# using thermal energy

## cooling systems

Its easy to see how thermal energy can be used to keep things warm. But did you know that thermal energy can also be used to keep things cool? Cooling systems such as air conditioners and refrigerators transfer thermal energy in order to keep homes and cars cool or to keep food cold. In a refrigerator, for example, thermal energy is transferred from the cool air inside the refrigerator to the warmer air in the kitchen. You read in this chapters "Transfer of Thermal Energy" lesson that thermal energy always moves from a warmer area to a cooler area, so how can it move from the cooler refrigerator to the warmer room? The answer is work. The refrigerator does work to transfer thermal energy in this way. Doing this work takes energy, which is usually provided by electricity. Figure 18.14 explains how a refrigerator does its work. For an animated demonstration of how a refrigerator works, go to this URL: The key to how a refrigerator or other cooling system works is the refrigerant. A refrigerant is a substance, such as FreonTM, that has a low boiling point and changes between liquid and gaseous states as it passes through the cooling system. As a liquid, the refrigerant absorbs thermal energy from the cool air inside the refrigerator and changes to a gas. As a gas, it releases thermal energy to the warm air outside the refrigerator and changes back to a liquid.

## warmair heating system

A warm-air heating system uses thermal energy to heat air. It then forces the warm air through a system of ducts. You can see a diagram of this type of heating system in Figure 18.13. Typically, the air is heated in a furnace that burns natural gas or heating oil. When the air is warm, a fan blows it through the ducts and out through vents that are located in each room. Warm air blowing out of a vent moves across the room, pushing cold air out of the way. The cold air enters an intake vent on the opposite side of the room and returns to the furnace with the help of another fan. In the furnace, the cold air is heated, and the cycle repeats.

## hotwater heating system

A hot-water heating system uses thermal energy to heat water and then pumps the hot water throughout the building in a system of pipes and radiators. You can see a diagram of this type of heating system in Figure 18.12. Typically, the water is heated in a boiler that burns natural gas or heating oil. There is usually a radiator in each room that gets warm when the hot water flows through it. The radiator transfers thermal energy to the air around it by conduction and radiation. The warm air then circulates throughout the room in convection currents. The hot water cools as it flows through the system and transfers its thermal energy. When it finally returns to the boiler, it is heated again and the cycle repeats.

## heating systems

Warming homes and other buildings is an obvious way that thermal energy can be used. Two common types of home heating systems are hot-water and warm-air heating systems. Both types are described below. You can watch an animation showing how a solar heating system works at this URL:

## external combustion engine

An external combustion engine burns fuel externally, or outside the engine. The burning fuel releases thermal energy that is used to turn water to steam. The pressure of the steam is then used to move a piston back and forth in a cylinder. The kinetic energy of the moving piston can be used to turn a turbine or other device. Figure 18.15 explains in greater detail how this type of engine works. You can see an animated version of an external combustion engine at this URL: http://science.howstuffworks.com/transport/engines-equipment/steam1.htm .

## combustion engines

A combustion engine is a complex machine that burns fuel to produce thermal energy and then uses the energy to do work. Two basic types of combustion engines are external and internal combustion engines.

## internal combustion engine

An internal combustion engine (see Figure 18.16) burns fuel internally, or inside the engine. This type of engine is found in most cars and other motor vehicles. It works in these steps, which keep repeating: 1. A mixture of fuel and air is pulled into a cylinder through a valve, which then closes. 2. The piston is pushed upward, compressing the fuel-air mixture in the closed cylinder. The mixture is now under a lot of pressure and very warm. 3. A spark from a spark plug is used to ignite the fuel-air mixture, causing it to burn explosively within the confined space of the closed cylinder. 4. The pressure of the hot gases from combustion forces the piston downward. 5. When the piston moves up again, it forces the exhaust gases of combustion out of the cylinder though another valve. Then the process repeats. In a car, the piston is connected by the piston rod to the crankshaft. The crankshaft rotates when the piston moves up and down. The kinetic energy of the moving crankshaft is used to turn the driveshaft, which causes the wheels of the car to turn. Most cars have at least four cylinders connected to the crankshaft. Their pistons move up and down in sequence, one after the other. You can watch animations of internal combustion engines in action at these URLs: http://auto.howstuffworks.com/engine1.htm

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

The function of a thermostat is to transfer thermal energy.

``````a. true

-->  b. false
``````

What happens to hot water as it moves through a homes hot-water heating system?

``````a. It transfers thermal energy to the rooms of the house.

b. It cools down and returns to the boiler.

c. It turns to steam, which runs the fan.

-->  d. two of the above
``````

The water in a hot-water heating system is heated by a furnace.

``````a. true

-->  b. false
``````

In a warm-air heating system, cold air in each room

``````-->  a. enters an intake vent near the ceiling.

b. blows out of a vent and across the room.

c. leaves the house through the chimney.

d. transfers thermal energy to the furnace.
``````

In a warm-air heating system, pipes carry thermal energy throughout the house.

``````a. true

-->  b. false
``````

What happens when the refrigerant of a cooling system absorbs thermal energy?

``````a. It melts.

b. It thaws.

c. It condenses.

-->  d. It evaporates.
``````

How are internal and external combustion engines similar?

``````a. Both burn fuel in a cylinder.

b. Both produce thermal energy.

c. Both have a piston that moves in a cylinder.

-->  d. two of the above
``````

Thermal energy from inside a refrigerator changes the refrigerant to a gas.

``````-->  a. true

b. false
``````

A combustion engine burns fuel to produce thermal energy.

``````-->  a. true

b. false
``````

In a functioning combustion engine, the piston has

``````-->  a. kinetic energy.

b. electrical energy.

c. chemical energy.

d. thermal energy.
``````

In any combustion engine, the engine does the work of moving a piston.

``````-->  a. true

b. false
``````

In a warm-air heating system, warm-air vents are always placed near the ceiling.

``````a. true

-->  b. false
``````

An air conditioner is an example of a cooling system.

``````-->  a. true

b. false
``````

Refrigerant changes to a liquid in the condenser of a refrigerator.

``````-->  a. true

b. false
``````

Steam ships have internal combustion engines.

``````a. true

-->  b. false
``````

The purpose of a radiator in a heating system is to produce thermal energy.

``````a. true

-->  b. false
``````

substance that absorbs and releases thermal energy in a cooling system

``````a. internal combustion engine

b. cooling system

-->  c. refrigerant

d. warm-air heating system

e. external combustion engine

f. hot-water heating system

g. thermostat
``````

Warm air moves through the ducts of heating system because of gravity.

``````a. true

-->  b. false
``````

device in a heating system that controls the furnace or boiler

``````a. internal combustion engine

b. cooling system

c. refrigerant

d. warm-air heating system

e. external combustion engine

f. hot-water heating system

-->  g. thermostat
``````

complex machine that produces thermal energy outside the machine and uses the thermal energy to do

``````a. internal combustion engine

b. cooling system

c. refrigerant

d. warm-air heating system

-->  e. external combustion engine

f. hot-water heating system

g. thermostat
``````

The transfer of thermal energy can be used to keep things cool.

``````-->  a. true

b. false
``````

heating system that includes a boiler, pipes, and radiators

``````a. internal combustion engine

b. cooling system

c. refrigerant

d. warm-air heating system

e. external combustion engine

-->  f. hot-water heating system

g. thermostat
``````

Thermal energy naturally moves from a warmer area to a cooler area.

``````-->  a. true

b. false
``````

The piston of a combustion engine moves because the crankshaft turns.

``````a. true

-->  b. false
``````

complex machine that produces thermal energy inside the machine and uses the thermal energy to do work

``````-->  a. internal combustion engine

b. cooling system

c. refrigerant

d. warm-air heating system

e. external combustion engine

f. hot-water heating system

g. thermostat
``````

refrigerator or air conditioner

``````a. internal combustion engine

-->  b. cooling system

c. refrigerant

d. warm-air heating system

e. external combustion engine

f. hot-water heating system

g. thermostat
``````

heating system that includes a furnace, ducts, and vents

``````a. internal combustion engine

b. cooling system

c. refrigerant

-->  d. warm-air heating system

e. external combustion engine

f. hot-water heating system

g. thermostat
``````

Types of home heating systems include

``````a. warm-air heating systems.

b. hot-water heating systems.

c. solar heating systems.

-->  d. all of the above
``````

How is thermal energy transferred in a refrigerator?

``````a. from the warm kitchen to the cool refrigerator

-->  b. from the cool refrigerator to the warm kitchen

c. from the cool refrigerator to the cold outdoors

d. two of the above
``````

Why must a cooling system do work to keep things cool?

``````a. It transfers thermal energy from a cooler to a warmer place.

b. It takes energy to reverse the normal direction of heat flow.

c. It takes energy to maintain the normal direction of heat flow.

-->  d. two of the above
``````

What happens to the refrigerant as it passes through a cooling system?

``````a. It freezes and lowers the temperature of the system.

-->  b. It changes between liquid and gaseous states.

c. It releases thermal energy into the refrigerator.

d. It keeps evaporating and has to be replace
``````

In an external combustion engine, thermal energy is used directly to

``````a. move the piston back and forth.

b. move the piston up and down.

-->  c. turn water into steam.

d. all of the above
``````

What happens first in an internal combustion engine?

``````a. Exhaust gases exit the cylinder.

b. The piston moves up or down.

-->  c. A fuel-air mixture enters the cylinder.

d. The piston rod turns the crankshaft.
``````

Thermal energy from a radiator travels throughout the air in a room by

``````a. conduction.

b. convection.