Chapter 2: Temp and transfer of thermal energy Flashcards
Define temp and state its S.I unit
Measure of the average K.E of the molecules in a substance.
A measure of the degree of hotness.
It is a scalar quantity.
S.I = K
What is Thermal energy (TE) and how is it transferred?
It is the non-mechanical (happens by itself) transfer of energy from an object of higher temperature to one of lower temperature
How does a liquid in glass thermometer measure temp?
Variation of a volume of fixed mass of liquid (e.g alcohol or mercury)
What condition is necessary for a physical property to be used to measure temperature within a specified range?
Physical property used must have each value of the property correspond to a unique value of temperature within the specified range
e.g, between 0-8˚C the density of water is not unique, so we cannot use it to measure temp
between 0-4˚C water increases in vol and thus decreases in density (mass is still the same)(ice floats on water)
Compare celsius scale to kelvin scale
Celsius:
- Uses ICE POINT and STEAM POINT of water as fixed points
- Ice point (0˚C) = temp pure ice melts
- Steam point (100˚C) = temp pure water boils
(all @ S.T.P)
Kelvin:
- Uses ABSOLUTE ZERO and TRIPLE POINT OF WATER as fixed points
- Absolute zero (0K) = theatrical value of the lowest temp any substance can reach
- Triple point of water (273.16K) = temp where water exists in all 3 phases
Why does the Kelvin scale use triple point of water?
It is precisely reproducible
State relation of celsius and Kelvin scales of temp
T(K) = T(˚C) + 273
0˚C = 273K (ice point)
State relation between the temp diff. of 1˚C and 1K
1K change in temp is EQUAL to 1˚C change
(doesn’t mean the values are the same, but the amount of change they go through is equivalent)
What do we assume when calibrating a liquid in a glass thermometer?
Length of liquid in tube changes linearly with the temp between the fixed points
Formula for calibration
T = (length of T - length of lower limit/ length of upper limit - length of lower limit) x (upper limit - lower limit) + lower limit
Suggest what is implied when two objects are in thermal equilibrium (the same temp)
There is no net transfer of thermal energy
Describe how conduction transfers TE
Through collisions between neighbouring particles
- particles bump into each other and pass the energy
Describe how convection transfers TE
Through movement of a fluid due to density differences
- fluid = gas or liquid
- first particle takes the energy and moves with the energy
Describe how radiation transfers TE
Through electromagnetic waves
- doesn’t need a medium
- can take place in a vacuum
State the 7 electromagnetic waves
1) Gamma
2) X-ray
3) UV
4) Visible light
5) Microwave
6) Infrared
7) Radio waves
Suggest why metals are good thermal conductors
In metals, conduction takes places quickly through the vibrating atoms that are closely packed tougher.
Free electrons also help as they travel through spaces in between the atoms and transfers thermal energy through collisions at the cooler ends.
- This is KPT:
1) mode of TE transfer
2) speed of transfer
3) movement/ arrangement of particles
4) maybe bonds - KMM:
1) Forces of attraction
2) movement of molecules / particles
Suggest why heating coils are placed at the bottom of the vessel and why air conditioners should be placed near the top
Hot fluid, being less dense, will rise while cooler fluid, being more dense, will fall.
In kettles, the heating coil is placed at the bottom to heat up cooler fluid at the bottom.
In rooms, air cons are placed near the top to cool down hot air near the ceilings.
This will set up a convection current in the kettle/ room.
Why does density change happen?
Hot fluid expands, becomes less dense and rises. Colder fluid remains the same, is more dense than the hot fluid and thus sinks.
Describe how TE is transferred from the sun to the earth
Radiation. There is no medium for conduction or convection
State 4 factors that affect the rate of radiation and describe their effect
1) Temperature (body and ambient):
- Greater temperature difference –> higher rate of radiation (vice versa)
2) Texture of body/ surface:
- Rough surface (better absorber/ emitter) –> higher rate of radiation (vice versa)
3) Colour of body surface:
- Black surface (better absorber/ emitter) –> higher rate of radiation (vice versa)
4) Surface area:
- Greater surface area –> higher rate of radiation (vice versa)
Compare absorbers, emitters and reflectors
Absorbers:
- take IN energy
Emitters:
- give OUT energy
Reflectors:
-TRANSFER energy
generally: good absorbers and emitters = bad reflectors
What terms do you use for radiation?
Emitter, absorber, reflector
What terms do you use for conduction?
insulator, conductor
What terms do you use for convection?
Difference in density
Understand and state direction for using terms
Inside cold –> prevent TE from ENTERING –> good thermal insulators/ poor absorbers
- Takes in energy slower through conduction/ radiation
Inside hot –> prevent TE from LEAVING –> poor thermal conductors/ poor emitters
- Gives out energy slower through conduction/ radiation
insulators/ conductors is only for method of TE transfer by conduction
Why is a frozen pack placed @ the top of the lunch box?
The cooled air from the top is denser and will sink to the bottom while the warmer, less dense air rises to the top.
This sets up a convection current to keep the whole box cool.
Explain why the food keeps fresh longer if the walls are rigid plastic instead of metal
plastic is a good thermal insulator, so thermal energy will enter the box slower via conduction
- must state speed and mode of TE transfer
Explain why the food keeps fresh longer is the walls are lined with shiny and silvery inner linings
The shiny and silvery linings are good reflectors of thermal energy. Thus, thermal energy enters the box through radiation @ a slow rate via radiation
What surface makes a good reflector?
Shiny/ silvery surfaces
