Heating Processes Flashcards
Loft insulation
- thick layer of fibreglass will load across the loft floor and ceiling
- reduces heat loss by conduction and convection
What is the specific heat capacity of water?
- 4200J/kg˚C
Why are ear sizes different in warmer and colder climate
- warm: larger ears with a larger surface area to allow them to lose heat by radiation easily
- cold: small ears with small surface area to minimise heat loss by radiation and conserve body heat
What is evaporation?
- when particles escape from a liquid
- particles can evaporate from a liquid at temperatures much lower than the liquid’s boiling point
Draught-proofing
- strips of foam and plastic around doors and windows to stop warm air blowing out and cold air in
- reduce heat lose by convection
What are U-values
- heat transfer faster through materials with higher U-values
- the better the insulator, the lower the U-value
What are heat sinks?
- devices designed to transfer heat away from objects they’re in contact with e.g. computer components
- have fins and large surface area to emit heat as quickly as possible§
What surfaces reflect a lot of infrared radiation?
- light, shiny surfaces, so they are poor absorbers and poor emitters
What is the main form of heat transfer in solids?
- conduction
What is specific heat capacity?
- specific heat capacity is the amount of energy needed to raise the temperature of 1kg of a substance by 1˚C
What is conduction and convection?
- the transfer of energy by particles
What happens when a solid is heated?
- particles vibrate faster
- forces between particles are overcome and particles start to move around (melting)
What happens when you heat up a fluid?
- heat transfer by convection
- the particles move faster and move further apart, so fluid expands, becoming less dense
- the warmer, less dense fluid rises above its colder denser surroundings
- as the warm fluid rises, cooler fluid takes its place
- as the process continues, there is a circulation of fluid (convection currents)
What is convection?
- convection occurs when the more energetic particles move from the hotter region to the cooler region and take their heat energy with them
What does increased surface area do to the transfer of heat?
- more infrared waves can be emitted (or absorbed) by the surface
- quicker transfer of heat
What happens to an object that is hotter than its surroundings and vice versa?
- it emits more radiation than it absorbs (as it cools down)
- it absorbs more radiation than it emits (as it warms up)
Where is infrared radiation emitted from?
- the surface of an object
What is payback time?
- the amount of time it takes to equal the initial cost of the insulation to the money saved on heating bills
What surfaces are better at absorbing infrared radiation?
- dark, matt surfaces
What is SLH of fusion?
- specific latent heat of fusion is the amount go energy needed to change 1kg of material from a solid to a liquid without changing its temperature
- Energy = mass x SLHF
How does condensation work?
- when a gas cools, the particles in the gas slow down
- they lose kinetic energy
- attractive forces between the particles pull them closer together
- if the temperature gets cold enough and the gas particles get close enough together, condensation takes place and the gas becomes a liquid
What are the three ways heat can be transferred?
- radiation
- conduction
- convection
why are non-metals good insulators?
- they don’t have free electrons, so warm up more slwoly
What happens when you heat a substance?
- you give particles more kinetic energy
- they vibrate/move faster
How can you reduce convection?
- stop the fluid from moving
- e.g. clothes, blankets and cavity wall foam insulation trap pockets of air
Explain the arrangement and energy of particles in liquids
- weaker forces of attraction between particles
- particles close together, but can move past each other
- form irregular arrangements
- more energy than particles in a solid
- move in random directions at low speeds
Why can’t convection occur in solids?
- particles can’t move, they can only vibrate on the spot
What surfaces emit more infrared radiation?
- dark, matt surfaces
Why do metals conduct heat very well?
- some of their electrons are free to move inside the metal
- heating makes the electrons move faster and collide with other particles in the metal, transferring energy
- it is much faster to transfer energy with free moving electrons
What is the equation for specific heat capacity?
- E (J) = m (kg) x c (J/kg˚C) x theta (˚C)
energy transferred = mass x specific heat capacity x temperature change
What is the main form of heat transfer in liquids and gases?
- convection
Give an example of how surface colour and texture affects infrared absorption
- solar hot water panels contain water pipes under a black surface
- radiation from the sun is absorbed by the black surface to heat the water in the pipes
- water can be used for washing or pumped to radiators
Explain the arrangement and energy of particles in solids
- strong forces of attraction hold the particles close together
- in a fixed regular arrangement
- particles dont have much energy so they can vibrate about their fixed positions
What is infrared radiation?
- the emission of electromagnetic waves
- all objects are continually emitting and absorbing infrared radiation
What is conduction?
- conduction of heat is the process where vibrating particles pass on extra kinetic energy to neighbouring particles
When do particles near the surface of a liquid escape?
- the particles are travelling in the right direction to escape
- particles have enough kinetic energy to overcome the attractive forces of the other particles in the liquid
How does evaporation work?
- the fastest particles, with the most kinetic energy, are most likely to evaporate from the liquid
- when the do, the average speed and kinetic energy of the remaining particles decreases
- this decrease in average kinetic energy means the temperature of the liquid falls
What happens when you heat a liquid?
- particles move faster
- eventually enough particles have enough energy to overcome their attraction to each other
- big bubbles of gas form in the liquid (boiling)
Why do car and motorbike engines have fins?
- increase the surface area so heat can be radiated away quicker
- engine cools quicker
What happens to the body when it is cold?
- hair on skin stand up to trap a thicker layer of insulating air
- limits the amount of heat loss by convection
Which state of matter can emit infrared radiation?
- solids
- liquids
- gases
How to calculate efficiency?
- efficiency = useful energy in / total energy in x 100%
- efficiency = useful power out / total power in x 100%
When is energy transfer by heating be increased?
- the bigger the temperature difference between body and its surroundings
Explain the arrangement and energy of particles in gases
- almost no forces of attraction between particles
- particles have more energy than in liquids and solids
- free to move
- travel in random directions at high speeds
Energy can be _______ but _______ or _______
- energy can be transferred but not created or destroyed
What are the flat spots on cooling graphs?
- when a substance is condensing or freezing, latent heat energy is being transferred, but not being used to change the temperature
- bonds are forming between particles, which releases energy
What is specific latent heat?
- specific latent heat of a substance is the amount of energy needed to change 1kg of it from one state to another without changing its temperature
What happens to the body when it’s too warm?
- body diverts more blood to flow near the surface of the skin so heat can be lost by radiation
Why is air a good insulator?
- liquids and gases conduct heat more slowly than solids as particles aren’t held so tightly together
What five factors increase rate of evaporation?
- higher temperature: average particle energy is higher, so more particles have enough energy to escape
- lower density: forces between particles usually weaker, so particles have more energy to overcome these forces
- larger surface area: more particles near enough to the surface
- greater airflow: the lower the concentration of an evaporating substance in the air its evaporating into, the higher the rate of evaporation. air above the liquid is replaced more quickly, so concentration in the air is lower
- lower humidity: lower concentration of water particles, so rate of evaporation will be higher
What are the flat spots on heating graphs?
- when a substance is melting or boiling, latent heat energy is being transferred, but not being used to change the temperature
- due to the energy being used for breaking intermolecular bonds
How does heat flow through solids by conduction?
- in a solid, particles are tightly held tighter. so when a particle vibrates, it bumps into other particles nearby and passes the vibrations on
- particles which vibrate faster than others pass on their extra kinetic energy to neighbouring particles
- process continues and all the extra kinetic energy is spread through the solid
- this causes a rise in temperature at the other side
Hot water tank jacket
- fibreglass wool reduces conduction and radiation
What is cavity wall insulation?
- foam squirted into the gap between bricks to stop convection currents being set up in the gap and radiation across the gap
- insulating form and air pockets help reduce head loss by conduction
What is SLH of vaporisation?
- the specific latent heat of vaporisation is the amount of energy needed to change 1kg of material from liquid to vapour without changing its temperature
- Energy = mass x SLHV
Thick curtains
- creates an air gap between room and window
- stops hot air reaching the glass by convection
- conduction and radiation also reduced
What is heat radiation?
- the transfer of heat energy by infrared (IR) radiation
