Heat Transfer Flashcards
What is heat
Heat is a form of energy and is therefore measured in joules.
What is temperature
It is a measure of how hot or how cold an object is. Usually measured using a thermometer usually in degrees Celsius.
How can heat energy be transported
Conduction, convection and radiation
Define conduction
The transfer of heat from molecule to adjacent molecule by vibrations
Define convection
The transfer of heat through a liquid or gas by the motion of molecules themselves
What is radiation
Radiation is the transfer of energy by electromagnetic waves, usually infrared waves
How does conduction work in metals
In metals free electrons move freely throughout the solid structure, free electrons will absorb heat causing them to move much faster. These fast free electrons collide with the metal’s atoms, these collisions cause the atoms to vibrate faster than before.
How does conduction work in non-metals
Atoms will absorb heat directly this causes the atoms to vibrate faster than before, these vibrations pass from atom to atom through the solid structure, this process is much slower than electron conduction in metals
How does convection work
Convection only occurs in gasses and liquids. Convection occurs when the fastest moving particles in a hot region of a gas or liquid move to a cool region, taking their heat energy with them.
What does heat cause
Expansion and therefore a decrease in density
Why are non-metals good insulators
Have no free electrons so take longer to conduct heat compared to metals
Why are metals good conductors
They have free electrons which move easily and can carry heat energy
Why is the best insulator a vacuum
There is no material present to conduct the heat.
What is the next best option for an insulator excluding a vacuum
Pockets of trapped air, eg a duvet, spray foam or a coffee cup
What causes convection currents?
Molecules gain kinetic energy, this extra energy causes the molecules to vibrate with greater amplitude. This warm water then expands. The density of the warm water is less than that of the cold water. The warm water rises and the cooler water flows downwards replacing it. This cycle repeats
What do all objects do (heat radiation)
All objects radiate heat through infrared waves.
Explain radiation
Radiation is a method of heat transfer that takes place without the need for any particles, so it can transfer heat even in a vacuum. The hotter an object is the more radiation it emits.
Which colours are best reflectors
Bright shiny objects
Which coloured objects are the best radiators (emitters) and absorbers of heat
Dark, Matt
What are the main sources of heat loss in a house
Conduction and convection, roof, outside walls, windows, doors and ground floors
How can houses prevent heat loss
Super-insulation, heat recovery ventilation unit, triple glazed windows, foam insulation
How does a thermos flask work
Double wall, vacuum between two walls, vacuum stops all heat transfer by conduction or convection through sides.
Walls facing the vacuum are silvered, shiny surfaces reduce heat transfer by radiation to a minimum.
Stopper made of plastic often filled with cork or foam to reduce heat transfer through it by conduction.
