April 19, 2023

Unit 8: Transfer of Thermal Energy

Discover PHYSICS

April 19, 2023

8.1 Transfer of Thermal Energy

Learning OutcomesIn this section, you’ll be able to:• Understand that thermal energy is transferred from a

region of higher temperature to a region of lower temperature

April 19, 2023

8.1 Transfer of Thermal Energy

What causes transfer of thermal energy?• Thermal energy is transferred only when there is a

difference in temperature.• Thermal energy always flows from a region of higher

temperature to a region of lower temperature.• There is no transfer of heat at thermal equilibrium.

April 19, 2023

8.1 Transfer of Thermal Energy

How is thermal energy transferred?

• Thermal energy is transferred by:• Conduction• Convection• Radiation

April 19, 2023

8.1 Transfer of Thermal EnergyKey Ideas1. Transfer of thermal energy takes place when there is a

temperature difference. Thermal energy is always transferred from a hotter region to a colder region.

2. When thermal equilibrium is reached between two bodies (i.e. both bodies are at the same temperature), there is no net flow of thermal energy between them.

3. There are three different processes of thermal energy transfer: conduction, convection and radiation.

April 19, 2023

8.1 Transfer of Thermal EnergyTest Yourself1. During winter, it is common for people to say ‘keep the

cold out of the house’. Is this statement correct? Comment.

Answer:The statement ‘keep the cold out of the house’ seems to

suggest that ‘the cold tends to move into the house’ which is not true.

In fact, it is the transfer of heat energy from the inside of the house to the outside that causes the temperature in the house to drop.

April 19, 2023

8.2 Conduction

Learning Outcomes

In this section, you’ll be able to:• Describe how energy transfer occurs in solid.

April 19, 2023

8.2 Conduction

What is conduction?

Definition:

Conduction is the process of thermal energy transfer without any flow of the material medium.

April 19, 2023

8.2 Conduction

Objective:To investigate the transfer of thermal energy through solids

Apparatus:bath, rods of the same dimensions but of different materials, stopwatch

Procedure:1. Coat the parts of the rods that are on the outside of the tank evenly with melted wax (see figure).2. Pour boiling water into the bath, so that the ends of the rods are submerged.3. Record the length of wax that melts in a given interval of time for each of the four rods.

Experiment 8.1

April 19, 2023

Observation:The wax melts the furthest along the copper rod, followed by iron, glass and wood.

April 19, 2023

8.2 Conduction

Experiment 8.1Two important conclusions can be drawn:

1. Thermal energy flows through the material of the rods without any flow of the material itself. This process is called conduction.

2. Different materials conduct heat at different rates. Those that conduct faster are called good conductors (e.g. copper) and those slower are called poor conductors (e.g. wood).

Note:Poor conductors are also known as insulators.

April 19, 2023

8.2 Conduction

How does conduction work?

• Conductors and insulators have different mechanisms to transfer of thermal energy.

• All solids are made up of tiny particles called atoms or molecules.

• Metals contain free electrons which move randomly between the atoms and molecules.

• Non-metals do not have free electrons.

April 19, 2023

8.2 Conduction

How does conduction work?

• When thermal energy is supplied to one end of a rod, the particles (atoms and molecules) at the hot end vibrate vigorously.

• These particles collide with neighbouring particles, making them vibrate as well.

• Kinetic energy of vibrating particles at the hot end is transferred to neighbouring particles.

April 19, 2023

8.2 Conduction

Good Conductor• In metals, another much

faster mechanism of thermal energy transfer takes place at the same time -free electron diffusion.

• The free electrons gain kinetic energy and move faster.

• The fast-moving electrons then diffuse into cooler parts of the metal.

How does conduction work?

April 19, 2023

8.2 Conduction

Insulators• In insulators, the transfer

of thermal energy is solely the results of vibrating atoms and molecules.

• There is no free electrons.

How does conduction work?

April 19, 2023

8.2 Conduction

Conduction in liquids and gases

• Thermal energy can be conducted from a hotter to a cooler region.

• Process of conduction is inefficient.• Liquid particles are further apart and collisions of

particles are less frequent and even lesser in gases.• Thus, transfer of kinetic energy from fast-moving

molecules to neighbouring molecules is slower.• Hence air is poor conductor of heat compared to water,

which is in turn is a poor conductor compared to most solids.

April 19, 2023

Objective:To test conduction of thermal energy in water

Apparatus:test-tube, ice, metal gauze, Bunsen burner,water

Procedure:1. Wrap a piece of ice with metal gauze and place it at the bottom of a test-tube.2. Fill the test-tube with tap water till it is almost full.

8.2 Conduction

Experiment 8.1

April 19, 2023

3. Heat the test-tube at the upper end, as shown in the figure.4. Observe the water being heated and the ice below it.

8.2 Conduction

April 19, 2023

8.2 Conduction

Key Ideas1. Conduction is the transfer of thermal energy without

any flow of the material medium.2. The two mechanisms for conduction are atomic or

molecular vibrations (for both metals and non-metals) and free electron diffusion (for metals only).

3. Liquids and gases are poor conductors of heat compared to solids.

April 19, 2023

Test Yourself1. Give an account of thermal energy conduction in

metals and non-metals.

Answer:In metals, conduction of heat is due mainly to the diffusion

of free electrons from a hotter region to a colder region. Conduction of heat can also take place with molecular vibrations.

In non-metals, conduction of heat only takes place due to molecular vibrations, where the K.E. of the vibrating molecules at the hot end is transferred to the neighbouring molecules.

8.2 Conduction

April 19, 2023

8.2 Conduction

Test Yourself2. Why are good conductors of thermal energy also good

conductor of electricity?

Answer:Good conductors such as metals have free electrons. It is

the presence of free electrons that enable metals to conduct both thermal energy as well as electricity.

Conduction of electric current is the flow of electric charges such as electrons.

April 19, 2023

8.2 Conduction

Test Yourself3. Is the heat transferred from a barbecue fire to a

person standing in front of it a good example of heat transfer by conduction? Explain.

Answer:A person standing in front of a barbecue fire and feeling

hot is not a good example of conduction since air is a poor conductor of heat.

In fact, we will learn later that we feel the hotness of the barbecue fire due to radiation of the heat energy.

April 19, 2023

8.3 Convection

Learning Outcomes

In this section, you’ll be able to:• Describe how energy transfer occurs in fluids.

April 19, 2023

8.3 Convection

What is convection?

Definition:

Convection is the transfer of thermal energy by means of currents in a fluid (liquids or gases).

April 19, 2023

8.3 Convection

Experiment 8.3ObjectiveTo show convection in water

ApparatusLarge, round-bottomed flask,

potassium permanganate crystals, Bunsen burner

Procedure1. Fill the flask with water. Carefully

place some potassium permanganate crystals at the bottom of the flask.

2. Place a Bunsen burner with a small flame under the flask and observe the crystals.

April 19, 2023

Unit 8.3: ConvectionExperiment 8.4

ObjectiveTo show convection in air

Apparatuslarge box with two chimneys on top, a

piece of clear glass on one side, candle, matches

Procedure1. Place the candle below one of the

chimneys. Light the candle.2. Introduce smoke into the other

chimney by placing a piece of smouldering paper over it and observe the movement of the smoke.

April 19, 2023

8.3: ConvectionHow does convection work?

• When fluids (liquids and gases) are heated, they expand and become less dense.

• The less dense fluids tend to rise from the heating source.

• Cooler fluids, being more dense, sink to replace the less dense fluids.

• This movement of fluid due to a difference in its density sets up a convection current.

April 19, 2023

8.3: Convection

How does convection work?

• Convection currents occur only in fluids such as liquids and gases but not in solids.

• Convection involves the bulk movement of the fluids which carry with them thermal energy.

April 19, 2023

8.3: ConvectionKey Ideas

1. Convection is the transfer of thermal energy by means of currents in a fluid (liquid or gas).

2. A convection current is the movement of fluid caused by the change in density in various parts of the fluid.

April 19, 2023

8.3: Convection

Test Yourself1. Why does it feel hot when you put your hands above a

small burning candle?Answer:

The hand feels hot because of convection. The air around the flame is being heated and becomes less dense and rises.

April 19, 2023

8.3: Convection

Test Yourself2. Describe briefly the mechanism for the transfer of

thermal energy in fluids.Answer:• When fluids are heated, they expand and become less

dense. The less dense fluid rises.• The cooler, denser fluids will replace the less dense

fluids.• This sets up a convection current.

April 19, 2023

8.4: Radiation

Learning OutcomesIn this section, you’ll be able to:• Explain energy transfer of a body by radiation.• State the factors affecting the rate of energy transfer

by radiation.

April 19, 2023

8.4: RadiationWhat is radiation?

Definition:

Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.

April 19, 2023

8.4: RadiationWhat is radiation?

• Radiation does not require a medium for energy transfer.

• It can take place in vacuum.

For example, the Sun is a major source of radiant heat.

April 19, 2023

8.4: RadiationWhat is radiation?

• The sun emits electromagnetic waves.• Part of this electromagnetic waves, called infrared

waves, make us feel warm.• Thermal energy from infrared waves is called radiant

heat.• All objects emit some radiant heat.• The hotter the object, the greater the radiant heat

emitted.

April 19, 2023

8.4: RadiationAbsorption of infrared radiation

• Infrared radiation is absorbed by all objects and surfaces.

• The absorption of radiant heat causes a temperature rise.

April 19, 2023

8.4: RadiationEmission of infrared radiation

• Infrared radiation is emitted by all objects and surfaces.• This emission causes the temperature of the objects

themselves to fall.• In general, good emitter of radiant heat is also a good

absorber of radiant heat.• Conversely, poor emitter of radiant heat is also a poor

absorber of radiant heat.

April 19, 2023

Unit 8.4: RadiationExperiment 8.6ObjectiveTo investigate the emission of infrared radiation

ApparatusTwo temperature sensors, data logger, two identical tins (one

black and one shiny), boiling water from two electric kettles

Procedure1. Connect the temperature sensors A and B to the data logger2. Set the sampling rate to ten seconds3. Pour boiling water into both tins at the same time until both

are filled to the brim.

April 19, 2023

4. Place the lid and the temperature sensors onto the tins. Temperature sensor A will monitor the temperature of the black tin, while temperature sensor B records the temperature of the shiny tin.

5. Start recording the temperature. Observe the temperature time graph of both sensors.

6. Stop recording after ten minutes.

Experiment 8.6

April 19, 2023

8.4: RadiationFactors affecting rate of infrared radiation

1. Colour and texture of the surface• Dull, black surfaces are good absorbers of infrared

radiation than shiny, white surfaces

• Dull, black surfaces are better emitters of infrared radiation.

April 19, 2023

Unit 8.4: Radiation

Factors affecting rate of infrared radiation

2. Surface temperature• Rate of infrared radiation also

depends on surface temperature

• The higher the temperature of the surface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.

April 19, 2023

Unit 8.4: RadiationFactors affecting rate of infrared

radiation

3. Surface area• The larger surface area will emit

infrared radiation at a higher rate.

April 19, 2023

Unit 8.4: RadiationKey Ideas

1. Radiation is the continual emission of thermal energy in the form of infrared waves.

2. Radiation is emitted from the surface of all bodies and does not require a medium of thermal transfer.

3. Dull, black surfaces are better emitters of infrared radiation than shiny, white surface.

4. The factors affecting rate of energy transfer by radiation are: colour, and texture of the surface, surface temperature and surface area.

April 19, 2023

Unit 8.4: RadiationTest Yourself1. Give two everyday examples of thermal energy

transfer by radiation.

Answer:a. Feeling the hotness of Sun’s radiation – Sun’s

thermal energy reaches earth by radiation.b. Standing next the a BBQ fire will make you feel

hot. The thermal energy reaches you by radiation.c. Placing your hand next to a hot object such a jar

of hot water. Thermal energy reaches your hand by radiation.

April 19, 2023

Unit 8.4: RadiationTest Yourself2. State briefly how thermal energy is transferred by

radiation.

Answer:Hot objects emit thermal energy in the form of

infrared radiation, which is a type of electromagnetic waves.

The hotter the object, the higher the rate of radiation.

April 19, 2023

Unit 8.4: Radiation

Test Yourself

3. State three factors that affect the rate of transfer of thermal energy by radiation.

Answer:• Colour and texture of the surface.• Surface temperature.• Surface area.

April 19, 2023

Unit 8.5: Applications of Thermal Energy Transfer

Learning Outcomes

In this section, you’ll be able to:• Understand and identify how thermal energy is

transferred by conduction, convection and radiation in everyday life.

April 19, 2023

Unit 8.5: Applications of Thermal Energy Transfer

Common applications of conductionUses of good conductors of heat

1. Cooking utensils – made of metals eg. Stainless steel or aluminium

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of conductionUses of good conductors of heat

2. Soldering iron rods – the tip is made of copper

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of conductionUses of good conductors of heat

3. Heat exchanges.

A heat exchanger transfers thermal energy from hot dirty water to cold clean water. Copper tubes are used to aid rapid transfer of thermal energy from the hot dirty water to the cold clean water.

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of conductionUses of bad conductors of heat (insulators)

1. Handles of appliances and utensils – made of plastics or wood

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of conductionUses of bad conductors of heat (insulators)

2. Table mats – made of cork

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of conductionUses of bad conductors of heat (insulators)

3. Sawdust.• Used to cover ice blocks because of its

insulating property4. Wooden ladles

• Useful for stirring or scooping hot soup.5. Woolen clothes

• Used to keep body warm

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of conductionUses of bad conductors of heat (insulators)

6. Fiberglass, felt and expanded polystyrene foam.

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of convection1. Heating water in Electric kettles – the heating coil

is placed at the bottom to aid the heating of water by convection.

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of convection2. Household hot water system – the heater is located

at the bottom of the system.

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of convection3. Air conditioners.

• Air conditioners are installed near to the ceiling of rooms to facilitate setting up convection currents as cooler air sinks

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of convection4. Refrigerators

• Freezing unit is placed at top to cool the air and facilitate the setting up of convection currents.

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of radiation1. Teapots

• Shiny teapots can keep tea warm for a longer time than black teapots.

• It can also keep cold liquids cool for a longer time than black containers.

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of radiation2. Greenhouses – infrared radiation emitted by the

contents in the greenhouse is trapped in the greenhouse.

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferCommon applications of radiation3. Vacuum flasks• Stopper is made of poor conductor.

• The vacuum between the double-glass wall minimises conduction and convection.

• The glass walls are silvered and highly reflective to minimise heat loss due to radiation.

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferWorked Example 8.2The figure shows a typical vacuum flask designed to keep liquids hot. Part of the vacuum flask is enlarged. State and explain the function of each of the parts labelled A to C

SolutionA: Thin silvering wall to minimise thermal energy loss by radiationSince shiny surfaces are poor absorbers of radiant heat.B: Vacuum to prevent thermal energy loss by conduction and convention (both require material medium for energy transfer)C: Hollow plastic stopper. Plastic is a poor conductor of heat, minimising thermal energy loss by conduction.

April 19, 2023

Unit 8.5: Applications of Thermal Energy TransferKey Ideas1. Some everyday applications of thermal energy

transfer involving conduction include cooking utensils and table mats.

2. Some everyday applications of thermal energy transfer involving convection include household hot water systems and electric kettles.

3. Some everyday applications of thermal energy transfer involving radiation include vacuum flasks and greenhouses.

April 19, 2023

Unit 8.5: Applications of Thermal Energy Transfer

Test Yourself

1. A saucepan with a thick copper base contains water and is placed on a flat electric hot plate.a. State the process by which energy is

i. transferred from the hot plate to the water,ii. spread through the water.

b. The sides of a saucepan are often polished. How does this reduce energy loss?

Answer:a(i) Conduction.(ii) Convection.b. The sides are polished to reduce heat loss due to

radiation. Polished and shiny surfaces are poor emitters of radiation

April 19, 2023

Unit 8: Transfer of Thermal Energy