# Circuits .. what is this question asking?

1. Aug 8, 2015

### Fat_Squirrel

1. The problem statement, all variables and given/known data
A battery has an emf E and internal resistance r. A variable resistor R is connected across the terminals of the battery. Find the values of R such that

(a) the potential difference across the terminals is a maximum;
(b) the current in the circuit is a maximum;
(c) the power delivered to the resistor is a maximum.

2. Relevant equations
I=E/(R+r)

3. The attempt at a solution
Im not sure if question (a) is talking about max voltage we can run on the circuit, or is just talking about the EMF in which case it never changes.

b) I assume when R=0 we get max current, no resistant means max current.
C) Max Power transfer theorem says when R=r we get max power.

Any help with a) greatly appreciated.

2. Aug 8, 2015

### nsaspook

The question asks about the terminal voltage (the external connection points to the battery) in question (a).

3. Aug 8, 2015

### Fat_Squirrel

Cheers.

Ok, in that case its when the current through the circuit is zero, so basically when we have a switch or a resistor with infinite resistance?

4. Aug 8, 2015

### Fat_Squirrel

That makes sense.
I assume I'm okay on the other two?

5. Aug 8, 2015

### nsaspook

Yes, for perfect circuits and batteries.

6. Aug 8, 2015

### Fat_Squirrel

Thanks heaps. Feeling stupid. Not good when I'm stuck on one of the first questions. Appreciate the quick response. Now I can enjoy my wine in peace.

7. Aug 8, 2015

### nsaspook

Question (c) while valid is not the best way to transfer power 'from' a battery or DC power supply. In practical circuits the load resistance is usually higher than the source internal resistance to minimize internal resistance power loss (lost to heat) with a greater percentage of power going to the load.

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