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

  1. Jan 11, 2006 #1
    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?


    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!


  2. jcsd
  3. Jan 11, 2006 #2

    Tom Mattson

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    Staff Emeritus
    Science Advisor
    Gold Member

    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.
  4. Jan 11, 2006 #3
    Ok, but how would I go about showing formulas correctly? Otherwise they just get confusing to read.
  5. Jan 11, 2006 #4

    Doc Al

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    Staff: Mentor

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