Heat Transfer Flashcards
(22 cards)
what are thermometers used for
to measure the temperature which depends on how much the particles are vibrating
what is internal energy
the total amount of heat energy in an object
what is the difference between heat and temperature
hear is the total internal energy in a substance temperature is how much the particles are vibrating
how many kelvin is 0 degrees
273K
how many degrees is absolute zero
-273 degrees
explain expansion
when materials are heated it causes the particles to move more (in metals they vibrate more and with liquids they move faster) due to the energy from the heat. They take up more space from the particles moving causing the material to expand
explain conduction in metals
in metals there are lots of free electrons. these electrons move about at random in the metal and hold the positive ions together. When the metal is heated in one area, the free electrons in that area gain more kinetic energy and move faster. These electrons diffuse and collide with other free electrons and ions in the cooler parts of the metal. As a result, kinetic energy is transferred to these electrons and ions causing the heat to travel.
why are metals better conductors than non-metals
in a solid that isn’t metal, every can only be transferred through vibrations of the atoms that shake each other. This is much less effective than using free electrons in metals.
what are insulators
materials that don’t conduct heat
explain how convection works
convection only occurs in fluids (gas and liquids) and is due to convection currents in the fluids. the currents are caused by the fluids rising when they are heated (they become less dense) and then the fluids sink when they cool down (they become more dense). Convection currents transfer thermal energy from the hotter parts to the cooler parts.
why do fluids rise when heated
the fluids expand when heated, this means that the particles move about more and take up more space. This causes the density to decrease because the same mass of fluid occupies a bigger volume. so if you heat part of a fluid it will rise.
explain thermal radiation
every object emits thermal radiation, the hotter an object is, the more it emits. Thermal radiation can also be called infra-red radiation because it is beyond the red part in the visible spectrum. we can’t see the infrared rays so we can use special cameras to see objects in the dark from their thermal radiation. the sun emits solar radiation
what surfaces are the best emitters of radiation
dark matt surfaces emit the most radiation while light shiny surfaces emit the least. this is because when radiation hits matt surfaces the infrared ray will be scattered and absorbed while on a shiny surface it is just reflected and absorbed
what surfaces absorb radiation better
dark matt surfaces absorbs radiation better than light shiny surfaces because the cavities in the matt surface trap and absorb the radiation
if you had two beakers filled with the same amount of water but one had a black matt surface and one had a shiny light surface which would warm up quicker and why
the black matt one because the cavities in the surface would trap and absorb the radiation while the shiny surface would reflect it
what sort of blanket would be best to stop someone from getting cold
a light and shiny blanket as it emits the least radiation
why are car radiators painted black
because black emits the most heat, so when thermal energy is transferred from the car engine the radiator can quickly emit it so the engine doesn’t get too hot
what happens to the rate of heat transfer from a car engine to a radiator when the cooling fan is switched on
the rate of heat transfer speeds up. This is because when it isn’t switched on the air around the heater warms up but doesn’t move away, making the whole area around the radiator a similar temperature. When the cooling fan is switched on it moves the air away so the temperature gradient remains the same.
explain how the vacuumflask keeps the drink at the same temperature
- the vacuum prevents conduction and convection between walls
- the plastic cap is a good insulator
- the glass surface is silvered to stop emitting radiation
- the glass is a bad conductor so there isnt good conduction through the glass
- the insulated support is an insulator
why is cavity walls insulation better than just air
while air is a good insulator, if you have large spaces of it then convection currents could be formed. Cavity walls trap the air in a space between them and reduces heat loss, it is filled with insulation to stop heat leaving the house. and there are two other layers of insulation
what is evaporation
evaporation is when particles leave the surface of a liquid using their kinetic energy, it happens more with hot liquids beacuse there is more energy but can happen at any temperature. The particles that leave the surface have more energy that other particles and when they leave the average energy will decrease but evaporation will still occur
why does a bimetallic strip curve when it’s heated
It curves because one of the sides will have a faster rate of expansion when it’s heated. Because they are bonded the only way for one of the strips to expand faster than the other one will cause the strips to curve.
