Heat is transferred by infra red, which is emitted by the surfaces of all solids, liquids and gases.
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Describe heat transfer by conduction
Heat is transferred in solids by conduction, either by the passed on vibration of particles or the movement of free electrons.
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Describe heat transfer by convection
The main method of heat transfer in fluids, caused by the currents of rising and falling particles due to temperature.
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How does IR cool objects?
An object that is hotter than its surroundings emits more radiation than it absorbs, so it cools down.
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What surface emits and absorbs radiation best?
Dark, matt surfaces emit and absorb the most IR.
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Explain the particle arrangement in a solid
Strong forces of attraction hold the particles in fixed regular arrangements where they vibrate because they don't have much energy.
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Explain the particle arrangement in a fluid
Weaker forces of attraction allow the particles to to move freely at varying speeds.
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How does conduction happen in solids?
If the solid is not a metal, the vibration (the extra kinetic energy) is passed onto the neighbouring particles. If the solid is a metal, the free electrons have lots of kinetic energy so move faster and collide with other free electrons.
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Describe convection.
Convection occurs when more energetic particles move from the hotter regions to the cooler regions- taking their heat energy with them.
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What happens when something condenses?
When the gas cools the particles slow down and lose kinetic energy, so the attractive forces pull them together, if they get close enough they condense.
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What happens when something evaporates?
When the particles in a liquid have enough kinetic energy they can overcome the attractive forces between molecules and escape.
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How does evaporation have a cooling effect?
The fastest particles are the ones that escape, so they reduce the average speed and kinetic energy of the substance, so the energy of the remaining liquid falls.
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Name 4 things that increase the rate of evaporation
If the temperature is higher, if the density is lower, if the surface area is larger, if the airflow is greater.
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Name 4 things that increase the rate of condensation
If the temperature is lower, if the temperature of nearby surfaces is lower, if density is higher, airflow is less.
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What does the rate of heat transfer depend on?
The surface area of the object, the proportion of the object in contact with its surroundings, whether nearby objects are insulators or conductors.
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Name 4 ways of limiting heat transfer
Doubled walls, with a vacuum, silvered walls, insulating foam and an insulating stopper.
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What is the formula for Payback time?
Payback time= initial cost / annual saving
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List four ways to prevent heat loss in a house
Loft insulation, double glazing, cavity wall insulation and draught proofing.
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What do U Values show?
U Values show how fast heat can transfer through a material, the lower the U Value the better insulator the material is.
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What is specific heat capacity?
Specific heat capacity tells you how much energy a substance can store.
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Describe objects with a high specific heat capacity.
Materials which need to gain lots of energy to warm up also release lots when they cool down.
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