Transfer of heat energy and limiting factors Flashcards
From where does infrared radiation come and how does it interact with objects?
All objects emit and absorb infrared radiation (THEY CAN ONLY DO ONE AT A TIME), and the hotter the object, the more it radiates in a given time
What properties do different materials have with regard to infrared radiation?
Dark, matt surfaces are good absorbers and good emitters (either one at a time)
Light, shiny surfaces and poor absorbers and poor emitters (either one at a time), and good reflectors
What does the heat energy contained in an object equal?
What does the temperature measure?
Heat energy = TOTAL kinetic energy of all the atoms in an object
Temperature = AVERAGE kinetic energy of all the atoms in an object
What is the structure of a solid like?
What happens when the solid is heated?
The vibrating atoms are held in a rigid 3D structure by ‘spring-like’ bonds
The atoms vibrate more (the temperature increases), the structure bonds become bigger and the solid expands
With enough heat, some bonds snap and the rigid structure becomes more ‘floppy’, and becomes a liquid
What is the structure of a liquid like?
What happens when the liquid is heated?
Loose structure that moves around fluidly, but the atoms stay connected by spring-like bonds
Vibration of atoms increases, and all the bonds snap, leaving individual atoms moving around independently
In a bar of plastic being heated at one end, A, why does the other end, B, eventually become warm?
Due to conduction (in a non-metal solid)
Atoms at A vibrate more, and the vibrations/kinetic energy passes along the spring-like bonds until it gets to B. The atoms at B have more KE and so the temperature has increased
In a bar of aluminium being heated at one end, A, why does the other end, B, eventually become warm?
Due to conduction (in a metal)
Metal atoms, held together by spring-like bonds, are surrounded by electrons that can move freely (free/delocalised electrons). At A, the KE of the electrons increases, and they will diffuse (move) to B, taking the KE with them
If a liquid or gas (fluid) is heated at one part, how does the heat spread throughout the whole fluid?
Convection
Hot fluids are less dense than the same fluid cooled, because materials expand with heat. Less dense fluids rise to the top of more dense fluids, so, when some fluid is heated, it rises, and the gap it leaves is replaced by cold fluid, creating a circular motion, and spreading the hot fluid throughout
What properties make a material a good insulator?
They contain trapped air
This is because air is a poor conductor of heat, and so can only transfer heat through convection. If the air cannot move, then convection can’t happen. It should be white or silver, to reduce heat lost by radiation
How does evaporation work?
The particles at the SURFACE of a liquid have varying levels of kinetic energy. The particles with the highest amount can break their spring-like bonds and escape from the liquid and enter the air
Why does evaporation cause cooling?
How can we increase this cooling effect?
When the particles with the MOST KE escape, that leaves the rest with a lower AVERAGE KE
Providing more KE, by blowing on the liquid or moving it around. This means that the particles that previously were almost able to escape but could not quite make it, now can. This further reduces the average KE
What factors increase the rate of evaporation?
Surface area, temperature and presence of wind/movement
In general, what factors affect the rate at which an object transfers energy?
Surface area and volume (the higher the surface area, the higher the rate)
Material of the object
The nature of the surface with which the object comes into contact
What does a U-value measure?
How does one read a U-value?
How effective a material is as an insulator
The lower the U-value, the better the material for insulation
How do water solar panels work?
They contain water that is heated by radiation from the Sun. The water is used to heat buildings or provide hot water for taps, showers, etc.
