# Change in Internal Energy Watch

1. Determine the change in internal energy for air trapped in a cylinder of radius 0.05m by
a vertical, frictionless piston of mass 10kg when 300J of heat is transfer to the air which
causes the piston to be raised by 0.2m. Take atmospheric pressure to be 100kPa.
2. (Original post by diko1996)
Determine the change in internal energy for air trapped in a cylinder of radius 0.05m by
a vertical, frictionless piston of mass 10kg when 300J of heat is transfer to the air which
causes the piston to be raised by 0.2m. Take atmospheric pressure to be 100kPa.
10*g+100kPa*pi*r^2 is the force
Times 0.2 gives us W
Then you do dU=Q-W

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3. (Original post by verier)
10*g+100kPa*pi*r^2 is the force
Times 0.2 gives us W
Then you do dU=Q-W

Posted from TSR Mobile
OP just to expand on that, the force they've specified is the external forces exerted by piston and atm pressure, however we need the internal force/work exerted by the air in the piston. but since we assume the process is quasi static that means the pressure exerted by the air in the piston is also equal to the external force at any point during the expansion which is why its ok to use 10*g+100kPa*pi*r^2 as the force times the distance

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