# Lab Question and Homework Help

1. Apr 27, 2015

### Dhooy7

1. The problem statement, all variables and given/known data
So I am doing a mini-lab and I measured the temperature of water before heating it up and after. I determined delta T or the temperature gain to be https://www.physicsforums.com/file:///C:/Users/BADGER~1/AppData/Local/Temp/msohtmlclip1/07/clip_image002.gif [Broken]=20.555https://www.physicsforums.com/file:///C:/Users/BADGER~1/AppData/Local/Temp/msohtmlclip1/07/clip_image004.gif [Broken]

Now it states to calculate the efficiency of the heat-transfer process asuuming the power outuput of the oven is 1000W.

2. Relevant equations
E=P xt

3. The attempt at a solution

I was thinking about calculating the energy output which is 30,000 J

Then use the definition of a calorie. Not sure if there is an equation for this or not. It is the delta T or temperature gain multiplied by the amount of liters which is 0.250 L. The energy input is 5.139 Calories and converting that to joules is 21501.576 J. Then you subtract the two for the energy efficiency. Is this correct?

Someone said to use this equation: Q=mCΔT

I don't have a mass. Which way is correct? Am I on the right track or not?

Last edited by a moderator: May 7, 2017
2. Apr 27, 2015

### SteamKing

Staff Emeritus
For some reason, the images attached to your post aren't working correctly.

Then state the definition for the calorie unit verbatim.

It's not clear how you are calculating efficiency here. Hint: efficiency is usually quoted as a percentage.

If this is a lab, you should have all your formulas squared away before you start to do the experiment. You shouldn't have to rely on what "someone" said.

If you have an amount of water in liters, you can find out what the mass of the water is. That's why you learned what "density" is for. (At least, I hope you did.)

Last edited by a moderator: May 7, 2017
3. Apr 28, 2015

### andrevdh

I would assume the efficiency would be: (amount of heat transferred to the water)/(total amount of energy used by the process) x 100
Certainly you would need Q=mcwΔT to calculate the first term.
The second term is the amount of electric energy that was converted to heat by the oven,
of which just Q was transferred to the water.