# Homework Help: Battery charged by a generator

1. Apr 8, 2010

### cyy91

1. The problem statement, all variables and given/known data
a generator of e.m.f 20V and internal resistance of 0.5 ohm is used to charge a car battery of emf 12V and internal resistance 0.10 ohm.They are connected in series together with a resistance R whose value is adjusted to give a charging current of 2.0A.

a) value of R
b) power of generator
c) total rate of dissipation of electrical energy
d) rate at which the battery stores chemical energy
e) efficiency of the whole operation

2. Relevant equations
P=VI
emf = I(R+r)

3. The attempt at a solution
i tried combining the 2 emfs and it sounds really funny, so i tried subtracting instead and still can't ring a bell.

How exactly can i calculate the rate at which a battery stores chemical energy?
anyone kindly provide me with an appropriate formula thanks.
better yet explain the whole question so i can answer other similar questions which caused me the same headaches...=S

2. Apr 8, 2010

### collinsmark

Hello cyy91

As a first step, draw out the diagram for this configuration. Seeing the diagram visually might make this problem seem a little easier.

You need to calculate the power flowing into the battery's emf section (hint: it involves the battery's emf and the current flowing through it [albeit in the reverse direction as normal] ).

I think that once you start with the diagram, and work through what components are dissipating/using what power, things will fall into place.

3. Apr 9, 2010

### cyy91

thx fr da help collinsmark...truly helps...