Term
| Describe thermal conduction in as much detail as possible. |
|
Definition
Thermal conduction occurs when particles collide and energy is transferred between particles.
Conduction is only effective in solids because the particles are close together.
Particles in liquids and gases are too far apart to collide often enough. |
|
|
Term
| Describe thermal convection in as much detail as possible |
|
Definition
Hot liquids or gases have a lower density because the particles are more spread out. The hot liquid or gas then rise or move upwards through the colder, denser liquid/gas.
Convection can obviously not happen in solids because the particles cannot move from place to place. |
|
|
Term
| Describe thermal radiation in as much detail as possible. |
|
Definition
| All objects emit (give off) and absorb (take in) heat energy as infra-red radiation or waves. |
|
|
Term
| What features of a good house prevent heat loss due to conduction? |
|
Definition
Double glazing - the trapped air between the glass panes is an excellent insulator Cavity wall insulation - Between the two layers of brick which make up the houses outer wool there is a layer of rock wool which again contains trapped air, an excellent insulator. Loft insulation - again in the loft a layer of insulation (a material with trappped air in it) stops conduction though the roof. |
|
|
Term
| What features of a good house prevent heat loss due to convection? |
|
Definition
Doors and windows are kept closed so that air cannot move freely. There are no gaps around the windows or doors again to stop air moving freely. |
|
|
Term
| What features of a good house prevent heat loss due to radiation? |
|
Definition
| Paint the walls of the house a shiny pale (white basically) colour rather than dull dark (black) to reduce the emission of infra-red radiation. |
|
|
Term
| Describe the features of a sankey diagram |
|
Definition
The width of the arrows is equal to the amount of energy. Useful energy arrows go horizontally across the page. Wasted energy arrows go vertically off the main arrow. |
|
|
Term
| Describe efficiency in as much detail as possible. |
|
Definition
Efficiency describes how much energy put into a machine is converted into useful energy. Efficiency = useful energy out / energy in x 100 For example, a typical light bulb needs 100 J of electrical energy every second and only 10 J of that is converted into useful light energy, the rest is wasted thermal energy. The efficiency of a light bulb is therefore only 10%! |
|
|
Term
| How does shape affect the rate of heat loss? |
|
Definition
The greater the surface area the more heat energy that move hot to cold places. Elephants have large ears not to hear better but to cool themselves down faster because of the large surface area. Small animals have a large surface area to a small volume and hence lose heat energy quicker than large animals. |
|
|
Term
|
Definition
Power decribes how quickly an object converts energy. Power is measure in Watts. 1 Watt means 1 joule of energy is converted every second. 1 kilowatt (kW) = 1000 Watts (W) |
|
|
Term
What units do we pay for electricity in? How do we work out how much electricity we've used? |
|
Definition
We pay for electricity in kilowatthours (kWh) Number of kWh = power of machine x time used in kilowatts in hours To convert watts into kilowatts divide by 1000 |
|
|
Term
|
Definition
Evaporation is when a liquid becomes a gas at a lower temperature than its boiling point.
The bonds between the particles at the surface of a liquid are weakest.
If a particle at the surface has lots of energy it can break its bond and escape from the liquid.
This reduces the temperature of the liquid. |
|
|
Term
| How can you increase the speed at which evaporation occurs? |
|
Definition
Increase the temperature of the liquid
Increase the surface area of the liquid
Have a wind blow across it.
|
|
|
Term
| How can you change the speed of heat transfer by conduction. |
|
Definition
Metals are excellent conductors because they contain 'free electrons' which allows heat energy to be transferred quicker.
Materials with trapped air in them are excellent insulators. This slows down the speed at which heat energy is transferred |
|
|
Term
| How can you increase the speed of heat transfer due to convection? |
|
Definition
| Leaving doors or windows open makes it easier for gases to move and for heat to escape due to convection. |
|
|
Term
| How can you increase the speed of heat transfer due to radiation. |
|
Definition
Dull black objects are good at emitting and absorbing heat energy which will increase the speed of heat transfer.
Shiny white/silver objects are really bad at emitting and absorbing heat energy which will slow down the speed of heat transfer.
Increasing the temperature of an obejct increases the speed of heat transfer.
Increasing the surface area of an object will increase teh speed at which it either absorbs or emits heat energy. |
|
|
Term
|
Definition
A U value describes how fast heat moves through an object.
A high u value means it allows heat to transfer wuickly.
A double galzed window will have a lower U value than a single glazed window because heat travels through it slower. |
|
|
Term
| How can you stop heat escaping from a house. For each example explain what method it stops and how. |
|
Definition
Closing doors and windows/filling gaps - stops convection because gases can't escape
Wall and loft insulation - material contains trapped air which is an excellent insulator and slows conduction down.
Paint the house white - it stops the house absorbing heat in the summer and emitting heat in the winter because white object are very bad at emitting and absorbing infra red radiation. |
|
|
