Thermal physics exam questions - would you check my answers please?

  • Thread starter Brewer
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  • #1
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I've been revising for my exams with past exam papers supplied by my uni, but they don't supply the answer schemes for them. Would some of you guys look at them for me if I put the question and the answers I get?

Thanks

1) i) calculate the change in length of a 1.5m long copper bar when its temp is raised from 303K to 353K
ANS: 1.275*10^-3 m

ii) calculate the amount of energy to completely melt a 10g ice cube at a temp of 273K
ANS: 3300J

iii)calculate change in entropy in part (ii)
ANS: 12.09 J/K

iv) state 4 assumptions of kinetic theory of gases
ANS: molecules evenly spaced, move in random directions, random velocities, all have the same mass

v) assuming temp constant at 303K and independant of height calculate atmospheric pressure at top of Everest
ANS: 37241 Pa

vi) estimate rate energy escapes from sun (surface temp ~ 6000K) in form of thermal radiation. state assumptions
ANS: 4.52*10^26 W assuming sun is spherical and is a black body emitter

vii) state the entropy form of the second law of thermodynamics
ANS: no process can occur that would result in an increase in the entropy of the universe. Universally the entropy must stay the same or increase.

2) i) State the first law of thermodynamics explain any symbols
ANS: U = Q + W, U=change in internal energy, Q=change in heat, W=change in work

ii) 10kJ of heat are supplied to a gas which then expands at a constant pressure of 5kPa, from an initial volume of 10 litres to a final volume of 40 litres. Calculate the change in internal energy
ANS: -140000J

iii) Explain what is meant by specific heat capacity of a substance
ANS: The specific heat capacity of a substance is the smount of heat required to raise the temperature of 1kg of substance by 1 degree

iv) Explain why the heat capacity of a substance at constant pressure (Cp) is larger than the heat capacity at constant volume (Cv)
ANS: Because Cv is not temperature dependant, but Cp is (complete guess this one)

v) Show that for n moles of ideal gas, Cp-Cv = nR
ANS: I have no idea for this one. I can it relates to PV=nRT somehow but I can't think how.

Please help me!

Thanks

Brewer
 

Answers and Replies

  • #2
Tom Mattson
Staff Emeritus
Science Advisor
Gold Member
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Brewer, no one wants to sit down and work out all these problems so that they can tell you if you got the right answer. If you want help then show your work. That way we can easily tell you if you've made an error by examining your solution.
 
  • #3
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Ok, but how would I go about showing formulas correctly? Otherwise they just get confusing to read.
 

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