# TPR FL1 CP 5 How does gas expand without doing work?

#### ThujaOccidentalis

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The passage literally says "work done by the system [expansion] is positive", which is why I chose positive.

The question stem does mention ideal gas, but I still can't mentally grasp how gas magically expands without doing any work on the environment...I mean it is EXPANDING. Would someone please give me sort of a step by approach/reasoning?

EDIT: This is under adiabatic conditions. I understand that Q is zero, but still can't grasp how W is zero when the gas is physically expanding.

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#### laczlacylaci

2+ Year Member
The passage literally says "work done by the system [expansion] is positive", which is why I chose positive.

The question stem does mention ideal gas, but I still can't mentally grasp how gas magically expands without doing any work on the environment...I mean it is EXPANDING. Would someone please give me sort of a step by approach/reasoning?

EDIT: This is under adiabatic conditions. I understand that Q is zero, but still can't grasp how W is zero when the gas is physically expanding.
Can you screenshot the last sentence of the passage, as indicated in the key?

#### aldol16

2+ Year Member
Well, we don't have the passage but one way is expanding against a vacuum. Since W = P*delta V where P is the external pressure (i.e. the pressure against which the gas expands), when a gas expands against a vacuum, P(ext) = 0 and therefore W = 0. If that expansion is adiabatic as well, then the change in energy is 0 as well.

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#### ThujaOccidentalis

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Thanks aldol16, that makes sense. Also, here is the last sentence.

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