# What is the resistance?

1. Oct 27, 2008

### shinobi12

1. The problem statement, all variables and given/known data
trrttA battery has an emf of 15.0 V. The terminal voltage of the battery is 11.6 V when it is delivering 20.0 W of power to an external load resistor R.
(a) What is the value R?
(b) What is the internal resistance of the battery?

2. Relevant equations

3. The attempt at a solution
I'm sure how to start this problem. I'm sure I get rolling if I can just get some momentum started. Please Help.

2. Oct 27, 2008

### christensen

Hopefully these equasions will help. If you get stuck just reply.

V = Voltage (Volts)
I = Current (Amps)
R = Resistance (Ohms)
P = Power (Watts)

V = IR (Ohm's Law)

I = V/R (Ohm's Law Rewritten)

P = VI

P = V2/R

P = I2R

Last edited: Oct 27, 2008
3. Oct 27, 2008

### thepanda

Keep in mind, the internal resistance of the battery can be modeled as a resistor in series with an ideal voltage source.

4. Oct 27, 2008

### LowlyPion

Draw a circuit diagram with the external and internal resistor.
You have a voltage divider there don't you?

And you know Power = I2R