Heat Engines and Thermal Processes

Heat Engines and Thermal Processes

Warm-up: Research- Find an example of a heat engine and describe how it works.

Thermal Reservoir and efficiency

• Object that supplies or receives thermal energy with essentially not change in temperature.( like water)

• Virtually no energy loss

• Energy in = Energy out

• Q_h= W + Q_c

• W=Q_h- Q_c

•  $e=\frac{W}{Q_h}$  e is efficiency

• or $e=\ \frac{Q_h-Q_c}{Q_h}\$  

A heat engine with an efficiency of 24% performs 1250 J of work. Find (a) the heat absorbed from the hot reservoir and (b) the heat given off to the cold reservoir.

Open Ended

What is the efficiency of an engine that exhausts 870 J of heat in the process of doing 240 J of work?

Engine A has an efficiency of 66%. Engine B absorbs the same amount of heat from the hot reservoir as engine A, but exhausts twice as much heat to the cold reservoir. Is the efficiency of engine B greater than , less than, or the same as the efficiency of engine A?

Constant Volume process

• do no work

• if W=0 then  $\Delta E=Q$  

• Change in thermal energy is equal to heat

Constant Pressure Processes

• Work is done

• Force exerted by a gas=P(pressure) A(area)

•  $W=P\Delta V$  

A gas with a constant pressure of 150 kPa expands from a volume of .76 cm3 to a volume of .92 cm3. How much work does the gas do?

Open Ended

A gas with a constant pressure of 270 kPa does 36000 J of work as it expands. What was the change in volume of the gas?

Research: How does a weather glass work?

Isothermal Process

• Change at constant temperature

• Constant temp means no change in energy  $\Delta E=0$  

•   $\Delta E=Q-W$  therefore Q=W

Adiabatic Process

• No change in thermal energy

• Q=0 therefore

•  $\Delta E=-W$  

If a system's thermal energy decreases by 470 J in an adiabatic process, how much work was done by the system?