# Emf and Terminal Voltage

nautica
Okay here is the question:

a) what is the internal resistance of a voltage source if its terminal voltage drops by 2 V when the current supplied increases by 5 A.

b) Can the emf of the voltage source be found with the information supplied?

I know that E = V - Ir, and I am pretty sure that a simple algebraic equation can be put together. So what formulat am I missing??

Thanks
Nautica

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HallsofIvy
Homework Helper
You know that E = V - Ir so &Delta;E= &Delta;V- &Delta;I r. You are told that &Delta;E= 2volts when &Dekta;I= -5 amps. Of course, &Delta;V= 0 since the voltage source is fixed.
Thatis: 2= -5r. Can you solve for r now? .

The answer to b is NO. In effect V is the "constant of integration" when you "integrate" dE= -rdI. There is no way to find that from the information given.

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nautica
I am in College Physics, we can not use calculus for an answer. So is there a reason algebraically.

thanks
nautica

If u know the potential diff b/w two terminals, then u can find the emf.

With this information u cannot find

nautica
So ther is no way to find emf without Potential difference???

thanks
nautica

nautica
Okay For a) here is what I got

V=E-Ir

or dV=dE - dIr

2V = -5r

so r = -.4Ohms

BUT, my instructor said this was wrong b/c there I did not no the Rload and there are too many unknowns for this to be solved.

I can see how part B could not be solved but this looked so simple for part a)

He said this was the other equation I should have considered, but I do not see why it was needed.

I = E / (Rload + r)

Thanks
nautica