# Physics - Work done

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Thread starter 1 year ago
#1
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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Thread starter 1 year ago
#2
Bump
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1 year ago
#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)
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Thread starter 1 year ago
#4
(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)
Thanks for the help! But tbh, I am still confused :/ I don't understand why that is the case and how that relates to my previous question :/
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1 year ago
#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.
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Thread starter 1 year ago
#6
(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.
Thank you very much! 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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1 year ago
#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!
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1 year ago
#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"
Last edited by Idg a damn; 1 year ago
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Thread starter 1 year ago
#9
(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 am not too sure - I have just quoted below what you kindly mentioned "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."

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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Thread starter 1 year ago
#10
(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"
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?
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1 year ago
#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?
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 ( 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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Thread starter 1 year ago
#12
(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"
Oh wait to edit my reply back:

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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Thread starter 1 year ago
#13
(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.
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
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1 year ago
#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
From the perspective of the surroundings:

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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1 year ago
#15
Do you understand what I said?
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Thread starter 1 year ago
#16
(Original post by Idg a damn)
Do you understand what I said?
I think so! So the first one makes sense I think but for the second part"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."

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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1 year ago
#17
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1 year ago
#18
Did you see that the force vector has moved in the direction opposite to itself?
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Thread starter 1 year ago
#19
(Original post by Idg a damn)
Did you see that the force vector has moved in the direction opposite to itself?
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 so therefore the pushing force by the surroundings is in the negative direction and so work done by the surroundings is negative?

Dunno if I am correct - assumed that cause of as mechanics
Last edited by h26; 1 year ago
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1 year ago
#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
What you said is correct.
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