Transfer of thermal energy Flashcards
How is heat transferred in solids (conduction)?
the particles which vibrate about a fixed spot gain energy, colliding with the particles next to them and transferring thermal energy
Define ‘conduction’
heat transfer from a hotter to a colder place through a material either by:
- vibration of its atoms
- free electrons (for metals)
Why are metals good thermal and electric conductors?
their delocalised electrons can move throughout the metal
How does conduction occur via delocalised electrons?
electrons gain ke from hot end of metal, moving and colliding with atoms, increasing their KE and temp.
Why are gases poor thermal conductors?
the particles are far apart, making it hard for energy to be transferred from one particle to another
Why are most liquids poor thermal conductors?
since particles are free to flow, making it hard for energy to be transferred easily as it is a lattice
What is one experiment to test conductivity?
put boiling water in a box with holes
place rods of different materials in these holes
place drawing pins held with petroleum jelly on the far ends of rods
see which drawing pins fall of first due to conduction of heat from water
Define ‘convection’
method of thermal energy transfer in liquids and gases
How does convection work in liquids/gases? What is the name given to this movement?
hot fluid expands, becomes less dense and rises
cold fluid sinks as it is more dense
the colder fluid is then heated and rises again, forming a convection current
What is a convection current?
movement of fluid that transfers heat energy upwards
Describe one experiment to test convection
fill a beaker with water, then place a potassium manganate crystal into the beaker
place a bunsen burned flame right under the crystal
you will see the convection from the movement of color in the beaker
Define ‘radiation’
heat transfer from a hot to a colder place by infrared electromagnetic waves
Which object emit thermal/infrared radiation?
all of them :)
Does infrared radiation require a medium?
no, like all electromagnetic waves
Which colours and textures are good emitters and absorbers of heat energy?
dark colours (black)
dull textures
Which colours are poor emitters and absorbers of heat energy?
white/shiny colours
Which colours are good reflectors of heat energy?
white/shiny colours
In terms of energy, what is necessary for an object to be at a constant temperature?
transfer energy away from object at same rate that it receives energy
How does the Earth’s average temperature remain constant?
infrared radiation energy per second absorbed from sun is balanced by i.r energy per second emitted to space
What is one experiment to test good/bad emission of infrared radiation?
place two metal cans (one silver, one matte black) with hot water, a thermometer and a lid on them
observe that the black one cools down faster since they emit better
What is one experiment to test good/bad absorption of infrared radiation?
take two metal cans (one matte black, one silver) with water, a thermometer and a lid on them
place them equidistantly from an electric radiant heater
black will heat up faster since it absorbs heat better
What are two factors affecting rate of emission of radiation?
- surface temperature
- surface area
The hotter an object is, the ____ infrared radiation it emits.
more
A bigger object (larger s.a) emits ____ radiation than a smaller object (ignoring other factors)
more
objects with _____ surface areas will absorb more radiation than those with ______ surface areas.
large
small
