# Energy question

1. Apr 26, 2005

### DB

How much energy in Joules is used by a 100 W light bulb in 2 hours?

$$W=\frac{J}{s}$$

$$100 W=\frac{x_{j}}{7200_s}$$

$$720000=Joules$$

My teacher said I probably got the right answer but I did the wrong work. I think I did so too. Can some1 help me out?

Thanks

2. Apr 26, 2005

### xanthym

Your answer is "correct". However, it's better to work problems using equations with variables representing the various quantities.
In words:
{Power} = {Energy}/{Time}
Now let {P = Power (in J/sec)}, {E = Energy (in J)}, and {T = Time (in sec)}, so that:
P = E/T
::: ⇒ E = P*T

P = (100 Watts) = (100 J/sec)
T = (2 hours) = (7200 sec)
E = (100 J/sec)*(7200 sec) = (720000 Joules)

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Last edited: Apr 26, 2005
3. Apr 26, 2005

### ShawnD

I would argue that your approach is better than the teacher's approach; especially when using a Texas Instruments calculator. TI calculators have an equation solver, so you don't actually need to manipulate the formula. The less you screw with an equation, the less likely you are to make silly algebra errors.

Last edited: Apr 26, 2005
4. Apr 26, 2005

### ShawnD

Check out my method of writing formulas

http://myfiles.dyndns.org:8080/math/shawn_equation_style.jpg

Notice how nothing is moved around from the original formula. The value I'm trying to find isn't even isolated.
You're in a physics class, so do physics. Math is for the losers in the math class :tongue:

Last edited by a moderator: Apr 21, 2017