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# Heat Engine Example (22.5):

## Heat Engine Example (22.5):

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## Presentation on theme: "Heat Engine Example (22.5):"— Presentation transcript:

Slide1

Slide2

Heat Engine Example (22.5):

A particular heat engine has a mechanical power output of 5.00 kW and an efficiency of 25.0%. The engine expels 8.00 x 10

3

J of exhaust energy in each cycle. Find:

The energy taken in during each cycle.

The time interval of each cycle.

Slide3

In one complete cycle, a heat engine extracts heat

Q

h

from a thermal reservoir, does work W and does not eject any heat into the environment. This heat engine is impossible because its operation violates

Conservation

of energy (1st Law) only

2nd

Law only

Both

1st Law and 2nd Law.

Slide4

The energy input to an engine is 3.00 times greater than the work it performs. What is the thermal efficiency?

3.00

1.00

0.33

Impossible to determine

Slide5

The energy input to an engine is 3.00 times greater than the work it performs. What fraction of the energy input is expelled to the cold reservoir?

0.333

0.667

1.00

Impossible to determine

Slide6

To increase the efficiency of a heat engine, it would be better

to increase the temperature of the hot reservoir while keeping the temperature of the cold reservoir constant.

to decrease the temperature of the cold reservoir while keeping the temperature of the hot reservoir constant.

to decrease the temperature of the hot reservoir while keeping the temperature of the cold reservoir constant.

to increase the temperature of the cold reservoir while keeping the temperature of the hot reservoir constant.

none of these.

Slide7

Slide8

A device containing an ideal gas executes the cycle shown. This is a heat enginerefrigerator Neither because the net work done by/on the device is zero

V

P

Start/finish

Slide9

The outdoor portion of a central air-conditioning unit has a fan that blows air across the condenser coils. If this fan breaks, why won’t the air conditioner cool the house properly?

Slide10

If you left the door to your refrigerator open, after a few hours your kitchen would feel

Warmer.

Cooler.

Cozy.

Roomier.

It depends on the efficiency of your refrigerator.

Slide11

By what factor does the cost of heating your home change when you replace your electric heater with an electric heat pump that has a COP of 4.00? Assume the heat pump motor is 100% efficient.

4.00

2.00

0.500

0.250

Slide12

It is impossible to transfer a given quantity of heat from a cold reservoir to a hot reservoir.

true.

false.

Slide13

An air conditioner is brought into the center of a dorm room and turned on. After several minutes, the temperature of the

room

Increases

Decreases

remains

constant

Slide14

Slide15

Carnot Engine Pre-Question:

Draw a P-V diagram for the following cyclic process:

Isothermal expansion

Isothermal compression

Slide16

Slide17

Carnot Example (22.3):

A 35%-efficient Carnot engine is run in reverse (i.e. as a fridge). What is the COP?

Slide18

It is not difficult to imagine a process that produces more work than the Carnot cycle. For example:How then, is the Carnot engine “ideal”?

P

V

W

Slide19

What must be true for a Carnot engine to have an efficiency of 1?

Slide20

You wish to maximize the COP of a Carnot heat pump. You should:

Maximize the temperature of the hot reservoir.

Maximize the temperature of the cold reservoir.

Minimize

the temperature of the hot reservoir.

Minimize

the temperature of the cold reservoir

.

Slide21

Problem 1 (22.12):

A heat pump has a COP of 4.20 and requires a power of 1.75 kW to operate.

How much energy does the heat pump add to your home in one hour?

W

hat is the pump

is

doing? Where does the energy come from?

If the heat pump were reversed to act as an air conditioner, what would the COP be?

Problem 2

(22.27)

:

Argon enters a turbine at a rate of 80 kg/min, a temperature of 800 C, and a pressure of 1.5MPa. It expands adiabatically as it pushes on the turbine blades and exits at pressure 300

kPa

.

Calculate its temperature at the exit.

Calculate the (maximum) power output of the turning turbine.

The turbine is one component of a model closed-cycle gas turbine engine. Calculate the maximum efficiency of this engine.

Slide22