First Law of Thermodynamics - easy q's

[dtavob]

Homework Statement

The internal energy of a gas is 600 J. The gas is compressed adiabatically, and its volume decreases by 400 cm3. If the pressure applied on the gas during compression is 4 atm, what is the internal energy of the gas after the adiabatic compression?

Homework Equations

Change Q = Change U + Change W

The Attempt at a Solution

Since it's adiabatic then change in Q = 0...

So it should be change U = - change W...
(Ufinal - Uinitial) = - (4*4) ----I also tried changing atm to pascals, but the answer is still wrong.

What am i doing wrong? Anyone please.

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Homework Statement

A gas has an initial volume of 7.05 m3. It is expanded to six times its original volume through a process for which P = αV3, with α = 4.00 N/m11. How much work is done by the expanding gas?

Homework Equations

W = P*delta V

The Attempt at a Solution

So P = 4*7.05^3 = 28.2

delta V = (Vfinal - Vinitial) = (7.05*6) - 7.05 = 35.25 m3

so W = 994.05 J. The correct answer is 3.20e6 J, but I don't understand what I'm doing wrong. Anyone please?

 

[eczeno]

hello,

in the first problem you are not converting cm^3 to m^3 correctly. and you do need to use Pa for pressure. you must be consistent with your units throughout any formula.

if i understand the second problem correctly, the volume and pressure are changing at the same time and are related by P = a*V^3 (i assume the 3 is an exponent). for this, one needs calculus.

W = \int_{V_1}^{V_2}{P}dV

 

[dtavob]

Woah. Thank you so much. I guess I was just too hungry to think right =/.

 

[eczeno]

cheers.