Ever wondered why a metal bridge looks like it’s breathing on a hot summer day?
💡 In Simple Words: Heat is energy that moves from a hotter object to a cooler one, while temperature tells us how fast the tiny particles inside a material are buzzing. When something gets hotter, its particles jiggle more and push each other apart, making the object expand.
What is Temperature?
Temperature is a measure of the average kinetic energy – that’s the energy of motion – of the particles (atoms or molecules) in a substance. Think of a thermometer like a ruler that measures how fast the particles are dancing. The higher the temperature, the faster the dance.
How We Measure Temperature
- Degree Celsius (°C) – the scale you see on school thermometers.
- Kelvin (K) – the scientific scale where 0 K is absolute zero, the point where particles stop moving.
What is Heat?
Heat is the form of energy that flows because of a temperature difference. It’s not “hotness” itself; it’s the energy that moves from a higher‑temperature body to a lower‑temperature one. Imagine water spilling from a full bucket into an empty one – the water (energy) moves because of a difference in level (temperature).
Units of Heat
- Joule (J) – the SI unit, same as the unit for work.
- Calorie (cal) – 1 cal = 4.184 J, often used in chemistry and nutrition.
Heat vs. Temperature – Quick Comparison
| Aspect | Temperature | Heat |
|---|---|---|
| What it measures | Average kinetic energy of particles | Total energy transferred because of temperature difference |
| Units | °C, K | J, cal |
| Nature | Scalar property of a system | Energy in transit |
| Example | Ice at 0°C vs. water at 0°C | Heat absorbed when ice melts to water |
Thermal Expansion – Why Things Grow When They Get Hot
Thermal expansion is the tendency of matter to change its shape, area or volume when its temperature changes. The particles vibrate more when heated and need a little more space, so the material expands.
Linear Expansion Formula
For a solid rod, the change in length (ΔL) is given by:
ΔL = α L₀ ΔT
where:
- α = coefficient of linear expansion (how much a material expands per degree)
- L₀ = original length
- ΔT = change in temperature (final minus initial)
Worked Example
A steel rod 2.00 m long is heated from 20°C to 100°C. The coefficient of linear expansion for steel is 12 × 10⁻⁶ °C⁻¹. Find the increase in length.
Step 1: Find ΔT = 100 °C – 20 °C = 80 °C.
Step 2: Plug into the formula:
ΔL = (12 × 10⁻⁶ °C⁻¹) × (2.00 m) × (80 °C) = 1.92 × 10⁻³ m = 1.92 mm.
So the rod becomes about 2 mm longer – enough that railway tracks need expansion joints.
Real‑World Examples
- Railway tracks have small gaps to allow for expansion in summer.
- Thermostat bimetal strips bend because two metals expand at different rates.
- Bridge decks often have expansion bearings to avoid buckling.
How Heat Moves – The Three Modes
Conduction is the direct transfer through a material (like a metal spoon getting hot in soup). Convection is the transfer via moving fluids (warm air rising). Radiation is transfer through electromagnetic waves (the Sun’s heat reaching Earth).
Key Points to Remember
- Temperature measures average particle speed; heat is energy in motion.
- Heat flows from hot to cold until temperatures equalise.
- Linear expansion: ΔL = α L₀ ΔT; area expansion uses 2α, volume expansion uses 3α.
- Materials have different α values – metals usually expand more than glass.
- Engineers design expansion joints, bimetallic strips and bridges keeping thermal expansion in mind.
📝 Likely Exam Questions
- Define temperature and heat. How are they different?
Temperature is the measure of average kinetic energy of particles; heat is the energy transferred due to a temperature difference. - State the formula for linear expansion and explain each term.
ΔL = α L₀ ΔT; ΔL is change in length, α is coefficient of linear expansion, L₀ is original length, ΔT is temperature change. - A copper wire 1.5 m long expands by 0.27 mm when heated from 25°C to 75°C. Find α for copper.
ΔL = 0.27 mm = 2.7 × 10⁻⁴ m; ΔT = 50°C. α = ΔL/(L₀ ΔT) = 2.7 × 10⁻⁴ / (1.5 × 50) = 3.6 × 10⁻⁶ °C⁻¹. - Explain why railway tracks have small gaps between them.
Because steel expands on heating; gaps (expansion joints) accommodate the increase in length, preventing buckling. - List and briefly describe the three modes of heat transfer.
Conduction – transfer through direct contact; Convection – transfer by fluid motion; Radiation – transfer via electromagnetic waves.