I have forgotten most of the formulea in Heat and Thermodynamics. I would appreciate if some one can check the following for me please.(adsbygoogle = window.adsbygoogle || []).push({});

1. A 2.2 Kg. block of ice melts to water at 0 degree celcius. By how much has the internal energy of the ice changed? Neglect the small change in volume.

Q = M L where L is the latent heat of water = 3.33810^5 J/Kg

Is this correct?

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2. Suppose that 2.3 mol of argon gas is heated from -45 degrees to 90 degrees celcius. Find the change in internal energy of the gas and the work it does if the heating occurs at a.) constant volume and b.) constant pressure

At constant volume,

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work done, dW= p dV = 0

From first law of TD,

du = dq-w = dQ

This is how I found dQ.

dQ= M Cv (Tf - Ti) where M = number of moles * molecular weight of Ar.

I found in the internet Cv(Ar)= 12.5 J/K at 15 degree celcius & 1 atm.

Since Cv is a function of temperature, what value do I use for Cv in my problem?

For constant pressure

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From pv=nRT, I have work done, W = PdV = nR dT = nR (T2-T1)

To find the change in internal energy,

du = Q - W = M Cp (T2-T1) - nR (T2-T1)

I believe I can use Cp- Cv = R to find Cp.

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3. An average adult person generally emits energy at the rate of approximately 105 W while sedentary. At what rate is the persons entropy changing?

No idea what formula to use. Any advise??

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4. Suppose you leave a 100 W light bulb on continuously for 'h' hours. If the electricity generated and delivered by the power company to the bulb at 30% efficiency, how much thermal energy does the power plant exhaust to the environment due to you oversight?

Does this mean power plants loose 70% energy to the environment?

This is how I worked out this problem

Power * Time (in sec) * 70% = 180 MJ!! This seems so unreal. Am I right

Thanks,

Regards,

GAmma.

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# Homework Help: Formulea in Heat and Thermodynamics

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