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# Thermal energy unit 5 question Watch

1. I'm stuck on part C:

For all of C I used p1.v1/T1 = p2.v2/T2. (See pic)

I used this for ci) and got an answer of 20kPa. I used fact that p1 is 15 kPa using PV equals constant etc and t1 as 300.

The answer is actually 200kPa for part a. Where did I go wrong!?

Also I can't do part ii or iii) do I read of the graph at 400kpa and get v1 and use that for part ii and apply the same method for part iii)

If so then using that constant rule for iii) I got 2.4x10^-4 but the markscheme used a completely different method. They used point A to get v1 p1 and we know t1. Then used that to get v2 giving 2.5x10^-4...

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2. Likewise, I cannot derive any other answer for part Ci.) other than 20000 Pa. Perhaps there is something simple that we are both missing? However I cannot see any reason why we should both be out by a factor of ten.
For parts ii and iii you would continue to use the same method as for part i. The graph is showing isothermal changes at a temperature of 300K, whereas the questions in part C are concerning a temperature of 400K.

Deep breaths.
3. (Original post by tigerboobs)
Likewise, I cannot derive any other answer for part Ci.) other than 20000 Pa. Perhaps there is something simple that we are both missing? However I cannot see any reason why we should both be out by a factor of ten.
For parts ii and iii you would continue to use the same method as for part i. The graph is showing isothermal changes at a temperature of 300K, whereas the questions in part C are concerning a temperature of 400K.

Deep breaths.
Using your method for ii and iii what answers did you get? I believe the markscheme is wrong for ci.

*Taking deep breaths*

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4. Also, might I add - for parts i, ii, and iii you can use any point on the graph to obtain values for p1v1/t1 - that is the very essence of p1v1/t1 = a constant. They all give the same value. It does not matter that they have used point A, and you have used a different point.
If you were still using point C, then your equation should've looked something like this
150*8x10^-4*400/300*640
I am not sure why you are getting 2.4x10^-4. Perhaps you have simply made an error on your calculator.
5. For parts ii and iii, I got 4x10^-4 and 2.5x10^-4, respectively.
6. (Original post by tigerboobs)
Also, might I add - for parts i, ii, and iii you can use any point on the graph to obtain values for p1v1/t1 - that is the very essence of p1v1/t1 = a constant. They all give the same value. It does not matter that they have used point A, and you have used a different point.
If you were still using point C, then your equation should've looked something like this
150*8x10^-4*400/300*640
I am not sure why you are getting 2.4x10^-4. Perhaps you have simply made an error on your calculator.
Thanks, I see what they did for ciii) I applied your method and got those answers posted above and I used it for i) giving 20000Pa again.

Thanks again!

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