How Do You Calculate Resistor Values in a Circuit?

[shinobi12]

Homework Statement

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?

Homework Equations

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.

 

[christensen]

Hopefully these equations 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

 

[thepanda]

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

 

[LowlyPion]

Homework Statement

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?

Homework Equations

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.

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

And you know Power = I²R