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Hi, so the bits highlighted are what I don't get.
compression means system volume is decreasing so how is it positive for compression?
Would really like some help!! Thanks!!
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compression means system volume is decreasing so how is it positive for compression?
Would really like some help!! Thanks!!
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#3
They are just saying, if dV and p dV are negative, then - p dV will be positive.
(I think - I always got in a tangle over = & - signs)
(I think - I always got in a tangle over = & - signs)
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(Original post by old_teach)
They are just saying, if dV and p dV are negative, then - p dV will be positive.
(I think - I always got in a tangle over = & - signs)
They are just saying, if dV and p dV are negative, then - p dV will be positive.
(I think - I always got in a tangle over = & - signs)
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#5
Ok - I'll write some random stuff and see if any helps.
dV: if the volume is getting smaller, the change of volume is negative, i.e. dV is negative during compression.
work done by gas: I prefer to think of a gas expanding, perhaps a steam engine pushing the piston out and driving the wheels - this is +ve dV and the gas is doing work.
So if an expanding gas does work, a compressed gas has work done on it, so you'd say the work done by the gas is negative.
Helpful?
dV: if the volume is getting smaller, the change of volume is negative, i.e. dV is negative during compression.
work done by gas: I prefer to think of a gas expanding, perhaps a steam engine pushing the piston out and driving the wheels - this is +ve dV and the gas is doing work.
So if an expanding gas does work, a compressed gas has work done on it, so you'd say the work done by the gas is negative.
Helpful?
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(Original post by old_teach)
Ok - I'll write some random stuff and see if any helps.
dV: if the volume is getting smaller, the change of volume is negative, i.e. dV is negative during compression.
work done by gas: I prefer to think of a gas expanding, perhaps a steam engine pushing the piston out and driving the wheels - this is +ve dV and the gas is doing work.
So if an expanding gas does work, a compressed gas has work done on it, so you'd say the work done by the gas is negative.
Helpful?
Ok - I'll write some random stuff and see if any helps.
dV: if the volume is getting smaller, the change of volume is negative, i.e. dV is negative during compression.
work done by gas: I prefer to think of a gas expanding, perhaps a steam engine pushing the piston out and driving the wheels - this is +ve dV and the gas is doing work.
So if an expanding gas does work, a compressed gas has work done on it, so you'd say the work done by the gas is negative.
Helpful?
This was very helpful.But for your last point, isn't it the opposite way round?
So for the case of compression: The surroundings(the piston i think ) does work on the gas (system) , and work done on a gas is negative.
This is given here: the bit in red
Could you please kindly explain this? Thanks very much!!
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#7
I think we've both said it the same way?
You're a good student to worry so much - but I think we agree - sorry - I'm babysitting and she's crying!
You're a good student to worry so much - but I think we agree - sorry - I'm babysitting and she's crying!
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#8
The system , ie the air in the container, is being pushed, hence work is being done against the air.
By convention, when work is being done against a system, or an object, the work calculated is negative.
Hence, the system is indeed doing " negative work"
By convention, when work is being done against a system, or an object, the work calculated is negative.
Hence, the system is indeed doing " negative work"
Last edited by Idg a damn; 1 year ago
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(Original post by old_teach)
I think we've both said it the same way?
You're a good student to worry so much - but I think we agree - sorry - I'm babysitting and she's crying!
I think we've both said it the same way?
You're a good student to worry so much - but I think we agree - sorry - I'm babysitting and she's crying!
I said work done on the gas is negative. Just quoted what I said below:
"But for your last point, isn't it the opposite way round?
So for the case of compression: The surroundings(the piston i think ) does work on the gas (system) , and work done on a gas is negative. "
work done by the gas on the surroundings is not really the same as work done on the gas by the surroundings.
This is what is provided by the book:
So I am not sure if we are saying the same thing haha.
Apologies for all the headache - I am quite a worrier and try to understand things perfectly and not memorise it and that got me in trouble at school! I understand you must be very busy with other reposibilities
so no worries at all if you can't reply as soon enough
Last edited by h26; 1 year ago
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(Original post by Idg a damn)
The system , ie the air in the container, is being pushed, hence work is being done against the air.
By convention, when work is being done against a system, or an object, the work calculated is negative.
Hence, the system is indeed doing " negative work"
The system , ie the air in the container, is being pushed, hence work is being done against the air.
By convention, when work is being done against a system, or an object, the work calculated is negative.
Hence, the system is indeed doing " negative work"
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#11
(Original post by h26)
Thanks! But what what do you mean by "against" in this context so work being done by the system on the surrounding or work being done on the system by the surroundings?
Thanks! But what what do you mean by "against" in this context so work being done by the system on the surrounding or work being done on the system by the surroundings?
Conventionally, if the direction of the force vector upon an object is opposite to the movement that object, hence moving ( the vector, not the object) in the direction opposite to itself , then work done by that force is negative.
Last edited by Idg a damn; 1 year ago
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(Original post by Idg a damn)
The system , ie the air in the container, is being pushed, hence work is being done against the air.
By convention, when work is being done against a system, or an object, the work calculated is negative.
Hence, the system is indeed doing " negative work"
The system , ie the air in the container, is being pushed, hence work is being done against the air.
By convention, when work is being done against a system, or an object, the work calculated is negative.
Hence, the system is indeed doing " negative work"
so work being done on the sytem (air/gas) by the surroundings is the same thing as saying work is being done against the system and this is negative.
so therfore compression must be negative but it is apparently positive.
Could you please kindly let me know your thoughts on this whenever possible? Thanks!
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(Original post by Idg a damn)
What I meant by "against" is that the force by the surroundings is pushing the air in the container in the direction opposite to its movement, thus going against it.
Conventionally, if the direction of the force vector upon an object is opposite to the movement that object, hence moving in the direction opposite to itself , then work done by that force is negative.
What I meant by "against" is that the force by the surroundings is pushing the air in the container in the direction opposite to its movement, thus going against it.
Conventionally, if the direction of the force vector upon an object is opposite to the movement that object, hence moving in the direction opposite to itself , then work done by that force is negative.
Thanks, so compression would be an example.
Could you please kindly let me know your thoughts about my other reply if you have time?
Really appreciate the help
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#14
(Original post by h26)
Haha we replied at exactly the same time
Thanks, so compression would be an example.
Could you please kindly let me know your thoughts about my other reply if you have time?
Really appreciate the help
Haha we replied at exactly the same time
Thanks, so compression would be an example.
Could you please kindly let me know your thoughts about my other reply if you have time?
Really appreciate the help
When the system is compressed, the force vector caused by the surroundings is moving along its direction ( remember how force is a vector quantity hence has direction?) as it pushes the air in its direction, causing the air , and the force vector to move along the force's orientation, hence work done by the surroundings is positive.
On the other hand, when the system expands, the force vector ( which is caused by the surroundings,in this case still pushing the air) is moving in the direction opposite to itself as the piston is pushed by the air. Hence work done by the surroundings is negative in this case.
Last edited by Idg a damn; 1 year ago
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(Original post by Idg a damn)
Do you understand what I said?
Do you understand what I said?
so the force vector which is caused by the surroundings is instead compressing air on the other side of the piston? Wait nevermind I don't really udnerstand the second part
I am also trying to relate it to the formulas in my notes but that isn't going too great at the moment
Attachment 794602
Last edited by h26; 1 year ago
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(Original post by Idg a damn)
Did you see that the force vector has moved in the direction opposite to itself?
Did you see that the force vector has moved in the direction opposite to itself?
Dunno if I am correct - assumed that cause of as mechanics
Last edited by h26; 1 year ago
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#20
(Original post by h26)
I mean the force by the surroundings is still in the same direction but the direction of motion has changed as the particles are expanding and getting bigger so the direction of motion is opposite the pushing force by the surroundings.
Dunno if I am correct - assumed that cause of as mechanics
I mean the force by the surroundings is still in the same direction but the direction of motion has changed as the particles are expanding and getting bigger so the direction of motion is opposite the pushing force by the surroundings.
Dunno if I am correct - assumed that cause of as mechanics
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