Thermal Energy Transfers Flashcards
What does energy transfer by conduction through a material depend on?
Thermal conductivity
What is thermal conductivity?
Thermal conductivity is a measure of how well a material conducts energy when it is heated.
A good insulator has a ____ thermal conductivity?
low
The energy transfer per second through a layer of insulating material depends on:
- Temperature difference across the matieral
- Material thickness
- Thermal conductivity of the material
What is infrared radiation and where is it emitted from?
Infrared radiation is a part of the electromagnetic spectrum of waves, it is one of many types of electromagnetic radiation emitted from the sun.
All objects, no matter their temperature…
emit and absorb infrared radiation.
An object at constant temperature…
emits and absorbs infrared radiation at the same rate
What are the 3 ways that heat can be transferred? (+states of matter)
- Conduction (solids)
- Convection (liquids and gases)
- Raditiation (vacuum / gases)
The bigger the temperature difference between hotter and cooler places…
the faster the energy transfers
Compare solids, liquids, gases and vacuums as conducturs
- Solids are the only material that can be good conductors
- Liquids and gases are insulators
- Conduction cannot happen in a vacuum
why are solids the best conductors?
Because their particles are closely packed
Why are metals such good conductors?
Because they have free electrons, these can move around freely, bumping into lots of particles, causing them to vibrate as well
What is conduction?
Conduction is the process where vibrating particles transfer energy to neighbouring particles
What is cold?
the absence of heat
Why is air a good insulator?
because the particles in air are far apart and therefore can’t vibrate and pass energy to neighbouring particles like solids.
Why is a vacuum a good insulator?
Vacuums have no particles meaning there is no medium for the energy to pass through = no transfer
What is convection?
Convection is where energetic particles move away from hotter to cooler regions
What happens when you heat a region of a gas or liquid? What does this do to it’s density?
What does this mean?
The particles move faster and the space between individuals particles increases
This decreases the the density of the region being heated
Liquids and gases can flow, so the warmer less dense regions rise above the cooler denser regions, if there is a constant heat source, a convection current can be created
What is the name for the waves (radiation) that travel to us from the sun?
infrared electromagnetic waves
What colour absorbs and emits radiation the best?
matt black materials
What colour absorbs and emits radiation the worst
shiny white/silver materials - they reflect heat
What factors make a material transfer heat well? (4)
- large surface area (big ears - animals + radiator fins)
- Matt black colour (absorbs and emits radiation best)
- Metal (transfers heat by conduction - close particles + free electrons)
- Large temperature difference between object and environment
What factors make a material transfer heat badly? (4)
- Small surface area - less contact between object + surroundings
- Shiny white/silver colours (reflects radiation)
- Air pockets/vacuum/insulators - stops conduction from occurring
- Small temperature difference makes heat transfer slower
What are methods used to reduce energy loss in the home? (6)
- Loft Insulation
- Cavity walls and insulation
- Draught proofing
- Carpeting
- Double glazed windows + curtains
- Hot water tank jackets
Loft insulation?
Fiberglass “wool” laid across loft floors reduces conduction through the the ceiling to the roof
Cavity walls and insulation?
Two layers of brick with a gap between them reduces conduction, Insulating foam (with a very low thermal conductivity) traps layers of air to minimise convection
Draught proofing?
Strips of foam and plastic around doors and windows stop air going out which reduces convection
Carpeting?
Laid across a floor to reduce conduction
Curtains and double glazed windows?
Thick curtains reduce conduction and radiation through the windows. Double glazing = two layers of glass with an air gap between to reduce conduction
Hot water tank jacket?
Lagging such as fibre glass wool reduces conduction
Why does having free electrons mean metals are good conductors?
The movement of the free electrons means the transfer of energy or current happens quicker than simply from particle to neighbouring particle, making metals good conductors
Why does large surface area mean a good heat transfer?
Large surface area means more particles exposed to the environment or object that heat is being transferred too. This means more particles are transferring heat at the same time = a good heat transfer
