# Change in Internal Energy

Announcements Posted on
Why bother with a post grad? Are they even worth it? Have your say! 26-10-2016
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

Posted from TSR Mobile
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

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: May 14, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### Who is getting a uni offer this half term?

Find out which unis are hot off the mark here

### Brexit: Why you should study law

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.