Homework Help: Internal Resistance of a Battery

1. Mar 21, 2005

JoakimvE

I am currently doing a lab trying to calculate internal resitance of a battery. Do I use the formula V=IR?

2. Mar 21, 2005

ramollari

What do you denote by R? The circuit connected to the battery also has resistance?
$$Emf = I(R_{batt} + R_{circ})$$

3. Mar 21, 2005

Quarkycharm

Using a variable resistor, take different values of R and find the values of the current (I) and voltage (V). Draw a graph of V against I, find the slope as it'll give you -r (which is the internal resistance of the battery) .

4. Mar 21, 2005

JoakimvE

you mean to say, that an ammeter or voltmeter would come to effect my results?

5. Mar 21, 2005

JoakimvE

and that Internal Resistance is relative to the electric current.

6. Mar 21, 2005

Nylex

What do you mean? . Surely you know that if you increase resistance, current will decrease and voltage will increase?

7. Mar 21, 2005

JoakimvE

yes... ofcourse i know that... but i mean, is Internal Resistance of a Battery a constant or not?

8. Mar 21, 2005

Nylex

Yes, it is constant.

9. Mar 21, 2005

JoakimvE

okay...
damn, my values are confusing the hell out of me...

10. Mar 21, 2005

Quarkycharm

JoakimvE,
Hey it's simple. What are the steps of the experiment you're conducting to find internal R? Are you using a variable resistor or only adding resistors to the circuit?

11. Mar 21, 2005

Integral

Staff Emeritus
This depends on the meters you are using. If you are using a modern digital meter, then then I doubt that you will be able to detect their presence. If you are using an old fashioned analog meter with a D'Arsonal/needle movement, then yes, they will have a measurable effect.

An ammeter is an addional resistance in series and a voltmeter is a resistance in parallel to the measured resistance.

12. Mar 22, 2005

JoakimvE

variable resistor

13. Mar 22, 2005

JoakimvE

okay, thanks a lot. im understanding this way more now (even though im not much of an electrician :zzz:).

anyhow, i guess the main problem is how much i varied the resistance by. i just varied the electric field (E)... im guessing i cant calculate the resistance i added.

14. Mar 22, 2005

Quarkycharm

Emf = V(terminals) + V(lost)

You actually don't need to.. First, short circuit the battery to find its emf. Set up your circuit with the variable resistor, voltmeter and ammeter connected properly. Now increase the resistance by moving the slide and record 5 reading for voltage and current (from the voltmeter and ammeter)

Plot a graph of V against I and calculate the slope as it would represent -r The straight line you've drawn is represented by this equation:

V = (-r)I + E
E: battery emf
-rI: voltage lost due to internal resistance
V: voltage across terminals, measured by voltmeter

Hope that cleared things out for you, if you need me to explain it more let me know, I'd be glad to help.

15. Mar 22, 2005

JoakimvE

no i got it. thanks a lot.