Chapter 10 - Questions

Questions
Answer by commenting(:

1. Spoons of different materials are used to stir a hot drink. Which of the following spoons will heat up the fastest?

A) Wooden spoon
B) Steel spoon
C) Plastic spoon
D) Glass spoon


2. When convection current is set up in a room, which one of the following does not happen?

A) Density of hot air increases
B) Cool air moves down
C) Air expands when heated
D) Hot air rises of the top


3.People sitting round an open fire feel warm because heat is transferred mainly by

A) Convection
B) Radiation
C) Freezing
D) Conduction


4. Which methods of transer of thermal energy does not require a material medium?

A) Radiation
B) Conduction
C) Convection
D) Conduction & Convection


5. In which of the following is most of the heat transferred by conduction?

A) Baking of bread in an oven
B) Heating of soldering iron
C) Boiling of water in an electric kettle
D) Deeping frying of chicken in oil

Chapter 10 - Applications of Thermal Energy Transfer

Application of Thermal Energy Transfer


Common application of conduction
Good conductors of heat

• Cooking utensils: made of aluminium or stainless steel where direct heating is involved
• Soldering irons rods: made of iron with top made of copper, as copper is a much better conductor of heat than iron
• Heat exchangers: used in large laundry facility, help save energy

Bad conductors of heat (insulators)

• Handles of appliances and utensils: Can be picked up without scalding our hands
• Tables mats: made of cork so hot kitchenware can be placed on them without damaging the table-top
• Sawdust: used to cover ice blocks because of its good insulating property
• Wooden ladles: used for stirring or scooping hot soup that has just been cooked
• Woollen clothes: used to keep people warm on cold days
• Fibreglass, felt and expanded polystyrene foam: trap large amounts of air which are employed as insulators in the walls of houses, ice boxes and refrigerators.
  

Common applications of convection

• Electric kettles: water near the heating coil is heating up, expands and becomes less dense when the power switch is switched on. The heated water will then rises while the cooler regions in the upper part of the water will descend to replae the heated water.
• Household hot water systems:
• Air conditioners: air conditioners is always installed near to the ceiling of a room to facilitate setting up of convection current.
• Refrigerators: freezing unit is usually placed at the top to cool the air and facilitate setting up of convection currents.
  

Common applications of radiation

• Teapots: shiny surface are bad emitters of radiation, they can keep tea warm for a longer time and keep cool liquids for a longer time too.
• Greenhouses: the greenhouses traps heat which warms up the soil and plants. Over time, the temperature in the greenhouse will increase.
• Vacuum flasks

1. Hollow plastic stopper: made of plastic which is a poor conductor of heat
2. Trapped air: conduction through the trapped air above
3. Vacuum: conduction and convection through the sides of the flask are prevented by the vacuum between the double-glass walls of the flask
4. Thin silvered walls of glass: minimise heat lose through radiation, walls are silvered so that it will reflect the radiant heat back into the hot liquid. Convection and evapouration can only occur when the plastic stopper is removed during use.
5. Double-walled glass bottle
6. Outer case
7. Hot liquid
8. Cork flask in place

Chapter 10 - Radiation

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


Rate of heat transfer is affected by:

• Colour and texture of the surface
• Surface temperature
• Surface area

Colour and texture of the surface
Dull black surfaces are better absorbers and emmiters of infrared radiation than shiny smooth surfaces.

Surface temperature Rate of radiation increases as the temperature increases.

Surface area The rate of radiation increases aas the surface area increases.

Short clip(:
http://www.youtube.com/watch?v=wz6wzOtv6rs

Chapter 10 - Convection

Convection

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


Convection current in water
Water at the bottom that is heated expands. Its density decreases and the warm water rises.The cool water at the top being denser moves down and gets heated in turn.

Covection current in air
During day time, lands heats up faster than water. Air above the land heats up and rises, cool air above the sea rushes in.
During night time, lands cools faster than see. Cool air aboce the land moves to the sea, warm air above the sea rises.

Short clip(:
http://www.youtube.com/watch?v=5pG-tkbQgMo

Chapter 10 - Conduction

Conduction

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


Liquid
Water at the top of the test tube boils, but the ice at the bottom remains. This shows that water is a poor conductor of heat.



Gas
The match stick does not ignite. This shows that air is a poor conductor of heat.



Solid
The one end of the metal rod is heated, the other end will also get heated up after a while. This shows that metal is a good conductor of heat.

However, not all solids are good conductor of heat.
Examples:

• Wood
• Rubber
• Plastic
• Glass

Short clip(:
http://www.youtube.com/watch?v=77R4arwD8G8

Chapter 10 - Introduction

Introduction

Thermal energy or heat is the form of energy that is transferred from a region of higher temperature to a region of lower temperature.

Thermal energy is transferred by:

• Conduction
• Convection
• Radiation