# transfer of thermal energy

Both conduction and convection transfer energy through matter. Radiation is the only way of transferring energy that doesnt require matter. Radiation is the transfer of energy by waves that can travel through empty space. When the waves reach objects, they transfer energy to the objects, causing them to warm up. This is how the suns energy reaches Earth and heats its surface (see Figure 18.10). Radiation is also how thermal energy from a campfire warms people nearby. You might be surprised to learn that all objects radiate thermal energy, including people. In fact, when a room is full of people, it may feel noticeably warmer because of all the thermal energy the people radiate! To learn more about thermal radiation, watch "Radiation" at the URL below.

## convection

Convection is the transfer of thermal energy by particles moving through a fluid. Particles transfer energy by moving from warmer to cooler areas. Thats how energy is transferred in the soup in Figure 18.7. Particles of soup near the bottom of the pot get hot first. They have more energy so they spread out and become less dense. With lower density, these particles rise to the top of the pot (see Figure 18.8). By the time they reach the top of the pot they have cooled off. They have less energy to move apart, so they become denser. With greater density, the particles sink to the bottom of the pot, and the cycle repeats. This loop of moving particles is called a convection current. Convection currents move thermal energy through many fluids, including molten rock inside Earth, water in the oceans, and air in the atmosphere. In the atmosphere, convection currents create wind. You can see one way this happens in Figure 18.9. Land heats up and cools off faster than water because it has lower specific heat. Therefore, land is warmer during the day and cooler at night than water. Air close to the surface gains or loses heat as well. Warm air rises because it is less dense, and when it does, cool air moves in to take its place. This creates a convection current that carries air from the warmer to the cooler area. You can learn more about convection currents by watching "Convection" at this URL: .

## kqed darfur stoves project

Everyday, women living in the refugee camps of Darfur, Sudan must walk for up to seven hours outside the safety of the camps to collect firewood for cooking, putting them at risk for violent attacks. Now, researchers at Lawrence Berkeley National Laboratory have engineered a more efficient wood-burning stove, which is greatly reducing both the womens need for firewood and the threats against them. For more information on these stoves, see http://scien MEDIA Click image to the left or use the URL below. URL:

## thermal insulators

Particles of gases are farther apart and have fewer collisions, so they are not good at transferring thermal energy. Materials that are poor thermal conductors are called thermal insulators. Figure 18.7 shows several examples. Fluffy yellow insulation inside the roof of a home is full of air. The air prevents the transfer of thermal energy into the house on hot days and out of the house on cold days. A puffy down jacket keeps you warm in the winter for the same reason. Its feather filling holds trapped air that prevents energy transfer from your warm body to the cold air outside. Solids like plastic and wood are also good thermal insulators. Thats why pot handles and cooking utensils are often made of these materials.

## thermal conductors

Conduction is usually faster in liquids and certain solids than in gases. Materials that are good conductors of thermal energy are called thermal conductors. Metals are excellent thermal conductors. They have freely moving electrons that can transfer energy quickly and easily. Thats why the metal pot in Figure 18.5 soon gets hot all over, even though it gains thermal energy from the fire only at the bottom of the pot. In Figure 18.6, the metal heating element of the curling iron heats up almost instantly and quickly transfers energy to the strands of hair that it touches.

## conduction

Conduction is the transfer of thermal energy between particles of matter that are touching. When energetic particles collide with nearby particles, they transfer some of their thermal energy. From particle to particle, like dominoes falling, thermal energy moves throughout a substance. In Figure 18.5, conduction occurs between particles of the metal in the pot and between particles of the pot and the water. Figure 18.6 shows additional examples of conduction. For a deeper understanding of this method of heat transfer, watch the animation "Conduction" at this URL: http://w

## instructional diagrams

This diagram shows convection currents. Convection is the transfer of heat from one place to another by the movement of fluids. The heat source lies at the bottom of the diagram. The heat generated by this source causes the air next to it, to warm up. Warm air is lighter than cool air, and hence it rises up. As it rises up, it moves away from the heat source and cools down. As it cools down, it gets heavier and sinks towards the heat source. This cycle continues and causes a convection current.

This diagram shows the phenomena of the transfer of thermal energy. It happens by the convection of hot and cold air. The sun heats up the air, making it warm and less dense. Less dense air tends to go up, cooling down as doing it. Cool air becomes more dense and tends to sink, and wind does the job of making the air travel through different places, warming or cooling as he goes.

## questions

material that allows little if any conduction of thermal energy

``````a. conduction

b. thermal conductor

c. convection

-->  d. thermal insulator

f. convection current

g. density
``````

Conduction is usually slowest in

``````-->  a. gases.

b. solids.

c. liquids.

d. flames.
``````

transfer of thermal energy by waves that can travel through space

``````a. conduction

b. thermal conductor

c. convection

d. thermal insulator

f. convection current

g. density
``````

Which of the following materials is a thermal insulator?

``````-->  a. plastic

b. iron

c. copper

d. steel
``````

flow of particles in a fluid due to differences in temperature and density

``````a. conduction

b. thermal conductor

c. convection

d. thermal insulator

-->  f. convection current

g. density
``````

In which substance can thermal energy be transferred by convection?

``````-->  a. air

b. sand

c. wood

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

Thermal energy is transferred through the ocean by

``````-->  a. currents.

b. waves.

c. winds.

d. tides.
``````

material that is good at transferring thermal energy by conduction

``````a. conduction

-->  b. thermal conductor

c. convection

d. thermal insulator

f. convection current

g. density
``````

amount of mass in a given volume of matter

``````a. conduction

b. thermal conductor

c. convection

d. thermal insulator

f. convection current

-->  g. density
``````

Matter is not needed for the transfer of thermal energy by

``````a. conduction.

b. convection.

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

transfer of thermal energy between particles of matter that are touching

``````-->  a. conduction

b. thermal conductor

c. convection

d. thermal insulator

f. convection current

g. density
``````

transfer of thermal energy by particles moving through a fluid

``````a. conduction

b. thermal conductor

-->  c. convection

d. thermal insulator

f. convection current

g. density
``````

Conduction occurs when particles of matter flow.

``````a. true

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

Insulation can keep a house cool on a hot day.

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

b. false
``````

Thermal energy is always transferred from cooler to warmer objects.

``````a. true

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

Land and sea breezes are examples of convection currents.

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

b. false
``````

Only hot objects radiate thermal energy.

``````a. true

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

A pot resting on a hot stovetop heats up because of

``````a. convection.

-->  b. conduction.

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

Your hand feels cold when you hold an ice cube because

``````a. the ice radiates cold to your hand.

b. the ice conducts cold to your hand.

c. your hand cools down by convection.

-->  d. your hand transfers thermal energy to the ice.
``````

In which of the following materials does conduction occur most quickly?

``````-->  a. iron

b. wood

c. plastic

d. oxygen
``````

Examples of thermal insulators include

``````a. down feathers.

b. Styrofoam.

c. air.

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

The transfer of thermal energy by convection occurs only in

``````a. gases.

b. solids.

-->  c. fluids.

d. liquids.
``````

Thermal energy is transferred throughout the ocean by

``````a. radiation.

b. conduction.

c. thermal conductors.

-->  d. convection currents.
``````

A sea breeze blows

``````-->  a. toward the land.

b. toward the sea.

c. only at night.

d. during both day and night.
``````

Conduction occurs only between particles that collide.

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

b. false
``````

Wood is an example of a good thermal conductor.

``````a. true

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

Home insulation prevents the transfer of cold into the house.

``````a. true

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

Warmer air rises because it is less dense than cooler air.

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

b. false
``````

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

b. false
``````

Convection currents carry thermal energy from the sun to Earth.

``````a. true

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

Fluid particles with more energy have greater density.

``````a. true

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

Metals are excellent thermal conductors because they have freely moving electrons.

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

b. false
``````

A land breeze is an example of a convection current.

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

b. false
``````

Thermal energy is transferred from a space heater to a person in front of it by conduction.

``````a. true

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

## diagram questions

What happens after the sun warms the land, which warms the air?

``````a. This cold dense air sinks

b. Nothing happens

-->  c. Less dense warm air rises

d. The air is cooled off by the land
``````

Where is the air the coolest?

``````a. On the ground

-->  b. Higher in the atmosphere

c. Low in the atmosphere

d. Closest to the sun
``````

What happens after less dense warm air rises?

``````a. wind

b. Air warms

-->  c. Air cools

d. Cold dense air sinks
``````

What happens to cool air when it reaches a hot radiator?

``````-->  a. Warm air rises

b. Cool air falls

c. Warm air falls

d. Cool air rises
``````

Why is the warm air rising in the room?

``````a. It doesn't

b. The cool air is less dense

c. It started there

-->  d. It's less dense than cool air
``````

Wind direction flows:

``````a. From warm air over land to cool air over land

b. Towards the sun

c. From low pressure to high pressure

-->  d. From high pressure to low pressure
``````

How many stages are there in the transfer of thermal energy?

``````a. 5

-->  b. 6

c. 3

d. 4
``````

To where does the wind blow?

``````a. high pressure area

b. into the earth

-->  c. low pressure area

d. towards the sun
``````

In a convection current how does cool air react?

``````-->  a. It sinks

b. It rises

c. It stays the same

d. It both rises and sinks
``````

What does the convection current do to air?

``````a. does not change the air

b. charges the air with elctricity

-->  c. cools air

d. warms air
``````

What happens when warm air rises?

``````a. warm air stays in the atmosphere

b. air keeps warming up

c. warm air doesn't rise

-->  d. air cools and sinks
``````

How many processes are there in the convection current?

``````a. 3

-->  b. 2

c. 1

d. 4
``````

How many stages are shown in this image?

``````a. 6

b. 7

-->  c. 4

d. 5
``````

In a convection current, what does warm air do?

``````a. It moves laterally.

b. It sinks.

c. It stays dormant.

-->  d. It rises.
``````

The red arrows in the diagram refer to:

``````a. Cool Molecules

b. Heat

c. Convection

-->  d. Warm Molecules
``````

Heat is lost when warm molecules:

``````a. Encounter cool molecules

b. Stay in place

-->  c. Rise to the surface of the liquid

d. Fall to the bottom of the kettle
``````

How many types of molecules are involved in the convection process?

``````a. 4

-->  b. 2

c. 3

d. 1
``````

At what point does air cool?

``````-->  a. After hot air rises

b. After hot air sinks

c. After cold air sinks

d. After cold air replaces hot air
``````

When does air start getting hot?

``````a. When it cools.

-->  b. When it rises.

c. When it sinks.

d. When cool air replaces hot air.
``````

How many processes are shown in the diagram?

``````a. 2

-->  b. 4

c. 3

d. 5
``````

How many processes are thereafter hot air rises?

``````-->  a. 3

b. 2

c. 4

d. 5
``````

Which part of the room will have higher temperature?

``````a. Near the floor

b. All parts of the room will have same temperature

c. On the wall near the furnace

-->  d. Near the ceiling and above the furnace
``````

What does hot air do?

``````a. It stays dormant.

b. It sinks.

c. It moves laterally.

-->  d. It rises.
``````

After heat rises in a convection cycle:

``````a. Cool air rises

b. The convection cycle ends

c. Heat falls

-->  d. Cool air falls
``````

Conduction is the transfer of heat energy by

``````-->  a. molecular contact

b. movement through a vacuum

c. density differences

d. electromagnetic waves
``````

Where is the best place to put a convection heater to warm up a room?

``````-->  a. Near the floor

b. Near the ceiling

c. On the ceiling facing down the room

d. It does not matter
``````

When the transfer of thermal energy occurs between particles that are touching each other, it exhibits...

``````a. insulation

-->  c. conduction

d. convection
``````

What type of heat transfer happens in the water?

``````a. Radiation

c. Conduction

-->  d. Convection
``````

Which form of thermal energy transfer does NOT require physical contact?

``````-->  a. Radiation

b. Conduction

c. Release

d. Convection
``````

How many forms of the transfer of energy are shown in the diagram?

``````-->  a. 3

b. 4

c. 1

d. 2
``````

How many stages are there in the process of boiling water?

``````-->  a. 3

b. 6

c. 4

d. 5
``````

What is the transfer of thermal energy between particles of matter that are touching?

``````a. Steaming

c. Convection

-->  d. Conduction
``````

Which process emits heat, causing the pot and water to warm?

``````a. convection

b. precipitation