Thermal Properties and Temperature

Section: Thermal Physics | Syllabus: Cambridge IGCSE Physics (0625)

Thermal Expansion of Solids, Liquids and Gases

When most substances are heated, they expand (get bigger); when they are cooled, they contract (get smaller). This is called thermal expansion.

heating a substance gives its particles more kinetic energy, so they move faster
- in a solid, the particles vibrate more strongly about their fixed positions
- in a liquid or gas, the particles move faster and, on average, spread further apart
this increases the average distance between particles, so the substance expands
cooling has the opposite effect - particles lose kinetic energy, move closer together on average, and the substance contracts
at constant pressure, all three states of matter expand on heating, but by very different amounts:
- solids expand the least
- liquids expand more than solids
- gases expand the most

small gaps are left between sections of railway track so the metal can expand in hot weather without buckling
bridges are built with expansion joints, which allow the bridge to expand and contract safely as the temperature changes throughout the year
overhead power cables sag more in summer, when they are longer due to thermal expansion, and become tighter in winter
if a structure is not given room to expand, thermal expansion can cause serious damage - for example, railway tracks can buckle, and pipes or window glass can crack, in very hot weather or if heated too quickly

Extended Only

in a solid, particles are held in fixed positions by strong forces between them and can only vibrate about those positions - heating increases the vibrations only slightly, so the increase in average particle separation, and the expansion, is small
in a liquid, the forces between particles are weaker, and particles are free to move around each other - heating lets them push slightly further apart on average, giving more expansion than in a solid
in a gas, the forces between particles are extremely weak, and the particles are already moving freely and are far apart - heating increases their speed and the space between them by a much greater amount, giving far more expansion than in a solid or liquid
this explains the order of expansion for the same rise in temperature: solid < liquid < gas

the particles in an object are constantly moving or vibrating, and have both kinetic energy (from their motion) and potential energy (from the forces between them) - the total of all this energy is called the object's internal energy
when an object is heated and its temperature rises, its internal energy increases

Extended Only

a rise in the temperature of an object corresponds to an increase in the average kinetic energy of all of the particles in the object
different substances need different amounts of energy to raise their temperature by the same amount - for example, a metal spoon left in a cup of tea heats up quickly, while the tea itself warms up much more slowly

the specific heat capacity (c) of a substance is the energy required, per unit mass, per unit temperature increase
- Formula:
  - c = (Δ E/m Δ θ)
- Where:
  - c = specific heat capacity (J/(kg°C))
  - Δ E = energy transferred (J)
  - m = mass (kg)
  - Δ θ = temperature change (°C)
rearranged, this gives the energy transferred for a given temperature change: Δ E = mcΔ θ
a substance with a high specific heat capacity (such as water, c ≈ 4200 J/(kg°C)) heats up and cools down slowly
a substance with a low specific heat capacity (such as copper, c ≈ 385 J/(kg°C)) heats up and cools down quickly

Substance	Approximate specific heat capacity (J/(kg°C))	Behaviour when heated
Water	4200	Heats up and cools down slowly
Aluminium	900	Moderate rate of heating
Copper	385	Heats up and cools down quickly

measure the mass, m, of the solid block
insert an electrical heater and a thermometer into holes drilled in the block
insulate the block (e.g. with cotton wool or foam) to reduce heat loss to the surroundings
record the initial temperature, then switch on the heater and start a stopwatch
record the voltage V and current I from the power supply
after a measured time t, switch off the heater and record the highest temperature reached, to find the temperature change Δ θ
calculate the energy transferred: Δ E = VIt
calculate the specific heat capacity: c = (Δ E/m Δ θ)

find the mass of liquid, m, by weighing an insulated container empty, then weighing it again after adding the liquid
place an electrical heater, a thermometer and a stirrer into the liquid
record the initial temperature, then switch on the heater and start a stopwatch, stirring continuously so the liquid heats evenly
record the voltage V and current I from the power supply
after a measured time t, switch off the heater and record the highest temperature reached, to find the temperature change Δ θ
calculate the energy transferred and the specific heat capacity as before: Δ E = VIt, c = (Δ E/m Δ θ)

stirring is essential for a liquid, but not needed for a solid, so the thermometer reads a temperature that is representative of the whole liquid, not just the region close to the heater
the container should be well insulated (e.g. an insulated calorimeter or vacuum flask) to reduce heat losses to the surroundings, which would otherwise make the measured value of c too high

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