Sunday, July 5, 2009

Extra qn. answers....

Since copper is a good conductor of heat, the space in between the copper gauze will have a higher temperature than the surroundings. The higher temperature inside is also due to the presence of the candle. When the surrounding temperature almost reach the ignition temperature of the methane, the methane in between the copper gauze will burn first. This is because the space inside the copper gauze have a slightly higher temperature. Thus, this will warn the miners to run out of the mine quickly and SAVE THEMSELVEs!!!!

Extra qn....

The figure shows a miner's safety lamp invented by Sir Humphry Davy. How does the copper gauze act as a safety device to prevent explotions if a flammable gas such as methane is present in the mine? (Hint: Copper is a good conductor of heat. Methane will burn only if the surroundings reach its ignition temperature.)

Sunday, June 28, 2009

Ans... for the Stuctured Qns....

1. (a) The freezer compartment is placed at the top because cold air, being denser, will sink to the bottom while hot air will rise to take its place, and is turned to cold air on top and again the cold air will sink, setting up a convection current.
(b) As hotter air from outside moves in, it will quickly rose up, to the freezer compartment being less dense than the cold air inside the refrigerator due to convection current. Thus, the hot air will not have enough time to warm up the contents below the freezer compartment.
(c) The cooling fin of the refrigerator is painted black because black is a better emitter of heat and it radiates heat readily.

2. (a) It is because polished surfaces are bad emitter of heat energy and thus it will prevent heat loss from the kettle to the surroundings.
(b) This is because of convection current. As hot water rises, cold water will sink to take its place and thus it is heated up by the heating element at the bottom.
(c) This is because plastic are a bad conductor, thus it prevents the user from getting scalded.

3. (a)(i) Radiation
(ii) Conduction
(b)(i) d.Black-painted copper
(ii) This is because copper is a good conductor of heat and therefore will quickly transfer the heat to the water. Secondly, black surfaces is a better absorbers of heat rather than white, polished surfaces.

4. (a) When birds fluff up their wings, it is trapping air, and as air is a bad conductor of heat, the heat from its body would not be lost to the surroundings cold air.
(b) This is due to the over-exposure of radiation from the sun, and the radiation will make the skin cells act abnormally.

Structured Qns...

1. (a) In a household refrigerator, the freezer compartment is placed at the top. Explain why this is so.
(b) When the refrigerator door is opened for a short time, the cold air inside is replaced by the warmer air outside. Why does this exchange of air have little effect on the temperature of the contents in the part of the refrigerator below the freezer compartment ?
(c) Explain why the cooling fins at the back of the refrigerator is black.

2. (a) Explain why a household electric kettle has a polished surface.
(b) What is the reason for placing the heating element at the bottom of the kettle?
(c) Most kettles have plastic handles. Why?

3. A solar panel is used to heat water.
(a) State the name of the process by which energy is transferred
(i) from the Sun to the outside of the pipe in the solar panel.
(ii) from the outside of the pipe to the water inside it.
(b) (i) Which of the following materials would be the most suitable for the pipe that contains the water
a. black plastic
b. white plastic
c. glass
d. black-painted copper
e. polished steel
(ii) Give two reasons for your choice of material.

4. Explain the following observations.
(a) Birds usually fluff up their feathers during cold weather
(b) Excessive sunbathing can cause skin cancer

Saturday, June 27, 2009

Answers for MCQs!!!

Part 1:
1. C
2. D
3. A
4. A
5. B

Part 2:
6. A
7. C
8. B
9. B
10. C

Part 3:
11. B
12. C
13. A
14. D
15. B

MCQss!!!! Part 3

11. The heat from the hot water in a metal radiator passes through the metal and then spreads around the room.What are the main processes by which the heat is transferred through the radiator and then spread around the room?12. Identify the procesess of thermal energy transfer.
Process I : Transfer of thermal energy that involves a change in fluid density.
Process II : Transfer of thermal energy from an energetic atom or molecule to a less energetic one when they collide.
13. A person sits in front of a warm fire. What is/are the main process(es) of thermal energy transfer?

A Radiation only

B Convection and radiation

C Conduction and radiation

D Conduction, convection and radiation

An experiment is carried out as shown.
Why does the ice take a long time to melt, even when the water at the top of the test-tube
is boiling?
A Ice is a poor radiator of heat.
B Convection cannot occur in water.
C The metal gauze prevents heat from melting the ice.
D Water is a poor conductor of heat.

15. Two cars were painted, one was in black, the other white. The cars were left under the sun to dry. The black paint dried more quickly than the white paint. Which property of black paint makes it dry more quickly?

A It is a better conductor of heat.

B It is a better absorber of heat.

C It is a better insulator of heat.

D It is a better reflector of heat.

MCQss!!! Part 2

6. The diagram shows four similar cans. Each can contains the same volume of water initially at 80 degree celcius. After five minutes.which can will contain the coolest water?
7. In a vacuum flask, which methods of heat transfer are prevented by the vacuum?
A conduction only
B convection only
C conduction and convection only
D conduction, convection and radiation

8. A teacher has a large tank of water in which he wants to set up a convection current.

Which of the following arrangements would do this?

A cooling at X

B cooling at Y

C heating atY

D heating at Z
9. Three blocks, at temperatures T1, T2, and T3, are in good thermal contact.
At which temperatures are the blocks in thermal equilibrium? 10. Four beakers containing the same amount of water at the same temperature are placed on hot metal plates. The lower surfaces of the metal plates are kept at the same temperature.

The plates are all the same size but are made from four different metals

The time taken to produce stated temperature rises of the water are given below.
Which metal is the poorest conductor?

MCQ!!! Part1

1. A small cork is fixed with wax to a metal plate. An electric heater is placed close to the plate. After a time, the waxmelts and the cork drops off.
How does heat reach the wax?
A by conduction only
B by conduction and convection
C radiation and conduction

D radiation and convection

2. Fibre is used for home insulation as shown
How does fibre prevent heat passing easily through the ceiling?

A Fibre allows air to pass through easily.

B Fibre is tightly packed

C Fibre is warm.

D Fibre traps air.

3. Which material is the best absorber of infra-red radiation?

A dark animal fur

B shiny metal

C window glass

D white paper

4. The diagram shows a metal saucepan containing water and
placed on a hot plate. After some time, the air at point X also
becomes hot.

What are the main ways by which heat travels from the hot
plate through the base of the metal saucepan' through the
water and through the air to Point X?
5. Density changes are responsible for which methods of thermal energy transfer?

A conduction only

B convection only

C radiation only

D conduction, convection and radiation

Wednesday, June 24, 2009

Applications of radiation

1. Teapots
2. Greenhouse
3. Vacuum flask
4. Heat transfer through
space from the sun

Thermal conductivity of substances


Thermal Conductivity
 k [J/(s-m-C)]


Thermal Conductivity
 k [J/(s-m-C)]

















Body fat








Applications of convection

1. Boiling water
2. Household hot water system

3. Air conditioners
4. Refrigerators

Applications of conduction

Uses of good conductors of heat:

1. Cooking utensils
2. Soldering iron
3. Heat Exchangers

Uses of bad conductors of heat (insulators):

1. Handles of appliances and utensils
2. Table mats
3. Sawdust
4. Wooden ladles
5. Woollen clothes
6. Fibreglass, felt and expanded
polystyrene foam

Factors affecting rate of infrared radiation

1. Colour and texture of the surface
Dull, black surfaces are better absorbers and emitters of infrared radiation than shiny, white surfaces
2. Surface temperature
The higher the temperature of the surface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.
3. Surface area
If two objects have the same mass and materials, the object with the larger surface area will have a higher rate of infrared radiation

Wednesday, June 17, 2009

Sunday, June 14, 2009

Absorption and Emission of infrared radiation

Absorption of radiant heat by all objects and surfaces will cause the temperature of the object to rise.
Emission of infrared radiation will cause the temperature of the object to fall.

In general,
good emitter is a good absorber (of radiant heat)
bad emitter is also a bad absorber (of radiant heat)

Good emitter and absorber:
Dull, black surfaces

Bad emitter and absorber:

Shiny, white surfaces

Saturday, June 13, 2009


What is radiation?
Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium. Radiation does not need a medium for energy transfer, therefore, radiation can take place in a vacuum. Thermal energy from the sun reach the earth by the process of radiation. The sun emits electromagnetive waves. The group of electromagnetive waves that makes us feel warm is called infrared waves. While the thermal energy from the infrared waves is called radiant heat.

Thermal radiation also refers to electromagnetic radiation emitted from the surface of an object due to the object's temperature. This shows that all objects will emit some radiant heat, depending on its temperature. The hotter the objects, the greater radiant heat it emitted.

Thermal radiation is generated when heat from the movement of charged particles within atoms is converted to electromagnetic radiation.

Examples of convection

Natural convection current:
Land and sea breeze- creation of breezes over land masses next to large bodies of water.

Water has a larger heat capacity than land, and subsequently holds heat better. It therefore takes longer to change its temperature, either upward or downward. Thus, during the day the air above the water will be cooler than that over the land. This creates a low pressure area over the land, relative to the high pressure area over the water, and subsequently one finds breezes blowing from the water to the land. On the other hand, during the night water cools off more slowly than the land, and the air above the water is slightly warmer than over the land. This creates a low pressure area over the water relative to the high pressure area over the land, and breezes will blow from the land to the water. Thx:


What is convection?
Convection is the transfer of thermal energy by means of currents in a fluid (liquids or gases). to put it simply, convection is the transfer of heat by the actual movement of the warmed matter in currents.
Convection current is caused by the change in density. Suppose we consider heating up a local region of air. As this air heats, the molecules spread out, causing this region to become less dense than the surrounding, unheated air. Being less dense than the surrounding cooler air, the hot air will subsequently rise, while the cooler air will sink because they are denser.
Convection currents can only occur in fluids such as liquids and gases but not in solids. This is because convection involves the bulk movement of the fluids which carry thermal energy with them.

Conduction in Liquids and Gases

In liquids and Gases, conduction is due to the collisions and diffusion of the molecules during their random motion. As the particles are spaced further apart, there are less frequent collisions in liquids and even lesser in gases. Thus the transfer of kinetic energy from fast moving molecules to neighbouring molecules is slower. This is why most liquids (except for molten mercury) are poor conductors of heat and gases are the poorest (or best insulators) of heat.

Conduction in metals and non-metals

In metals, conduction happens due to the combination of vibrations of molecules and free electron diffusion. While in non metals, the mechanism for conduction is only atomic or molecular vibrations.

What is atomic or molecular vibrations?
When thermal energy is supplied to one end of the rod, the particles (atoms or molecules) at the hot end vibrate vigorously. These particles will collide with neighbouring particles, making them vibrate as well. Thus, the kinetic energy of the vibrating particles at the hot end is transferred to the neighbouring particles.
This process is slow.

What is free electron diffusion?
When the coppper rod is heated, the free electrons in the copper gain kinetic energy and move faster as a result. These fast-moving electrons then diffuse or spread into the cooler parts of the metal. In the process, they collide with the atoms in the cooler parts of the metal and transfer their kinetic energies to them.
This is a much faster mechanism of thermal energy transfer.

This explains why the conduction in metals is faster than in non-metals. Thus metal is a good conductor of heat and non-metals is a poor conductor of heat.

metals, eg. copper, silver, steel and iron

non-metals, eg. glass, plastic, wood, brick, wool


What is conduction?
Conduction is the process of thermal energy transfer without any flow of the material medium.
It is also the transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient.
Conduction is most effective in solids, but it can happen in liquids and gases.
eg. a spoon in a cup of hot soup becomes warmer because the heat from the soup is conducted along the spoon.
Fun fact:
Have you ever noticed that metals tend to feel cold? Actually, they are not colder. They only feel colder because they conduct heat away from your hand. You perceive the heat that is leaving your hand as cold.

Thermal energy transfer

Thermal energy can be transferred by conduction, convection and radiation.
Often more than one of these processes occur in a given situation.

Transfer of thermal energy

Heat is the transfer of thermal energy from a hotter to colder region.
Thermal energy is only transferred when there is a difference in temperature.
Thermal energy always flows from a region of higher temperature to a region of lower temperature (temperature gradient) to equalize temperature differences (reach equilibrium).
When thermal equilibrium is reached, there is no net flow of thermal energy.