P2. Energy transfer by heating Flashcards
What does the energy transfer per second through a layer of insulating material depend on
The temperature difference across the material
The thickness of the material
The thermal conductivity of the material
How do you reduce the energy transfer as much as possible in any given situation?
- The thermal conductivity of the insulating material should be low as possible
- The thickness of the insulating layer should be as thick as is practically possible
What is the relationship between the temperature and infrared radiation?
The higher the temperature of an object, the more infrared radiation it emits in a given time
What is a perfect black body?
A perfect black body is an object that absorbs all the radiation that hits it
What is black body radiation?
The radiation emitted by a black body
What is the relationship between radiation and an object with a constant temperature?
An object that has a constant temperature emits radiation across a continuous range of wavelengths
Why do objects stop cooling in the terms of the rate of absorption and emission of radiation?
It absorbs more heat than it emits causing its temperature to increase until it reaches equilibrium.
What are the effect of coloured surfaces on absorbing and emitting ability?
Black - good absorber and good emitter
Dull/dark - reasonable absorber and reasonable emitter
White - Poor absorber and poor emitter
Shiny - Very poor absorber and very poor emitter
What factors affect the rate of emission of infrared radiation?
Colour
Reflectivity
Temperature
Surface area
What is the process of the greenhouse effect in terms of absorption, emission and wavelengths of electromagnetic radiation?
- Infrared radiation from the Sun warms the ground
- The ground becomes warm and emits longer-wavelength infrared radiation
- CO₂ molecules absorb and re-emit infrared radiation back to the ground.
What is the specific heat capacity of a substance?
The energy needed to raise the temperature of 1kg of the substance by 1°C
How do you calculate the energy transferred?
Energy transferred, E = mass, m × specific heat capacity, c × temperature change
How do you find the specific heat capacity?
Use a joulemeter and thermometer to measure the energy transferred and the temperature change for a measured mass then use the equation: Specific heat capacity, c= Energy transferred, E ÷ mass, m × temperature change
How can the rate of energy transfer from houses be reduced?
Loft insulation -fibreglass reduces the rate of energy transferred through the roof.
The air between the fibres helps to reduce the rate of energy transfer by conduction.
The greater the number of layers of incubation insulations the thicker the insulation will be so the rate of energy transfer through the roof will be less.
Cavity wall insulation - Reduces the rate of energy transfer through the outer walls of the house.
The cavity of an outer wall is the space between the two layers of brick that make up the wall. The insulation is pumped into the cavity.
It is a better insulator than the air it replaces. It traps the air in small pockets reducing the rate of energy transfer by conduction.
Aluminium foil between a radiator panel and the wall reflects radiation away from the wall and so reduces the rate of energy transfer by radiation
Double glazed windows - Have two glass panes with dry air or a vacuum between the panes. The thicker the glass and the lower its thermal conductivity is, the slower the rate of transfer of energy through it by conduction will be.
If the external walls of a warm building have thicker bricks and lower thermal conductivity, the rate of transfer of energy from the inside of the building to the outside will be lower and the cost of heating.
