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Thermodynamics Problems

Thermodynamics Problems. Preliminaries. Energy conservation for a thermodynamic system: If heat is gained by the system, ; if heat is lost by the system, . If the system does work on its surroundings, ; if the surroundings does work on the system, .

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Thermodynamics Problems

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  1. Thermodynamics Problems

  2. Preliminaries • Energy conservation for a thermodynamic system: • If heat is gained by the system, ; if heat is lost by the system, . • If the system does work on its surroundings, ; if the surroundings does work on the system, . • In order for there to be work, the system’s volume has to change. Otherwise, .

  3. Example: An ice cube is lying on a table and starts to melt. Since the cube absorbs energy from its surroundings, . Assuming the volume of water does not change, . The internal energy increases due to heat gain.

  4. Example: A hot cup of coffee cools down. Since the coffee cools down, heat is lost from the system. . There is no work. The internal energy of the cup of coffee decreases due to heat loss.

  5. Example: A child squeezes a balloon. Does the system’s internal energy decrease or increase? The child adds energy to the system. Ignoring heat transfer during the process, the work done on the balloon is positive, so the internal energy of the system increases.

  6. Example: A gas absorbs 200 J of heat and releases 120 J of heat as it does 50 J of work on the surroundings. What is the change in internal energy of the system?

  7. Example: A metal stamping machine in a factory does 2100 J of work each time it stamps out a piece of metal. Each stamped piece is then dipped in a 32.0-kg vat of water for cooling. By how many degrees does the vat heat up each time a piece of stamped metal is dipped into it? (Glencoe)

  8. Example: A 750-kg car moving at 23 m/s brakes to a stop. The brakes contain about 15 kg of iron, which absorbs the energy. What is the temperature increase of the brakes? (Glencoe)

  9. Example: What is the efficiency of an engine that produces 2200 W while burning enough gasoline to produce 5300 W? How much waste heat does the engine produce per second? (Glencoe)

  10. Example: A block of copper at 100.0°C comes in contact with a block of aluminum at 20.0°C. The final temperature of the blocks is 60.0°C. What is the ratio of the masses? (Glencoe)

  11. Example: A 0.600 kg sample of water at 90.0°C is mixed with 0.400 kg of water at 22.0°C. Assuming no heat loss, what is the final temperature of the mixture? (Glencoe)

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