Physics IGCSE - Thermal physics

Properties of solids

• Fixed shape + volume

• Particles close together

• Vibrate in fixed positions

• Cannot be compressed

Properties of liquids

• Fixed volume but no shape

• Particles close together but can slide over each other

• Not easily compressed

Properties of gases

• No fixed shape and volume

• Particles are far apart + move freely

• Easily compressed

Changes inbetween states

Relationship between temperature and the motion of particles

Temperature increases → particles energy increases → motion increases → moves faster

Evidence for the kinetic particle model of matter

The random motion of particles in a suspension → e.g. smoke particles can be viewed in microscopes

How forces and distance between particles affect the properties of matter → solids

Strong forces → particles close together → fixed shape

How forces and distance between particles affect the properties of matter → liquids

Weaker forces → particles slightly apart → flow

How forces and distance between particles affect the properties of matter → gases

Very weak/no forces → large distances → compressible + expandable

Brownian motion

Random movement of visible particles → due to collisions with smaller, fast moving particles in a gas or liquid

What causes gas pressure

Particle colliding with the walls of a container → creates force per unit area

Gas pressure when temperature increases at a constant volume

Particles move faster → collide with wall more frequently + greater force → pressure increases

Gas pressure when volume increases at constant temperature

Particles have more space → collide with wall less frequently → pressure decreases

Thermal expansion of solids at constant pressure

Expands slightly → particles vibrate more

Thermal expansion of liquids at constant pressure

Expands more than solids

Thermal expansion of gases at constant pressure

Expands the most → particles move more freely + further apart

Everyday applications of thermal expansion

• Gaps in railways → allow for expansion

• Thermometers → liquid expansion

• Bridges → have expansion joints

• Overhead cables → sag in summer due to expansion

Condensation in terms of particles

Gas particles lose energy → move closer together → form a liquid

Solidification in terms of particles

Liquid particles lose more energy → become fixed in place as a solid

Evaporation

Only most energetic particles at the surface escape the liquid → those particles overcome attractive forces → become gases

Evaporation causes cooling of a liquid

What happens to energy inputs during melting or boiling

Energy breaks bonds between particles → not to raise temperature → temperature stays constant during melting/boiling

Difference between evaporation + boiling

Evaporation	Boiling
Occurs at fixed temperature → throughout liquid	Happens at any temperature → only at surface

Factors affecting evaporation

• Higher temperature → faster particle motion

• Larger surface area → more particles escape

• Air movement → removes particles above surface → increase evaporation

Examples of good thermal conductors

Metals → e.g. copper, aluminium + silver

Examples of bad thermal conductors (insulators)

Wood, plastic, glass + air

What causes thermal conduction in solids

Vibrations of particles in a lattice → vibrations transfer energy to neighbouring particles → faster vibrations → higher temperature → more energy transfer

Why metals are good conductors

Delocalised electrons → move through the structure → electron transfers thermal energy quickly by colliding with ions + other electrons

Convection

Important method of heat transfer for liquids + gases

Convection in liquids and gases

Heated fluid expands → becomes less dense → rises → cooler + denser liquids sinks → creates a convection current → transfers thermal energy through liquids → particles gain energy → move faster + spread out → decreases density

Convection in liquids + gases in terms of density changes

When a fluid is heated → particles gain energy + move apart → decreases its density → convection current occurs → movement transfers heat throughout the fluid

Thermal energy transfer by thermal radiation → doesn’t require a medium → due to infrared radiation

How black + dull surfaces affect thermal radiation

Black, dull surfaces → good absorbers + transmitters

How white + shiny surfaces affect thermal radiation

White, shiny surfaces → poor absorbers + transmitters → better at reflecting thermal radiation

How the temperature of the Earth is affected

• Radiation absorbed by Earth

• Radiation emitted by Earth

Experiment to find good + bad emitters of thermal radiation

2 metal cans (black + white) filled with hot water → measure how quickly they cool → black can emits radiation faster (cools faster) → better emitter

Experiment to find bad + good absorbers of thermal radiation

Black + white surface under heat lamp → measure temperature over time → black surface heats up faster → better absorber

Basic examples of conduction

• Metal saucepan handles get hot → metals conduct heat

• Plastic/wood handles → insulators → prevent burns

Basic examples of convection

• Heaters → warm a room → sets up convection currents in the air

• Hot air rise → cools → sinks → circulates heat

Basic examples of radiation

• Black clothing feels hotter in sun → absorbs more radiation

• Shiny foil blankets → reflects heat to keep person warm