Topic 3.1: Thermal Concepts I

Question 1

What is temperature?

Answer 1

Temperature is the hotness or coldness of a body measured by a thermometer on a scale (°C or kelvin)

Question 2

What is temperature in terms of molecules?

Answer 2

In terms of molecules, temperature is a measure of the average kinetic energy of the random motion of molecules of a substance

Question 3

What is heat (thermal) energy?

Answer 3

Heat or thermal energy refers to the non-mechanical transfer of energy between a system and its surroundings

It is incorrect to say “thermal energy in a body”

Question 4

What direction does thermal energy flow?

Answer 4

Thermal energy is transferred (flows) from a HOT body at a HIGHER temperaure to a COLDER body at a LOWER temperature

Thermal energy can not be stored only transferred

Question 5

What are the requirements for something to be used in a temperature scale?

Answer 5

We need a property that changes (ideally in a linear way) with changing temperature

For a temperature scale we need fixed points

Question 6

For a fixed mass of GAS at constant pressure, what is volume proportional to?

Answer 6

For a fixed mass of gas at constant pressure, volume is proportional to the kelvin temperature or absolute temperature

Question 7

What are the three ways of thermal energy transfer?

Answer 7

1. Conduction
2. Convection
3. Radiation

Question 8

What is CONDUCTION?

Answer 8

Heat transfer from a hot (higher temp) to a colder place (lower temperature) through a material by vibration of its atoms (and free electrons for metals)

Question 9

What are insulators?

Answer 9

Insulators are materials with no free (delocalised) electrons and that can only conduct heat energy by atomic or molecular lattice vibrations

Question 10

Why are insulators bad thermal conductors?

Answer 10

Atoms at the hot end gain KE and increase the frequency of their vibrations. They collide with their neighbouring aoms, transfering KE and increasing their temperature

This process is slow, so insulators are poor thermal conductors

Question 11

What is CONVECTION?

Answer 11

Heat transfer from a hot to a colder place when the molecules of a fluid (liquid or gas) transfer the heat energy

Question 12

How does convection work?

Answer 12

1. Hot fluid (liquid or gas) expands, becomes less dense and rises
2. Colder fluid is more dense and sinks
3. This fluid is then heated and rises as in 1

This movement of fluid transfers heat energy upwards and is called a convection current

Question 13

What is RADIATION?

Answer 13

Heat transfer from a hot to a colder place by infrared electromagnetic waves

Question 14

Which materials are GOOD emitters and absorbers of heat energy?

Answer 14

Dark colours (black) and dull textures are good emitters and good absorbers of heat energy

Question 15

Which materials are POOR emitters and POOR absorbers of heat energy?

Answer 15

White or shiny colours are poor emitters and poor absorbers of heat

or

they are good reflectors of heat energy

Question 16

What is internal energy?

Answer 16

This is the total kinetic energy (due to random motion of molecules) AND intermolecular potential energy of the molecules of a body

Question 17

What is POTENTIAL ENERGY of MOLECULES?

Answer 17

Potential energy of molecules is due to their position in the (electric) force field between the molecules

Question 18

How does POTENTIAL ENERGY increase?

Answer 18

Potential Energy of the molecules increases when there is a change of state because molecular seperation increases

Work is done against attractive intermolecular molecular forces