# HELP! Physics problem! Circuits and resistance question!

#### Tweak155

##### Lifer
please help, ive spent too long trying to figure this out, its killing me...anyone know how to do this?

A solar cell generates a potential difference of 0.345 V when a 700ohm resistor is connected across it, and a potential difference of 0.373 V when a 840ohm resistor is substituted. What are (a) the internal resistance (ohm) and (b) the emf (in volts, V) of the solar cell?

thanks

BTW:
I have the correct answers but i have to show how to do it, the right answers are:
a.) 600 ohms
b.) .64 V

#### Darien

##### Platinum Member
V = EMF - I*r, small r = internal resistance

V = IR (ohms law)

so I = V/R

and

V = EMF - V/R * r

with your values, you get 2 equations and 2 unknowns

#### Tweak155

##### Lifer
V = EMF - I*r

Which V is that? You have to assume you dont know the answers to the problem...

#### Darien

##### Platinum Member
note: your answers of 600 ohms and .64 V are what you get after rounding.

#### Tweak155

##### Lifer
V=I(R+r)

.345=I(700+r)

.373=I(840+r)

The currents in the 2 situations arent the same...