kroner wrote:20. D and E are immediately out since the state of the gas is the same after a full cycle. Under adiabatic expansion the temperature of the gas drops. If you draw the P-V diagram of the cycle you'll find the net work of the gas on the environment is negative (which means the thermal energy transferred to the environment is positive).
24. The rule of thumb for counting radioactive decays over a short amount of time relative to the half life is that it approaches a Poisson distribution and so the variance is approximately equal to the expected count. The expected count here is 100 so the standard deviation is sqrt(100) = 10. The question asks what the chance is that the number of counts is more than 1 standard deviation above the mean, which is about 15% since it's nearly a normal distribution.
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