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Flashcards in Heat Transfer Deck (22)
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Where is heat energy transferred to?

Heat energy is always transferred from hot objects to cold objects, or from a hot object to its surroundings.

eg putting hot pot into cold water. As heat E from the pot is transferred to the water, the pot cools down. The water absorbs heat E from the pot and becomes warmer.

As heat E from water is transferred to the ice, the ice melts. As the water loses heat E its temp drops.


3 ways in which heat energy is transferred

Conduction, convection and radiation.


Explain conduction

In conduction, heat energy is passed from particle to particle, with the heat energy moving from hot to cold areas. The particles themselves stay in the same places, but as they vibrate, they bump into the neighbouring particles, passing on the heat energy.

Heat E is absorbed by particles first.


What are thermal conductors

Substances that heat energy travels through easily. Metals are good conductors because the electrons in the metals help transfer the heat energy.


What are thermal insulators

Non-metals, liquids and gases are generally poor conductors of heat. Materials which are poor conductors are called insulators, and can be used to prevent heat from escaping.


How do Batts, Insul-Fluf, polystyrene, wool and furs provide warmth?

Air is a poor conductor, so they provide warmth because air is trapped.

This is how the building industry make use of the insulating properties of air to keep buildings warm in winter and cool in summer.


Example of insulation being used for wetsuits

Wetsuits are made from a type of rubber than contains small bubbles of trapped air. Conduction is the process where heat E is transferred by the vibration of particles. Air is an insulator, and prevents the transfer of heat by conduction as air is a gas, meaning its particles are far apart. Rubber is also a poor conductor/good insulator, but its particles are closer together than air. The cells of trapped air mean the wetsuit material is a better insulator than rubber alone. This insulates the person from the cold water outside the suit.


Explain convection

Convection is the heat transfer by the movement of particles from one place to another.

When a liquid or gas is heated, the particles in them speed up and move further apart. As a result, the hot liquid or gas expands and has less density, causing it to rise. The colder, denser liquid or gas sinks to take the place of the warm liquid of gas.


Where does convection occur?

ONLY in liquids and gases as the particles in solids cannot move from place to place.


What is a convection current?

The movement of hotter, less dense particles rising and the colder, denser liquid/gas sinking to take the place of the warm liquid or gas. This transfers heat E from one area to another.


Why is hotter water removed from the jug?

Because due to convection currents, the hotter liquid is always at the top.


Why is the exterior of a kettle made from plastic?

Because it is a good insulator/bad conductor of heat E, meaning that heat E from the water inside will not pass through easily, keeping the contents of the jug hotter for longer and making the jug safe to handle.


What causes the movement of tectonic plates?

The convection currents in the magma in the mantle of the Earth.


Explain convection between the land and the sea?

Sea breezes are convection currents. Water has a higher specific heat capacity than materials that make up land. So, land heats up quicker than the sea. The air above the land is heated up quicker from the hotter land than the air above the sea from the cooler sea. The hot air above the land, being less dense, rises and is replaced by cooler, denser air from above the sea. At night, the direction is reversed as the sea is warmer than the land, as due to water's high heat capacity it cools down slower


What is radiation?

Method of transfer of heat energy from one place to another without the need for a medium. Radiant heat consists of infrared radiation (a part of the electromagnetic spectrum). Radiant heat travels at the speed of light (3x10^8 ms^-1) and can travel through a vacuum.

Absorbed radiant heat causes an increase in the temperature of the object.


What can radiant heat do when it falls on an object (3)

-pass straight through it eg non-reflective glass
-be absorbed eg dull surface
-be reflected eg shiny surface.


What surface is the best absorber and reflector?

Dull, dark surfaces absorb a large amount of radiation and reflect very little. So the surface heats up rapidly.

Shiny, light-coloured surfaces reflect a large amount of radiation and absorb very little. So the surface warms only slightly.


What surface is the best emitter?

Dark surfaces are good emitters (radiators) of radiant heat. eg if boiling water had been placed in a dark and a light coloured can, the dull black can would cool down the fastest.

Because absorbing more = able to emit more.


Eg of heat energy and surface area

Heat energy from a car engine needs to be quickly removed from the car to prevent overheating. The radiator is painted black so that it emits radiant heat. Its fins help to increase the surface area so that more radiant heat is emitted.


Eg of keeping heat in in a travel mug

Heat E must NOT be easily transferred so that the drink can be kept hot or cold.

-The double wall with vacuum in between prevent heat transfer by conduction and convection as both processes require particles for heat transfer to occur, and vacuums have no particles.
-Silver coating on inner wall to prevent heat transfer by radiation. Reflect radiant heat energy back in the direction it came.
-very little heat E is transferred near the opening, because the opening is small and has a cover. Plastic not metal stopper is used as plastic is a poor conductor, meaning heat E isn't easily transferred through the stopper.


Eg of heat transfer in greenhouses

Radiant energy passes through the glass and is absorbed by the plants and soil. Heat radiated by plants and soil is absorbed by the air close to them. The air expands and rises, the colder denser air fills its place and convection currents are set up and eventually heats the whole greenhouse. Vents are placed at the top of the greenhouse so that the temperature can be controlled.


Eg of heat transfer in solar heating panels

Solar heating panels have dull, black surfaces to absorb heat energy. Solar ovens have shiny reflecting surface which is shaped to focus radiated heat energy onto the object to be heated.