Chapter 2: Temp and transfer of thermal energy

Question 1

Define temp and state its S.I unit

Answer 1

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

Question 2

What is Thermal energy (TE) and how is it transferred?

Answer 2

It is the non-mechanical (happens by itself) transfer of energy from an object of higher temperature to one of lower temperature

Question 3

How does a liquid in glass thermometer measure temp?

Answer 3

Variation of a volume of fixed mass of liquid (e.g alcohol or mercury)

Question 4

What condition is necessary for a physical property to be used to measure temperature within a specified range?

Answer 4

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)

Question 5

Compare celsius scale to kelvin scale

Answer 5

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

Question 6

Why does the Kelvin scale use triple point of water?

Answer 6

It is precisely reproducible

Question 7

State relation of celsius and Kelvin scales of temp

Answer 7

T(K) = T(˚C) + 273

0˚C = 273K (ice point)

Question 8

State relation between the temp diff. of 1˚C and 1K

Answer 8

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)

Question 9

What do we assume when calibrating a liquid in a glass thermometer?

Answer 9

Length of liquid in tube changes linearly with the temp between the fixed points

Question 10

Formula for calibration

Answer 10

T = (length of T - length of lower limit/ length of upper limit - length of lower limit) x (upper limit - lower limit) + lower limit

Question 11

Suggest what is implied when two objects are in thermal equilibrium (the same temp)

Answer 11

There is no net transfer of thermal energy

Question 12

Describe how conduction transfers TE

Answer 12

Through collisions between neighbouring particles
- particles bump into each other and pass the energy

Question 13

Describe how convection transfers TE

Answer 13

Through movement of a fluid due to density differences
- fluid = gas or liquid
- first particle takes the energy and moves with the energy

Question 14

Describe how radiation transfers TE

Answer 14

Through electromagnetic waves
- doesn’t need a medium
- can take place in a vacuum

Question 15

State the 7 electromagnetic waves

Answer 15

1) Gamma
2) X-ray
3) UV
4) Visible light
5) Microwave
6) Infrared
7) Radio waves

Question 16

Suggest why metals are good thermal conductors

Answer 16

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

Question 17

Suggest why heating coils are placed at the bottom of the vessel and why air conditioners should be placed near the top

Answer 17

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.

Question 18

Why does density change happen?

Answer 18

Hot fluid expands, becomes less dense and rises. Colder fluid remains the same, is more dense than the hot fluid and thus sinks.

Question 19

Describe how TE is transferred from the sun to the earth

Answer 19

Radiation. There is no medium for conduction or convection

Question 20

State 4 factors that affect the rate of radiation and describe their effect

Answer 20

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)

Question 21

Compare absorbers, emitters and reflectors

Answer 21

Absorbers:
- take IN energy

Emitters:
- give OUT energy

Reflectors:
-TRANSFER energy

generally: good absorbers and emitters = bad reflectors

Question 22

What terms do you use for radiation?

Answer 22

Emitter, absorber, reflector

Question 23

What terms do you use for conduction?

Answer 23

insulator, conductor

Question 24

What terms do you use for convection?

Answer 24

Difference in density

Question 25

Understand and state direction for using terms

Answer 25

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

Question 26

Why is a frozen pack placed @ the top of the lunch box?

Answer 26

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.

Question 27

Explain why the food keeps fresh longer if the walls are rigid plastic instead of metal

Answer 27

plastic is a good thermal insulator, so thermal energy will enter the box slower via conduction

• must state speed and mode of TE transfer

Question 28

Explain why the food keeps fresh longer is the walls are lined with shiny and silvery inner linings

Answer 28

The shiny and silvery linings are good reflectors of thermal energy. Thus, thermal energy enters the box through radiation @ a slow rate via radiation

Question 29

What surface makes a good reflector?

Answer 29

Shiny/ silvery surfaces