IGCSE Physics : Topic 2 Thermal Physics

2.1 - Simple kinetic molecular model of matter

Molecular Model

Solid : 1. Strong forces of attraction between particles
            2. Have a fixed pattern (lattice)
            3. Atoms vibrate but can’t change position.

Liquid : 1. Weaker attractive forces than solids
             2. No fixed pattern
             3. Particles slide past each other.

Gas : 1. Almost no intermolecular forces
          2. Particles are far apart, and move quickly, gases spread out to fill up the container and exert equal pressure on all surfaces.
          3. They collide with each other and bounce in all directions.

Characteristics of molecules
•The hotter a material is, the faster its particles move, and the more internal energy they have.
•The pressure gases exert on a container is due to the particles colliding on the container walls.
•If the volume is constant, then increasing the temperature will increase the pressure.
•If you look at smoke through a microscope, you will see the particles move in a zigzag motion. This is known as Brownian motion. The smoke particles have very little mass but are larger enough to be seen. They collide with the air particles randomly and move in different directions, to give a random motion.
•Liquids and gases do not have a fixed shape because of their weak forces of attraction. Gases can be compressed because there is plenty of space between the particles; solids can’t because such space does not exist. The particles in a solid cannot move because they are held tightly together by the attractive forces, but can vibrate.

2.2- Evaporation and boiling

Occurs at any temperature
Occurs only at the B.P. of liquid
Occurs only at the surface
Occurs throughout the liquid

How to increase rate of evaporation- Increase the temperature, surface area, reduce humidity, blow air across the surface

2.3 -Thermal Properties

Thermal expansion of solids, liquids and gases
-Solids, liquids and gasses expand when they are heated as the atoms vibrate more and this causes them to become further apart, taking up a greater volume.

Everyday applications and consequences:

-Hot water is used to heat up a lid of a jar, to make it expand, so that it is easier to remove

-The liquid in thermometers expand and contract when temperature changes, the volume of the liquid taken up in the tube can be used to find out the temperature

-Bimetal thermostat: when the temperature gets too high, the bimetal strip bends, to make contacts separate until the temperature falls enough, then the metal strip will become straight again and the contacts touch, to maintain a steady temperature

-Overhead cables have to be slack so that on cold days, when they contract, they don’t snap or detach.

-Gaps have to be left in bridge to allow for expansion (rollers allow the bridge to expand) .

-Expansion is highest in gases, then liquids and lowest in solids

Measurement of temperature

-Liquid-in-glass-thermometer - Expand on contract when there is a change of temperature

-Thermistor thermometer- Becomes a much better conductor when its temperature rises. This means that a higher current flows from the battery.

-Thermocouple thermometer- the probe contains 2 different metals joined metals to form 2 junctions. The temperature difference causes a tiny voltage which makes a current flow.

Characteristics of liquid-in-glass-thermometers:

Sensitivity: To increase the sensitivity of thermometers you have to put the liquid in a narrower tube. This makes more distance for same amount of expansion of liquid. Mercury expands less than alcohol. Sensitivity can be increased by using a material that expands more during a temperature change.

Range: The maximum and minimum temperature of thermometers Mercury = -39 °C to 500 °C Alcohol = -115 °C to 68 °C

Responsiveness: How long it takes for the thermometer to react to a change in temperature (increased by making the glass bulb thinner or making the bulb smaller)

Linearity: If the sizes of the individual degrees are closer to each other then it is more linear.

Heat Capacity

Specific Heat Capacity = The amount of energy needed to increase 1kg of a substance by 1C

Experiment Planning- things needed to know- Power of heater, time, temperature change, mass

Latent heat

Latent heat of fusion/ vaporisation = energy transformed/ mass

Experiment planning- things needed to know- Power of heater, time, mass

2.3 Transfer of Energy

Conduction- Thermal energy is transferred from the hot end to the cool end due to the vibrations of atoms when they gain energy from the heat.

More thermal energy is transferred if:
-The temperature is increased
-Cross-sectional area of the bar is increased
-Length of bar is reduced

When a material is heated, particles move faster, push on neighbouring particles and speed those up too.


-Only occurs in fluids (liquid and gas)

-Theory: As a fluid (liquid or gas) warms up, the particles which are warmer become less dense and rise. They then cool and fall back to the heat source, creating a cycle called a convection current. As particles circulate they transfer energy to other particles. If a cooling object is above a fluid it will create a convection current

-Practical uses: Sea breeze, Hot water system (Heated water in the boiler rises to the top of the storage tank), Refrigerator ( Cold air sinks from the freezer compartment)


-Radiate heat through electromagnetic waves

Matt Black

Preventing heat loss- Vacuum Flask
1) Insulated stopper to reduce conduction and convection
2) Double-walled container with a gap between the walls. Air has been removed from the gap(vacuum) to reduce conduction and convection
3) Walls with silvery surfaces to reduce thermal radiation.

Next Topic

Topic 1 General Physics

Topic 2 Thermal Physics

Topic 3 Properties of Waves, Including Sound and Light

Topic 4 Electricity and Magnetism

Topic 5 Atomic Physics

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