# Homework Help: Electric power and emf

1. Jun 16, 2010

### daysrunaway

1. The problem statement, all variables and given/known data
A battery supplies a steady current of 5.6 A for 21 s. During this time 1410 J of chemical energy is transformed into electric energy, but 560 J of this electric energy is transformed into heat energy inside the battery. What is the total power output of the battery?

2. Relevant equations
P = E/t

3. The attempt at a solution
What is confusing me is: is the energy I should consider in the "total power output" the energy associated with the emf - 1410 J - or the energy associated with the terminal voltage - 1410 J – 560 J = 850 J? I don't understand why I should use one or the other.

The next question asks pretty much the same question but with emf and current, and doesn't even give a terminal voltage, so I am guessing I should use 1410 J, but I don't understand why this is. Could someone please help explain?

2. Jun 16, 2010

### kuruman

The way I read it, the battery puts out two kinds of power, thermal, that heats its surroundings, and electrical that does what it does in the circuit. Total power would be the sum of the two.