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1. Oct 15, 2015

### ElegantSir

1. The problem statement, all variables and given/known data
I have the question with a diagram posted in the thumbnail to make things easier.

2. Relevant equations
E = mc²

3. The attempt at a solution
The homework only had a single example for a question like this so im not 100% sure how to get the answer. So far I've added the 2 masses:
5.030151g and 5.011267g

Then I subtracted them both 5.030151 – 5.011267 = 0.018884g

Here is where im stuck. I know that from the example I need to do something like 1.8884 * 10^(input some number here) but I have no clue how to get the number that it should be to the power to. All I know is after this I need to multiply 1.8884 * 10 ^ (???) by 3.8 * 10 ^ 8 to calculate the total amount of energy produced using E = mc²

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2. Oct 15, 2015

### SteamKing

Staff Emeritus
Why are you assuming that the masses of the elements reacting are not in grams?

I think you are assuming, based on the wording of the question, that only 1 atom of D and T react, but that need not be the case. Only the ratios of the reactants matter.

Also, units should be carefully scrutinized here. If you take c in m/s, what mass units must you use in order to calculate energy in joules?

3. Oct 15, 2015

### ElegantSir

Im not quite sure what you mean by all of that. I assume you mean I need to convert grams to a different unit of measurement but im not sure what. Also what do you mean by
"I think you are assuming, based on the wording of the question, that only 1 atom of D and T react?"

4. Oct 15, 2015

### SteamKing

Staff Emeritus
Well, joules are the SI units of energy. Joules are also derived units. What is the definition of a joule?

https://en.wikipedia.org/wiki/Joule

It's obvious that 1 atom of deuterium does not have a mass of 2.014102 g, but a certain number of deuterium atoms does have this total mass. Ever heard of a fellow by the name of Avogadro?

If you're going to study physics, you've got to study units and understand how units are useful in deciphering physics problems.

5. Oct 15, 2015

### ElegantSir

Ahh I see where I went wrong. The example

6. Oct 15, 2015

### ElegantSir

Ahh I see where I went wrong. The example questions like to use numbers like 1.9 * 10^-4 rather than use the actual whole number so I got confused.

I would need to convert grams to kilograms which would mean the number would now become: 1.8884 * 10^-5
Now I plug it into E = mc^2 and get:

E = 1.8884 * 10^-5 * (3 * 10^8)^2
E = 1.7 * 10 ^ 12 (rounded of course)

Im assuming this is the correct answer now?

7. Oct 15, 2015

### SteamKing

Staff Emeritus
Always include the units in your calculation results. That's a habit you should acquire now.

8. Oct 15, 2015

### ElegantSir

Ahh yes it would be in Joules...Thanks for the help!