gradient will be -r, whilst the y intercept will be the EMF.