Unit 5 – Thermal effects
 5.01 Moving particles
 The kinetic theory of matter
 Solids, liquids and gases and their particles
 How the properties of solids, liquids and gases depend on the motion and arrangement of their particles (e.g. molecules)
 Brownian motion and why it occurs
 The link between internal energy, moving particles
(molecules) and temperature
 5.02 Temperature (1)
 The link between internal energy and temperature
 Measuring temperature: the principles
 The link between temperature and the motion of particles
(e.g. molecules)
 Thermocouple thermometers: how they work, and their advantages

Unit 5 – Thermal effects
 5.03 Temperature (2)
 Defining a temperature scale: fixed points
 Melting point and boiling point
 The structure and action of a liquid-in-glass thermometer
 Thermocouple thermometers: how they work and their advantages
 The sensitivity, range and linearity of a thermometer
 5.04 Expanding solids and liquids
 The thermal expansion of solids and liquids, its effects and its uses
 Comparing the expansions of solids, liquids and gases

Unit 5 – Thermal effects
 5.05 Heating gases
 How the pressure of a gas is caused by the motion of its particles (molecules)
 Why pressure increases with temperature for a gas at constant volume
 Why volume increases with temperature for a gas at constant volume
 Comparing the expansions of solids, liquids and gases
 How gas pressure is caused by momentum changes of particles (molecules)
 Explaining why, when heated (at constant pressure), gases expand much more than liquids and liquids more than solids
 5.06 Heating gases
 Good and poor thermal conductors
 Why some materials are better thermal conductors than others

Unit 5 – Thermal effects
• 5.05 Heating gases
• 5.06 Thermal conduction
• 5.07 Convection
• 5.08 Thermal radiation
• 5.09 Liquids and vapours
• 5.10 Specific heat capacity
• 5.11 Latent heat

5.01 Moving particles
Solids, liquids, and gases

5.01 Moving particles
Solids, liquids, and gases

5.01 Moving particles
Solids, liquids, and gases

5.01 Moving particles
Brownian motion: evidence for moving particles
 In 1827, a botanist named Robert Brown observed the random movement of particles in liquids.
 Observation:
 pollen grains wobbling and jiggling about in water.
 Explanation:
 the pollen grains are being bombarded by the molecules of water around them
 This movement is called Brownian motion .
 It proves that matter is made up of tiny particles.

5.01 Moving particles

5.01 Moving particles
Brownian motion: evidence for moving particles
 Brownian motion also explains how smoke particles move about in air.
 Observation:
 Smoke particles wobble about in zig-zag paths.
 Explanation:
 Air particles are bumping into the smoke particles.

5.01 Moving particles
Energy of particles
 The particles in solids, liquids, and gases have kinetic energy because they are moving.
 Solids – low kinetic energy – vibrates about fixed position
 Liquids – moderate kinetic energy – moves randomly
 Gases – high kinetic energy – moves are high speed
 The particles also have potential energy because there are forces of attraction trying to hold them together.
 Solids – low potential energy – fixed position
 Liquids – moderate potential energy – close together/ not fixed and glide over one another
 Gases – high potential energy – very far apart from one another

5.01 Moving particles
Energy of particles
 Internal energy = potential energy + kinetic energy of all the particles in a substance
 When:
 Substances get hotter  particles move faster  internal energy 
 Substances get colder
 particles move slower
 internal energy 
 Hot substances transfer energy to cold substances – called heat.
 Hot substance loses internal energy.
 Cold substance gains internal energy.

5.02 Temperature (1)
The Celsius scale
 Temperature is a measure of hotness .
 This measure is hotness comes from the average kinetic energy per particle in a substance.
 Hence:
1. Higher temperature = higher average kinetic energy per particle
2. Lower temperature = lower average kinetic energy per particle
 Temperature is NOT the same as heat.
Heat is a form of energy but temperature is not energy.

5.02 Temperature (1)
The Celsius scale
 Temperature is a measure of hotness .
 The Celsius scale aka the Centigrade scale is very commonly used.
 Unit: degree Celsius ( ° C)
 The Celsius scale has two fixed points:
1. The melting point of water: 0 ° C
2. The boiling point of water: 100 ° C
Practical activity 5.3 – Determining absolute zero

5.02 Temperature (1)
Absolute zero and the Kelvin scale
 The lowest temperature in the universe = –273 ° C
 At this temperature, particles have
 the lowest kinetic energy ;and
 move the slowest
 This is why this temperature is called absolute zero .
 The Kelvin scale starts at absolute zero – 0K .
Temperature in Kelvin = Temperature in °C + 273

5.02 Temperature (1)
Thermometers
 Temperature is measured using thermometers.
Liquid-in-glass thermometers
 Glass bulb
 contains liquid (alcohol or mercury)
 thin glass:
1.
increase sensitivity of thermometer – allow heat to enter and leave quickly
 Liquid in bulb
 expands when temperature rises and pushes a thread of liquid into the capillary tube.
 thin tube:
1.
increase sensitivity of thermometer – thread moves easily for a small change in temperature

5.02 Temperature (1)

5.02 Temperature (1)
Thermometers
Thermocouple thermometer
 Made up of:
1. Two different metals
2. Two different junctions – inserted into two substances of different temperatures
3. A meter – converts potential difference into temperature
 How it works:
1. Temperature difference causes a current to flow
 The higher the temperature difference, the greater the current flow.
 The greater the current flow, the greater the potential difference.
2. The meter converts the potential difference into temperature.

5.03 Temperature (2)
Fixing a temperature scale
 To create a temperature scale, two standard temperatures must be chosen i.e.
two fixed points .
 Calibrating an instrument means to put a scale on a measuring instrument so that it gives accurate readings.
 The fixed points chosen for the Celsius scale are: 0 °C and
100 °C .
 Step 1:
 Put the column into ice and measure the height of the increase in liquid/ mercury.
 Step 2:
 Put the column into boiling water and measure the height of the increase in liquid/ mercury.
 Step 3:
 Divide the distance between the two fixed points into 100 equal divisions. Each division is called 1 °C.

5.03 Temperature (2)
Liquid-in-glass thermometers
 Liquids are used in thermometers because they expand when heated.
 This expansion causes them to move up the column and give a reading.
 Sensitivity
 How far the liquid moves when heated
 Smaller column/ tube width will increase sensitivity
 Mercury expands less than alcohol – so the tube of a mercury thermometer needs to be narrower to have same sensitivity as a liquid-in-glass thermometer
 Thicker glass/ bulb will reduce responsiveness because it reduces heat absorption