Structured Question

Why is it more effective to wear a few layers of thin clothing than a single thick layer of clothing on a cold day?

There are many layers of air trapped if a person is wearing many layers of clothing.On the other hand, if the person is only wearing one layer of clothing, there is only one layer of air trapped. As air is a good insulator of heat, it will minimise heat loss from the body to the cold surroundings.Thus, it is more effective to wear a few layers of thin clothing than a single thick layer of clothing.

Questions with solutions

1. Which of the following is the poorest conductor of heat energy?

A. Air B. Brass C. A vacuum D. Water

2. Which of the following will be the best absorber of infra-red radiation?

A. Dark Animal Fur B. Shiny Metal Tray

C. White Plastic Bag D. Window Glass

3. An illustration of the use of a good conductor of heat is the

A. wooden pan handles

B. fibre-glass roof insulation

C. double glazing of windows

D. cooling fins on engines

4. A black plastic bag is filled with cold water and hung up in a sunny place. What causes the water temperature to rise?

A. Convection inside the black bag

B. The black bag absorbing radiation

C. The black bag acting as an insulator

D. The black bag emitting radiation

5. A small cork is fixed with wax to a metal plate.An electric heater is placed close to the plate.After some time, the wax melts and the cork drops of. How does heat reach the wax?

A. By Conduction only

B. By Conduction and Convection

C. By Radiation and Conduction

D. By Radiation and Convection

Answers

1.C

2.A

3.D

4.B

5.C

Examples

Examples of radiation:
1)Food stalls have bright lights shining at their food to keep food warm.
2)Rooms in cold countries have a metallic heating unit that emits heat.

Examples of conduction:
1)heat transferred through the walls of a steel radiator
2)heat exchangers is used to save energy. It is made of copper.
(heat exchanger)

Radiation

Factors affecting rate of infrared radiation:

1.Colour and texture of the surface

(comparison between dull,black surfaces and

shiny, white surfaces on previous post)

2.Surface temperature

(The higher the temperature of the surface of the objectrelative to the surrounding temperature, the higher the rate of infrared radiation)

3.Surface area

(Compare two objects with the same mass and material but with different surface areas. The object with the larger surface area will emit infrared radiation at a higher rate.)

Note

The higher the rate of infrared radiation means the amount of heat released from the object is high.

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.Thus, radition can take place in a vacuum.

Absorption and Emission of Infrared radiation:

Infrared radiation is absorbed and emitted by all objects.Absorption of infrared radiation causes a rise in temperature while emission of infrared radiation causes a decrease in temperature.

Dull,black surface absorb infrared radiation faster than shiny, white surfaces. They are also better emitters of infrared emitters.

Examples of convection currents

Land and Sea Breeze

Electric Kettle

Convection helps to make the Earth liveable as it removes excess heat from the Earth and transporting it high up into the atmosphere.

In general, conduction is the process where energy is transferred because of density differences in a liquid.

video on convection currents

Hi! This is a video I found on convection currents. It shows a demonstration and explains a convection current. However, I am not able to upload it on my blog so this is the URL:
http://www.youtube.com/watch?v=xnB-Y8wPK2w

Convection

Convection is the transfer of thermal energy by means of
currents in a fluid.

It only takes place

in liquids and

gases but not in

solids as convection

involves the movement

of fluids which carry

thermal energy while

thermal energy is

transferred from one

particle to another

through vibrations.

Does conduction take place in Liquids & Gases?

Conduction does take place in liquids and gases but they are inefficient.The particles are spaced further apart than solids so collisions between particles are less frequent.Thus, the transfer of heat from molecule to molecule is slower. This explains why liquids and air are poor conductors of heat.

Question
All liquids are poor conductors of heat except molten mercury. Why?

Answer
Molten mercury is a metal in liquid form and it has properties of metals. For example, it has free and mobile electrons, which is a property of metals. Therefore, it is a good conductor of heat.

why are metals good conductors of heat?

All solids are made up of atoms and molecules. However, metals contain free electrons which move randomly between the atoms and molecules.

One mechnism for conduction is atomic or molecular vibrations. When thermal energy is supplied to one end of a rod, the atoms or molecules at the hot end will vibrate vigrously. The particles will collide with neighbouring particles, causing them to vibrate as well. Thus, the kinetic energy is transferred from particle to particle.This is how the heat is transferred from the hot end of the rod to the cold end. This mechanism takes place in both metals and non-metals.

The other mechanism for conduction is free electron diffusion which only takes place in metals. When a metal rod is heated, the free electrons gain kinetic energy and they move faster. These electrons then diffuse or spread into the cooler parts of the rod. Thus, they collide with the particles in the cooler parts of the metal and transfer their kinetic energy to them.Thus, this explains why metals are capable of transferring thermal energy much faster than insulators.

Conduction

Conduction:

It is the process of thermal energy tansfer without any flow the material medium.

For example, look at the picture below. One end of the rod is placed over a candle. Over time, the whole rod is hot as heat is transferred from a hotter region (end of rod above flame) to a cooler region (other end of rod not above flame) by conduction.

Different materials conduct heat at different rates. In general, metals such as copper,silver and steel are good conductors of heat. Non-metals like glass, distilled water and water are insulators of heat.

Transfer of Thermal Energy

Thermal Energy:

It always flows form a region of higher
temperature to a region of lower temperature.

3 process of how thermal energy is transferred:
Conduction Convention Radiation

video on Convection

Hi! i found this video that shows various experiments on convection. However, i cannot seem to be able to upload it on my blog so this is the URL of the video :
http://www.youtube.com/watch?v=HIYCR7gXXFo&feature=related