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St. Anthony's Canossian Secondary School Sec 3NA Science (Physics)

Chapter 7 Transfer of Thermal Energy

Name: ___________________( ) Class: Sec _______ Date: _________

Candidates should be able to:

(a) show understanding that thermal energy is transferred from a region of higher

temperature to a region of lower temperature

(b) describe, in molecular terms, how energy transfer occurs in solids

(c) describe, in terms of density changes, convection in fluids

(d) explain that energy transfer of a body by radiation does not require a material

medium and the rate of energy transfer is affected by:

(i) colour and texture of the surface

(ii) surface temperature

(iii) surface area

(e) apply the concept of thermal energy transfer to everyday applications

CONDUCTION

1. What is heat?

Heat is a form of __________ which flows from a region of ____________

temperature to a region of ____________ temperature.

2. How is heat transferred from a region of higher temperature to lower

temperature?

Heat can be transferred in THREE ways:

a. Conduction b. Convection c. Radiation

- Takes place mainly in

_________

- Slowly in ___________

- Takes place mainly in

________ and _______

Takes place mainly in

________________ or

__________

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3. How do we explain conduction in molecular terms?

When one end of the rod is heated,

! The solid particles gain _____________ and _______________ faster.

! As they vibrate more vigorously, they _________________ with their less

energetic neighbouring particles and transfer some of their energy to

them. The process continues until the end of

the rod is heated.

! In this way, heat is conducted from the

_________________ end to

the________________ end.

4. What is the definition of conduction?

Conduction is the transfer of heat in which energy from the particles in the

warmer region is passed on to the particles in the cooler region without

_________________ of the medium (solid).

Think! Can conduction take place in a vacuum (empty space)? Why or why not? _________________________________

_________________________________

_________________________________

____________________

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5. Do different materials conduct heat differently?

Materials which allow heat to flow through it quickly are called

_________________. Materials which do not allow heat to flow through quickly are

__________________________________.

Good conductors " Poor conductors " Worst conductor

Solid Liquid, Gas Vacuum

Metal Non metals

6. Why are solids better heat conductors than gases?

The particles in solids are _________________ than those in

gases. For the same volume, there are _________________

particles in solids to collide against each other.

The _________________ of the particles is therefore passed on

to neighbouring particles faster in solids than in gases. Hence

solids are better heat conductors than gases.

7. Why are metals good conductors of heat?

Metals have vibrating atoms and _________________. When

these free electrons gain __________, they _______ around

faster and ____________ more with neighbouring electrons

and atoms.They help to pass on the energy more quickly

than vibrating atoms.

Applications of heat conduction

8. What are some applications in which heat conduction is increased?

Pots, pans and base of electric irons are made of

_________ to transfer heat quickly by conduction.

However, their ladles are made of heat

______________ such as plastic or wood.

9. What are some applications in which heat conduction is reduced?

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Good ______________ reduce heat transfer by conduction. For example,

Styrofoam cups, cork mat.

10. Why do people wear wollen

clothings to keep themselves

warm?

Wool traps air and __________ is a good insulator of heat. Hence it reduces the

transfer of heat from our body to the surroundings by conduction.

Fur and feathers of animals trap air and __________________________.

Check point 1

1. Metals are good conductors of heat whereas air is a poor conductor of heat

What are the two ways in which heat is conducted in metals?

2. Explain why a saucepan has a body usually made of copper.

3. Explain why a saucepan has a handle usually made of plastic?

4. The diagram shows the inside of a refrigerator. Explain why a layer of insulation is needed.

ICE BOX insulation

outer

case

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CONVECTION

11. Why are heating coils placed at the

bottom of the electric heater?

When water at the bottom of a pot gains

_______________, it ____________ and

becomes ______________.

# The hot water _________ to the top.

# The cooler water at the top is denser and hence __________ to the bottom.

# The cooler water is heated by the heat source when it reaches the bottom.

# It becomes _________ and rises while cooler water continues to sink to the

bottom and is heated.

# This circulation of water is called ____________________. It enables heat to

be transferred rapidly from the bottom to the top of a fluid.

12. Definition of convection

Convection is the transfer of heat by the _____________ of currents from one

region to another in a ___________ or a _____________.

Water is a poor ___________ of heat. It will take a long time for the water to be heated if we only depend on conduction. Hence we heat water from the __________ so that heat can be transferred quickly

through CONVECTION.

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13. Where can we observe convection currents?

Air conditioners positioned near the ceiling of a room

# Cold air, which is ___________, sinks

# Warm air which is ___________, rises

# Cycle repeats until room air temperature is the

same as the pre-set temperature.

Refrigerators

# The freezers of refrigerators are usually located at the

________ of the refrigerator.

# The cold air from the freezer ____________ to the

bottom of the refrigerator to cool things in it while

warmer air from the bottom ___________ to the

freezer where it is cooled.

Natural ventilation at home

Think! What happens if the air-con is placed near the floor? ____________________________________________________________________________________________________________

# Windows are installed closer to the

door to allow cooler and fresh air

to flow into the room.

# Ventilation outlets are installed

__________ up to allow warm,

stale air to leave the room.

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Checkpoint 2

1. The diagram shows some rice being boiled in a pan.

(a) Name the process by which heat passes through the bottom of the pan. (b) Why is the handle made of plastic?

(c) Explain how the water at the top of the pan gets hot.

2. The diagram shows the position of an air-conditioner in a room.

(a) Draw arrows to show the convection current set up in the air in the room. (b) Explain how the air-conditioner cools the room.

3. In an air-conditioned room, the metal door knob feels colder than the wooden

door. Yet both of them are at the same temperature. Why is this so?

plastic

handle

boiling water

rice aluminium

pan

heat

Air-conditioner

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RADIATION

14. Comparing conduction, convection and radiation!

Conduction and convection require a ____________ to transfer thermal energy.

Radiation is the third mechanism of thermal

energy transfer that does not require a medium,

it can take place in a ____________.

15. Definition of radiation?

Radiation is the transfer of _________________ by electromagnetic waves.

These waves are part of the electromagnetic spectrum – radiation are known as infra-red

radiation.

In fact, all objects emits radiation

For example, energy from the Sun

reaches us via __________.

Conduction and convection cannot

take place because there is

____________ between the Sun

and the Earth.

Draw 3 ways in which your hands can feel the heat from the fire through

conduction, convection and radiation.

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16. What is the rate of radiation dependent on?

Surface Temperature Colour and Texture of

surface

Surface Area

• The hotter the object, the

__________ energy it

radiates per unit time.

• Black, dull surfaces are good

____________ and

____________ of thermal

energy.

• Silver, smooth surfaces are

poor ____________ and

____________ of thermal

energy.

• Silver surfaces are also

good ____________ of

thermal energy.

• The greater the surface

area, the __________

energy it radiates per unit

time.

17. A black and white can both contains hot water. Which can contains

water at a higher temperature? Explain your answer.

A) The black can

B) The white can

C) Not enough information

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18. What is the colour of the solar heating panels? Why?

19. Why are the cooling fins of the refrigerators painted dull black?

20. Why are metal plates attached to the cooling fins?

21. Why are the metals teapots silvery and polished?

22. Explain why the wax at the dull black surface falls off first.

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23. Vacuum Flask

A vacuum flask is designed to keep hot liquids hot, and cold liquids cold.

Transfer of thermal energy is __________ by conduction, convection and radiation.

Vacuum reduce heat transfer via ____________ and ____________ because

____________________________________________________________

The plastic cap reduce heat transfer via ____________ because

_______________________. It stops ___________ and ____________.

The inner silvered surface reduce heat transfer via ____________ because

____________________________________________________________

The outer silvered surface reduce heat transfer via ____________ because

____________________________________________________________

The foam plastic reduce heat transfer via ____________ because

____________________________________________________________