Why do railway tracks look straight in winter but need checking in summer?
That tiny shift is all about heat, temperature and something called thermal expansion. Let’s unpack the three ideas you’ll see on every ICSE exam.
Heat is the energy that makes particles jiggle faster. Temperature tells us how fast they’re jiggling on average. When things get hotter, they usually get a bit bigger – that’s thermal expansion.
What is Heat?
Heat (capital H) is energy transferred because of a temperature difference. Think of heat like water flowing from a higher pipe to a lower one – it moves from hot to cold until everything evens out.
Heat can travel in three ways:
- Conduction: direct contact, like a metal spoon getting warm in a pot.
- Convection: movement of fluid, like warm air rising.
- Radiation: energy carried by invisible waves, like sunlight.
Understanding Temperature
Temperature is a measure of the average kinetic energy (the energy of motion) of the particles in a substance. If you imagine a crowd of people dancing, temperature is like the average speed of the dancers.
We use three common scales:
- Celsius (°C): water freezes at 0 and boils at 100.
- Kelvin (K): starts at absolute zero, the point where particles stop moving.
- Fahrenheit (°F): used mainly in the US.
Conversion you’ll need:
°C to K: K = °C + 273.
°C to °F: °F = (°C × 9/5) + 32.
How Heat Makes Things Expand
When a material absorbs heat, its particles vibrate more vigorously. Those bigger vibrations push neighbors a little farther apart, so the whole object gets larger. This is called thermal expansion.
There are three types:
- Linear expansion: length change in a rod or wire.
- Area expansion: change in surface area of a plate.
- Volume expansion: change in volume of a liquid or gas.
Linear expansion formula
ΔL = α L₀ ΔT
where:
- ΔL = change in length
- α = coefficient of linear expansion (how much a material expands per degree)
- L₀ = original length
- ΔT = change in temperature (final minus initial)
Worked example
Problem: A steel rail 25 m long is heated from 20 °C to 80 °C. The coefficient of linear expansion for steel is 12×10⁻⁶ °C⁻¹. Find the increase in length.
Solution:
- ΔT = 80 °C – 20 °C = 60 °C.
- ΔL = α L₀ ΔT = 12×10⁻⁶ × 25 × 60 = 0.018 m = 1.8 cm.
The rail becomes 1.8 cm longer – enough to cause a tiny buckle if not accounted for.
Quick Comparison: Heat vs Temperature vs Internal Energy
| Aspect | Heat (Q) | Temperature (T) | Internal Energy (U) |
|---|---|---|---|
| What it measures | Energy transferred because of a temperature difference | Average kinetic energy of particles | Total kinetic + potential energy of all particles |
| Units | Joules (J) | Kelvin (K) or Celsius (°C) | Joules (J) |
| How it changes | Added or removed by conduction, convection, radiation | Rises when heat is added, falls when heat leaves | Changes when heat is added or work is done on the system |
Key Formulas to Remember
- Heat gained or lost: Q = m c ΔT
m = mass, c = specific heat capacity (energy needed to raise 1 kg by 1 °C), ΔT = temperature change. - Linear expansion: ΔL = α L₀ ΔT
- Area expansion: ΔA = 2α A₀ ΔT (approx.)
- Volume expansion: ΔV = β V₀ ΔT, where β ≈ 3α for solids.
Summary Points
- Heat flows from hot to cold; temperature tells how hot something is.
- Higher temperature → faster particle motion.
- Thermal expansion is the macroscopic result of microscopic particle vibration.
- Always check the coefficient (α) for the material you’re dealing with.
- Remember the sign of ΔT: positive when temperature rises, negative when it falls.
📝 Likely Exam Questions
- Define heat and temperature. State one difference between them.
Heat is energy transferred due to a temperature difference; temperature is a measure of the average kinetic energy of particles. Difference: heat is transferred energy, temperature is a property of the material. - A copper wire 2 m long expands by 0.012 mm when heated from 25 °C to 75 °C. Find the coefficient of linear expansion of copper.
ΔL = 0.012 mm = 1.2×10⁻⁵ m, ΔT = 50 °C, L₀ = 2 m.
α = ΔL/(L₀ ΔT) = 1.2×10⁻⁵ / (2×50) = 1.2×10⁻⁷ °C⁻¹. - Explain why a bimetallic strip bends when heated.
A bimetallic strip consists of two metals with different α values bonded together. When heated, the metal with larger α expands more, causing the strip to curve toward the metal with smaller α. - Calculate the heat required to raise 500 g of water from 20 °C to 80 °C. (Specific heat capacity of water = 4.18 J g⁻¹ °C⁻¹)
Q = m c ΔT = 500 × 4.18 × (80‑20) = 500 × 4.18 × 60 = 125,400 J. - State two practical applications of thermal expansion.
(i) Expansion joints in bridges to accommodate length changes.
(ii) Thermometers, where liquid expands in a narrow tube to indicate temperature.