THE INTERNET PLASMA PHYSICS EDUCATION EXPERIENCE
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# Stored Energy Calculation

More Explanation...

### This page will show you How to figure out the temperature increase equation by Canceling Units

QUESTION: How hot will 2 cups of water get from 2 minutes in a microwave oven?

```
Joules
Specific Heat of Water = 4180 -----------
Kg - Deg. C

Power of Microwave = 750 Watts

Joule
Watt =  ------
Second

Weight of Water = 0.454 Kg

Time of Heating = 120 Seconds
```

You want to write an equation so the units cancel and you are left with degrees C. The Specific Heat of Water has Deg. C in the denominator, so we will have to divide by it.

```
Kg - Deg. C
X Deg. C = something * -----------
4180 Joules
```
"X" will be the increase in temperature (in degrees Celsius) we expect to see from applying the facts above. "something" has to have Kg in the denominator, so:
```
Kg - Deg. C      1
X Deg. C = something else * ----------- * -------
4180 Joules   0.454 Kg
```
Now we need to cancel Joules, so let's multiply by what a watt is:
```
Kg - Deg. C      1       Joule/second
X Deg. C = another thing *  ----------- * -------- * ------------
4180 Joules   0.454 Kg      Watt
```
Another way to write that is:
```
Kg - Deg. C      1          Joule
X Deg. C = another thing *  ----------- * -------- * ------------
4180 Joules   0.454 Kg   Watt*second
```
Joules and Kg cancel out; now let's get rid of Watts and seconds in the next step:
```
Kg - Deg. C      1          Joule
X Deg. C =  ----------- * -------- * ------------ * 750 Watts * 120 Seconds
4180 Joules   0.454 Kg   Watt*second
```
All the units now cancel on the right hand side of the equation (whew!) leaving Degrees Celsius. So the answer is : 750 * 120 / 4180 / 0.454 = 47.425 Degrees Celsius increase in temperature!

In the real world, "your mileage may vary." Try and think why that could be.