# Power and Efficiency Question

1. Nov 7, 2013

### oMovements

1. The problem statement, all variables and given/known data
An incandescent bulb is only 5.0% efficient. How much waste thermal energy is porudced by a 100.0W bulb in 1.0h? (Anwer=95J)

2. Relevant equations
P = E/T
Efficiency = Eout/Ein x 100%

3. The attempt at a solution
P = E/T
E = PT
= (100)(1)
= 100J

Eff=Eout/Ein
0.95=Eout/100
Eout= 95J

This is the correct answer, but why is time in hours? Shouldn't it be in seconds (1h=3600s) so the equation becomes:

E = PT
= (100)(3600)
= 360000

This is the wrong answer, but I don't understand why time isn't measured in seconds as the SI unit is Seconds for time.

Last edited: Nov 7, 2013
2. Nov 7, 2013

### cepheid

Staff Emeritus
Yes, 100 W = 100 J/s, so in an hour, the total energy used by the bulb should be

E = P*t = (100 J/s)*3600 s = 360,000 J

3. Nov 8, 2013

### oMovements

I did this but the answer comes out to be:
95 = Eout / 360000 x 100
0.95 = Eout / 360000
Eout = 342000

This would mean 342000J of thermal energy is produced by a 100W bulb
However, this answer is 95J which you only get when you use time in hours. So maybe the answer is wrong at the back of the book.

4. Nov 8, 2013

### cepheid

Staff Emeritus
Yeah, maybe. All I know is that a watt is a joule per second.

5. Nov 8, 2013

### CWatters

I agree the book answer is wrong.

As you say..

A 100W bulb consumes 100J/S or 360,000 J/H

If 95% is wasted then the answer is 0.95 * 360,000 = 342,000 Joules