Science Homework - How do I work this out?

Question

**PLEASE NOTE: I DO NOT WANT THE ANSWER**
I just want to know how to work it out =]

1) A student wants to find out the efficiency of a stereo system, rated at 20 watts sound output. She buys some batteries, which store 400,000 joules of energy of chemical energy. When the new batteries are put into the stereo, and its switched on to play a tape, the batteries run out after 5 hours.

a) How much energy has the stereo usefully given out?

b) What is the efficiency of the tape recorder

In class we learnt that
Efficiency = Useful energy out divided by total energy in



The last question is:
If the stereo is used on the radio setting, would you expect the batteries to last longer?

I don’t know if it takes more power to power the motor to spin the CD and the laser to read it or if it takes more power to receive the signal from the radio station???


Please help me

Thanks for your time

Answer 1

Energy and power... What a struggle.

Power = energy /time

Pin= Eb t

n=Pout/Pin

a) The energy the stereo usefully given out is
E =Pout time
E= 20 5 (3600)= 360,000 J

b) ..efficiency of the tape recorder
Pin= E battery / time

n=Pout/Pin

n=20/[400,000 /(5 (3600))]=.9
or n=90%

Note: For efficiency computations I used power instead of energy.