Chapter 8 Question #3 (The First Law)

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Jonathan Orozco 1A
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Chapter 8 Question #3 (The First Law)

Postby Jonathan Orozco 1A » Wed Jan 18, 2017 7:36 pm

Hi everyone, i was wondering if anyone could help me solve number 3 from the homework problems.

Question 3: Air in a bicycle pump is compressed by pushing in the handle. If the inner diameter of the pump is 3.0 cm and the pump is depressed with a pressure of 2.00 atm, (a) how much work is done in the compression? (b) Is the work positive or negative with respect to the air in the pump? (c) What is the change in internal energy of the system?

I have no idea on how to start this problem if anyone can help i appreciate it and thank you in advance.

Maggie Bui 1H
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Re: Chapter 8 Question #3 (The First Law)

Postby Maggie Bui 1H » Wed Jan 18, 2017 7:55 pm

First, I would write down what you're given in variables, ex. Pressure (P) =2.00atm. Then, relate the given with what you're looking for. In part a, you're looking for "work...done in the compression". Recall that work is force over a distance and pressure is force over an area. Page 22 in the course reader, which talks about expansion, would be helpful.

Maya Kandah 3M
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Joined: Wed Sep 21, 2016 2:59 pm

Re: Chapter 8 Question #3 (The First Law)

Postby Maya Kandah 3M » Wed Jan 18, 2017 8:15 pm

a) I would start with the equation w=-Pex*ΔV, where P is given as 2.00 atm. Then substitute ΔV for -A*d. The radius of the pump is 3.0cm/2=1.5cm and the pump is depressed 20.o cm thus, d=20.0cm. Plug these values into the equation ΔV=-A*d and the answer into liters. Once you get ΔV plug it into the equation for work and covert the final answer into joules.

b) Think about how work is done on the air in compression.

c) Use the equation ΔU=q+w. Remember that no heat is transferred so q=0 and substitute your answer from part a for w to find ΔU.

Hope this helps!

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