Light bulb efficiency A 9 W LED bulb, an 18 W compact fluorescent bulb and a 75 W incandescent bulb each produce 3 W of visible light energy. What are the efficiencies of each of these bulbs?

James Joule and the mechanical equivalent of heat Joule knew a mass above the ground had potential energy. He dropped an object on a cord, turning a paddle in water monitored by a very accurate thermometer. His conclusion was to connect energy conservation (potential and kinetic) to heat as a third form observed.

Conversions are expected Values on the temperatures scales (Fahrenheit, Centigrade/Celsius, and Kelvin) may be readily interconverted. Physics professors will want values to eventually be in Kelvin because that is the form in SI units (and Fahrenheit is dumb).

Temperature scales Which temperature is greater or smaller: 20 °F, 20 °C, 20 K? Temperatures are ranked from smallest to largest. A. 20 °F < 20 °C < 20 K B. 20 °C < 20 °F < 20 K C. 20 °C < 20 K < 20 °F D. 20 K < 20 °F < 20 °C

Temperature scales Which temperature is greater or smaller: 20 °F, 20 °C, 20 K? Temperatures are ranked from smallest to largest. A. 20 °F < 20 °C < 20 K B. 20 °C < 20 °F < 20 K C. 20 °C < 20 K < 20 °F D. 20 K < 20 °F < 20 °C

Temperature scales Three samples start at the same temperature. The three samples are then heated by DT = 5 °F, 5 °C and 5 K, respectively? Which one has the highest final temperature? DT is ranked from smallest to largest A. 5 °F < 5 °C = 5 K B. 5 °C = 5 °F < 5 K C. 5 °C = 5 K < 5 °F D. 5 K < 5 °F < 5 °C

Temperature scales Three samples start at the same temperature. The three samples are then heated by DT = 5 °F, 5 °C and 5 K, respectively? Which one has the highest final temperature? DT is ranked from smallest to largest A. 5 °F < 5 °C = 5 K B. 5 °C = 5 °F < 5 K C. 5 °C = 5 K < 5 °F D. 5 K < 5 °F < 5 °C

Specific heat A specific heat value reveals how much temperature will change when a given amount of a substance absorbs a given amount of heat. Water is a “benchmark” as one ml of water will absorb 1 cal of heat to raise its temperature by 1oC. 4.2 J to raise 1g of water by 1 degree C. Compare to work: take a 1 kg mass and raise by 10 m (about 3 stories), W = 1kg*9.8*10m ~ 100 J. If put this amount of energy into heat, how much does it raise the temp of the 1kg mass? It takes 4200 J to raise 1 kg by 1 degree C, so 100 J/(4200 J/deg C) = 0.023 deg C. Not much! This means that huge amounts of heat are required to change temperature and also that using heat to do work can mean a lot of work!

Symbols, signs, and definitions for heat and work Hot coffee is poured into a room-temperature mug and over time, they reach thermal equilibrium. What is the sign of Q for the coffee? Sign of Q for mug?

Signs of heat and work on a system Is the work W, the heat Q, and the change in thermal energy DEth, positive (+), negative (-) or zero (0) for the following situations? Does the temperature increase (+), decrease (-), or stay the same (0) W Q DEth DT You hit a nail with a hammer You hold a nail over a Bunsen burner You compress the air in a bicycle pump by pushing down on the handle very rapidly You turn on a flame under a cylinder of gas, and the gas undergoes an isothermal expansion A flame turns liquid water into steam High pressure steam spins a turbine Steam contacts a cold surface and condenses

Heat engines As heat flows from a reservoir at higher temperature to a sink at lower temperature, work may be removed.

Q20.4 During one cycle, an automobile engine takes in 12,000 J of heat and discards 9000 J of heat. What is the efficiency of this engine? A. 400% B. 133% C. 75% D. 33% E. 25%

A20.4 During one cycle, an automobile engine takes in 12,000 J of heat and discards 9000 J of heat. What is the efficiency of this engine? A. 400% B. 133% C. 75% D. 33% E. 25%

Q20.5 During one cycle, an automobile engine with an efficiency of 20% takes in 10,000 J of heat. How much work does the engine do per cycle? A. 8000 J B. 6400 J C. 2000 J D. 1600 J E. 400 J

A20.5 During one cycle, an automobile engine with an efficiency of 20% takes in 10,000 J of heat. How much work does the engine do per cycle? A. 8000 J B. 6400 J C. 2000 J D. 1600 J E. 400 J

Analyze heat engine A gasoline engine in a truck takes in 10,000 J of heat and delivers 2000 J of mechanical work per cycle. The heat is obtained by burning gasoline. Draw heat engine diagram What is the thermal efficiency of this engine? How much heat is discarded in each cycle?

The internal-combustion engine A fuel vapor can be compressed, then detonated to rebound the cylinder, doing useful work.

Refrigerators A refrigerator is essentially a heat engine running backwards. If a refrigerator uses 50 J of electricity while it removes 200 J of heat from inside the refrigerator, how much heat does it dump into the outside room and what is its COP?

Q20.6 A copper pot at room temperature is filled with room-temperature water. Imagine a process whereby the water spontaneously freezes and the pot becomes hot. Why is such a process impossible? A. It violates the first law of thermodynamics. B. It violates the second law of thermodynamics. C. It violates both the first and second laws of thermodynamics. D. none of the above

A20.6 A copper pot at room temperature is filled with room-temperature water. Imagine a process whereby the water spontaneously freezes and the pot becomes hot. Why is such a process impossible? A. It violates the first law of thermodynamics. B. It violates the second law of thermodynamics. C. It violates both the first and second laws of thermodynamics. D. none of the above

Q20.7 A Carnot engine takes heat in from a reservoir at 400 K and discards heat to a reservoir at 300 K. If the engine does 12,000 J of work per cycle, how much heat does it take in per cycle? A. 48,000 J B. 24,000 J C. 16,000 J D. 9000 J E. none of the above

A20.7 A Carnot engine takes heat in from a reservoir at 400 K and discards heat to a reservoir at 300 K. If the engine does 12,000 J of work per cycle, how much heat does it take in per cycle? A. 48,000 J B. 24,000 J C. 16,000 J D. 9000 J E. none of the above

Analysis of a proposed engine At the patent office a application arrives proposing a heat engine with the characteristics shown in the diagram. If the work is 1000 J, what is the heat exhausted and what is the efficiency? Is this a possible engine cycle?