Physics II Chapter 18 Concepts and Math Flashcards Flashcards


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1

jogging on the beach

  • she is jogging on the beach and thus doing work on the beach
  • she's sweating so she's giving off heat
  • delta U = Q -W
  • Q is negative because she's losing heat, "give off" is hint
  • W is positive because she's doing W on the beach

2

A gas expands, doing 100 J of work. How much heat must be added to this system for its internal energy to increase by 200 J?

  • Q= W + change in U
  • 100 + 200
  • Q = 300 J

3

One mole of an ideal monatomic gas is initially at a temperature of 274 K. (a) Find the final temperature of the gas if 3260 J of heat are added to it and it does 712 J of work. (b) Suppose the amount of gas is doubled to two moles. Does the final temperature found in part (a) increase, decrease, or stay the same? Explain.

a. change in U = Q - W

replace change in U with 3/2 nR (Tf-Ti)

solve for Tf

b. When there are more molecules to share the energy, the average energy gained per molecule is smaller, resulting in a decrease in the final temperature found in part (a).

4

Suppose 118 moles of a monatomic ideal gas undergo an isothermal expansion from to , as shown in FIGURE 18-28. (a) What is the temperature at the beginning and at the end of this process? (b) How much work is done by the gas during this expansion?

a. solve for temperature with PV = nRT

b. W = nRT ln (Vf/Vi), solve for W in kJ

5

What is the efficiency of an engine that exhausts 840 J of heat in the process of doing 360 J of work?

  • first calculate Qhot, W = Qhot - Qcold
  • then efficiency = W/W + Qc

heat and no cold? better make it hot! Woah, woah there Qc

6

An engine receives 660 J of heat from a hot reservoir and gives off 420 J of heat to a cold reservoir. What are (a) the work done and (b) the efficiency of this engine?

  • first calculate W, W = Qhot - Qcold
  • then efficiency = W/Qh

hot and cold? you gotta work. Yes QqqqqHueeee Bob.

7

A reversible refrigerator has a coefficient of performance equal to 9.50. What is its efficiency?

??

8

Determine the change in entropy that occurs when 4.3 kg of water freezes at 0 °C.

delta S = -mLf/T

change C to K

9

On a cold winter’s day heat leaks slowly out of a house at the rate of 20.0 kW. If the inside temperature is 22 °C, and the outside temperature is , find the rate of entropy increase

delta S equals (negative rate in W/T) inside plus (positive rate in W/T) outside

change C to K

10

An 88-kg parachutist descends through a vertical height of 380 m with constant speed. Find the increase in entropy produced by the parachutist, assuming the air temperature is 21 °C.

delta S = mgh/T

11

COP for aircon

By definition for an air conditioner, the COP is QC/W