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### Energy in kj or j when solving a problem?

Posted: Sun Jul 09, 2017 3:58 pm
Just for clarification. When solving a photoelectric effect problem, if the threshold energy is given in kj should it be converted to j when finding the kinetic energy?

### Re: Energy in kj or j when solving a problem?

Posted: Sun Jul 09, 2017 5:42 pm
I think it has to be changed to Joules always because when we cancel units for energy we are left with J, and if we leave it in kj we have to change the rest of the other units to get the kj in the final answer.

### Re: Energy in kj or j when solving a problem?

Posted: Sun Jul 09, 2017 7:09 pm
I agree. Also, I would just set up the problem in the standard units of measurement (J for energy, nm for wavelength, etc.) to avoid multiple conversions down the road.

### Re: Energy in kj or j when solving a problem?

Posted: Wed Oct 11, 2017 4:03 pm
Set up the problem using J because that is the standard unit for energy, but you can either leave your answer in J or convert to kJ if youu want.

### Re: Energy in kj or j when solving a problem?

Posted: Wed Oct 11, 2017 6:10 pm
I have a question regarding units. How do kg cancel out when divided by J?

### Re: Energy in kj or j when solving a problem?

Posted: Wed Oct 11, 2017 10:21 pm
504909047 wrote:I have a question regarding units. How do kg cancel out when divided by J?

J=kg*m^2*s^-2

So if, for example, you get a value joules divided by kg, then that would leave you with m^2*s^-2. Hope that helps!

### Re: Energy in kj or j when solving a problem?

Posted: Thu Oct 12, 2017 11:49 am
I would use J since it makes the calculations simpler (since if you used kj you would need to change all of the j to kj so that all the units remain uniform) and if the answer asks for kj you can convert it as the last step of the problem.

### Re: Energy in kj or j when solving a problem?

Posted: Thu Oct 12, 2017 4:02 pm
If we have kg or km should we convert to g and m?

### Re: Energy in kj or j when solving a problem?

Posted: Thu Oct 12, 2017 5:02 pm
Josh Moy 1I wrote:If we have kg or km should we convert to g and m?

I think it depends on the problem. Sometimes it asks you for specific units to end up with. In that case, you should leave it in the desired units the question asks for. However if the question doesn't ask, then I think you can leave it in whatever units you choose. I tend to leave it in the units the question gives to me. For example if the question gives me 15 km and wants a distance answer, I will probably leave the answer in km.

### Re: Energy in kj or j when solving a problem?

Posted: Thu Oct 12, 2017 5:47 pm
Josh Moy 1I wrote:If we have kg or km should we convert to g and m?

If the problem specifically asks you to convert to g or m then you should, but if it doesn't ask you to then I would suggest just keeping the units that you are given in the problem.

### Re: Energy in kj or j when solving a problem?

Posted: Thu Oct 12, 2017 8:50 pm
So should we put the answer in Joules even if the problem gives us the units in Kilojoules?