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# Physics adiabatic compression and cooling watch

1. Could someone please help me with the attached question. We haven't learnt this and my teacher told me this was right even though the mark scheme just says anti clockwise arrows in the right place. Also, how can you tell if work is done on a gas or by a gas. Can't seem to find this stuff in a text book Thank youAttachment 725984
2. (Original post by MissMathsxo)
Could someone please help me with the attached question. We haven't learnt this and my teacher told me this was right even though the mark scheme just says anti clockwise arrows in the right place. Also, how can you tell if work is done on a gas or by a gas. Can't seem to find this stuff in a text book Thank youAttachment 725984

I cannot see the attachment, so I am explaining based on interpretation of what you are asking.

When an ideal gas undergoes expansion, the area under the curve of a PV diagram, would give the work done by the gas. Let say the area gives 73 J, then work done by the gas is 73 J.

Note that

work done by the gas = negative work done on the gas

In this case, the work done on the gas is −73 J.

As the ideal gas undergoes compression, then the area under the curve of a PV diagram, would give the work done on the gas. Again say that the area gives 80 J, the work done on the gas is 80 J, then the work done by the gas is −80 J.
3. (Original post by Eimmanuel)
I cannot see the attachment, so I am explaining based on interpretation of what you are asking.

When an ideal gas undergoes expansion, the area under the curve of a PV diagram, would give the work done by the gas. Let say the area gives 73 J, then work done by the gas is 73 J.

Note that

work done by the gas = negative work done on the gas

In this case, the work done on the gas is −73 J.

As the ideal gas undergoes compression, then the area under the curve of a PV diagram, would give the work done on the gas. Again say that the area gives 80 J, the work done on the gas is 80 J, then the work done by the gas is −80 J.
Thank you, that makes sense. If it's a graph with arrows on (I tried to attach documents but I'm not sure whether it worked), how can you find whether work is done on or by the gas depending on what direction the arrow go in. Not sure if that makes sense so hopefully the attachment works.
4. (Original post by MissMathsxo)
Thank you, that makes sense. If it's a graph with arrows on (I tried to attach documents but I'm not sure whether it worked), how can you find whether work is done on or by the gas depending on what direction the arrow go in. Not sure if that makes sense so hopefully the attachment works.

Consider the first diagram, ask yourself:

Is W --> X an expansion or compression?

Spoiler:
Show

Is Y --> Z an expansion or compression?

Spoiler:
Show

The area under the curve WX is more than that of curve YZ. This means that work done by the gas is more than work done on the gas. So the net work done is work done by the gas.

If you follow the arrow, it goes in a clockwise direction. You can base on this to deduce that the net work done is work done by the gas.

If you reverse all the arrow direction, it goes in an anticlockwise direction. Then the net work done is work done on the gas.
5. Thank you so much, makes a lot more sense now. So for the second diagram would the answer to bi) would be anti-clockwise? I'm still not sure which part would be cooling and which would be the adiabatic compression. Would the curved part be compression( volume decreasing so pressure increases) or should it be a straight line like the one drawn but in the opposite direction ?
6. (Original post by MissMathsxo)
Thank you so much, makes a lot more sense now. So for the second diagram would the answer to bi) would be anti-clockwise? I'm still not sure which part would be cooling and which would be the adiabatic compression. Would the curved part be compression( volume decreasing so pressure increases) or should it be a straight line like the one drawn but in the opposite direction ?

Vertical line in a pV graph or diagram means that there is no change in volume.

You can make use of ideal gas law to find out which way should the arrow being drawn to imply cooling.

As for adiabatic compression, it must be a curve and (I think) you had guessed it correctly. Don’t need to think of anticlockwise or clockwise. Just think of decrease in volume.

For adiabatic compression, the decrease in volume also implies that there must be an increase in pressure. (You should understand why this is so. Use First law of thermodynamics to explain it.)

Note that we can have compression without a change in pressure – it is a horizontal line in the pV diagram which shows a decrease in volume.

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