P2 Heat & Radiation Flashcards
Infrared radiation
Can transfer thermal energy without a medium
An electromagnetic wave
Invisible
All bodies emit radiation
Longer wavelength than visible light
Shorter wavelength than radio waves
How surfaces interact with infrared radiation
Can reflect, absorb, or emit infrared radiation
Good absorbers are usually bad reflectors
How shiny surfaces interact with infrared radiation
Compared to dull surfaces of the same colour, shiny surfaces are:
Poorer absorbers
Poorer emitters
Better reflectors
How perfect black bodies interact with infrared radiation
Absorbs all of the radiation that incidents on it
Doesn’t reflect or transmit radiation
As a good absorber is a good emitter, black body objects are the best possible emitters
How white surfaces interact with infrared radiation
Good reflectors
Poor emitters
Poor absorbers
How black surfaces interact with infrared radiation
Good emitters
Good absorbers
Poor reflectors
Factors affecting the amount of radiation emitted
Surface area
Surface temperature
The higher the surface area or temperature, the higher the rate of emission
Factors affecting temperature
Intensity of radiation emitted
Wavelength of radiation emitted
When is temperature constant
radiation absorbed = radiation emitted
When does the temperature change
radiation absorbed ≠ radiation emitted
Diffusion of heat
Heat will always transmit from a hotter to a cooler area
Rate of emission is higher if an object is hotter than its environment
Factors affecting absorption and emission
Internal temperature
Surface area
External temperature
The greenhouse effect process
- The sun emits short wavelength infrared radiation that enters the atmosphere and travels towards the Earth’s surface
- The Earth absorbs some of this radiation, but long wavelength radiation is reflected back into the atmosphere
- Greenhouse gases (e.g. carbon dioxide, methane, water vapour) can’t absorb the frequency of radiation emitted by the Sun. But they can absorb the longer wavelength reflected radiation
- The gases then re-radiate this energy in all directions, including back towards Earth
- This increases the temperature at the Earth’s surface
Emission to balance energy on Earth
Absorbed sunlight is balanced by heat radiated from the Earth’s surface and atmosphere
Most heat escapes from areas just north and south of the equator, where the surface is warm, but there are few clouds
Along the equator, persistent clouds prevent heat from escaping
The poles radiate little heat
Absorption to balance energy on Earth
340 Watts per square metre of solar energy falls on the Earth
29% is reflected back into space - primarily by clouds, but also by other bright surfaces and the atmosphere itself
~23% of incoming energy is absorbed in the atmosphere by atmospheric gases, dust, and other particles
The remaining 48% is absorbed at the surface
Thermal conductors
Materials which allows the conduction of thermal energy
Thermal insulators
Materials which prevent or do not readily conduct thermal energy
The greenhouse effect
The process of gases trapping heat to keep the Earth warm enough for life
The greenhouse effect can cause global warming if it becomes too strong
Examples of thermal insulators
Polystyrene foam
Water
Mineral wool
Plastic
Examples of thermal conductors
Copper
Aluminium
Brass
Silver
Formula for specific heat capacity
specific heat capacity(J/Kg°C) = (change in energy(J)) / (mass(Kg) * change in temperature(°C))
Conductivity of metals and non-metals
Metal are better conductors than non-metals
Conductivity of copper and steel
Copper is a better conductor than steel
Conductivity of glass and wood
Glass is a better conductor than wood
Specific heat capacity
The amount of energy needed to increase a kilogram of a substance by 1°C
Relationship between mass and change in temperature
Rate of energy transfer decreases as mass increases
Thermal conductivity
The rate at which a material conducts thermal energy
Factors affecting rate of energy transfer
Temperature difference across material
Thickness of material
Thermal conductivity of material
Relationship between thickness and thermal conductivity
Thermal conductivity decreases as thickness increases
Relationship between temperature and radiation emitted
Amount of radiation emitted increases as temperature increases
Radiation emitted from an object with a constant temperature
An object at a constant temperature emits radiation at a continuous range of wavelength
Relationship between heat and wavelength
Wavelength decreases as heat increases
Factors affecting change in temperature
Amount of energy supplied
Mass
Type of substance
Methods to reduce rate of emission from homes
Loft insulation
Cavity wall insulation
Double-glazed windows
Thick bricks (with low thermal conductivity)
Aluminium foil behind radiators
Methods to heat homes
Electric/gas heater
Oil/gas central heating
Solid fuel stove/fireplace
Cavity insulation
Insulation, with a lower thermal conductivity than air, between two layers of brick of an external wall
