P2 - Heat Transfer Flashcards
What is Conduction?
Conduction is the heat transfer in solids e.g. the handle of a pan getting hot
What is Convection?
Convection is the heat transfer in fluids (liquids, gases) e.g. convection currents in the mantle
What is radiation?
A form of heat transfer where thermal energy (heat) is given out as infrared waves e.g. the sun heating up your face. Radiation can be reflected and happens is gases and vacuums
What is thermal conductivity?
A measure of how well a material conducts thermal energy by conduction
Process of convection currents
Air is heated. As it warms, it’s density decreases. The warm air rises and cooler denser air takes its place. A convection current is set up and will heat all the air in the room eventually
Definition of absorb
To take something in (e.g. heat)
Definition of reflect
To make something bounce back
Definition of radiate
To give out something
Definition of transmit
To move through
Worst emitter, absorber, best reflector
Shiny white, silver
Matt and shiny comparison
Matt is dispersed over a larger area due to a more rough surface. Shiny reflects waves straight away.
What is black body and black body radiation?
A black body is a theoretical object that absorbs all the light that hits it so appears perfectly black when cold. Black body radiation is the radiation emitted by it.
What is specific heat capacity?
The thermal energy needed to raise the temperature of 1 kg of a substance by 1 °C
What is infrared radiation?
electromagnetic waves between visible light and microwaves in the electromagnetic spectrum
Change in thermal energy (J) equals…
mass (kg) × specific heat capacity (J/kg °C) × change in temperature (°C)
°C to K
+273
How does loft insulation work?
Loft insulation such as fibreglass contains trapped air and so forms an insulating layer between the loft and the rest of the house. This helps to reduce heat loss through the roof.
How does cavity wall insulation work?
In between 2 layers of bricks there is a cavity. The air trapped in the cavity acts as an insulator and reduces any heat loss due to conduction through the bricks. Heat loss can also occur due to convection within the cavity. Foam insulation can be pumped into the cavity to prevent this.
How does double glazing work?
Double glazing adds an extra pane of glass. The air trapped between the two panes acts as an insulator and reduces heat loss by conduction.
Hows does foil behind radiators work?
Heat loss from a radiator can be reduced by placing shiny foil between the wall and the radiator. The foil prevents heat radiation from reaching the wall by reflecting it back into the room.
How do draught excluders work?
Draught excluders are hairy or spongy strips that can be used to close the gaps around doors and windows to prevent air from convection currents escaping (draughts)
How do curtains work?
Fitting curtains and closing them can prevent draughts leaving a house and so reduce heat loss.
How do you calculate payback time?
Total expenditure / annual saving
Explain The Greenhouse effect
The atmoshpere prevents the suns radiation from leaving and reflects it back to the earth, making it warmer
Impacts of greenhouse effect
Sea level rises, more frequent storms, rainfall patterns change
Heat balance
A system stays at a constant temperature if the same amount of energy is absorbed than emitted.
Peak intensity, wavelength
Longer the wavelengh, the lower the peak intensity
