Understanding Heat: Conduction, Convection, and Radiation
Simple explanations, real-world examples, step-by-step experiments, and quick calculations for Class 7 students.
What is Heat?
Heat is energy that transfers between objects because of a difference in temperature. Heat always flows from a hotter object to a cooler object until both reach the same temperature (thermal equilibrium).
In this post we'll learn the three main ways heat moves: conduction, convection, and radiation — plus easy examples and experiments you can do at home or in class.
Conduction — heat transfer through direct contact
Definition: Conduction is the transfer of heat through a material without the material itself moving. It happens mostly in solids, especially metals.
How it works (simple)
When one end of a metal rod is heated, the particles there vibrate faster and pass on that energy to neighboring particles. This chain of energy transfer moves heat along the rod.
Formula (basic idea)
Qualitative: More contact area, higher temperature difference, and better thermal conductivity give faster conduction.
Q = (k · A · ΔT · t) / Lwhere Q is heat (J), k is thermal conductivity (W/m·K), A is cross-sectional area (m²), ΔT is temperature difference (K), t is time (s), and L is length (m).
Worked numeric example
Suppose a metal rod (length 0.5 m, area 0.002 m²) connects a hot plate at 100 °C to a cooler end at 30 °C. Thermal conductivity k ≈ 50 W/m·K. How much heat flows in 60 s?
- ΔT = 100 − 30 = 70 K
- Use Q = (k·A·ΔT·t)/L
- Q = (50 × 0.002 × 70 × 60) / 0.5
- Q = (50 × 0.002 × 70 × 60) / 0.5 = (50 × 0.14 × 60) / 0.5 = (50 × 8.4) / 0.5 = 420 / 0.5 = 840 J
Answer: About 840 J of heat transfers in 60 seconds (note: this is a simplified calculation; real systems have losses).
Simple experiment (classroom or home)
- Take a metal spoon and a wooden spoon.
- Place both handles outside a warm tea cup and their heads inside the tea.
- After a minute, carefully feel which handle becomes warm. The metal conducts heat quickly; the wooden spoon remains cooler.
Safety: Do this with warm, not boiling, liquids and with adult supervision.
Convection — heat transfer by fluid movement (liquids & gases)
Definition: Convection transfers heat by the bulk movement of a fluid (liquid or gas). Warm fluid becomes less dense, rises, and cooler fluid moves in to replace it, forming convection currents.
Real-world examples
- Boiling water: hot water rises from the bottom, cooler water sinks.
- Sea breeze: air warmed over land rises and draws cooler air from the sea.
- Central heating: warm air circulates around a room.
Worked example (conceptual)
Imagine heating one side of a rectangular aquarium. The water near the hot side warms, rises, and cooler water flows in, creating a circulation loop that distributes heat across the tank. This loop speeds up as ΔT increases.
Simple experiment
- Fill a clear glass with cold water.
- Carefully add a drop of food coloring near the bottom at one side.
- Gently warm the opposite side of the glass with a hair dryer at low setting or a warm cloth (adult supervision).
- Watch how colored water rises and sets up currents — this shows convection.
Safety: Don’t use boiling water. Use warm water and keep hair dryers on low with adult help.
Radiation — heat transfer by electromagnetic waves
Definition: Radiation is energy transferred by electromagnetic waves (infrared, visible light, etc.). It does not need particles or a medium — it can travel through empty space.
Key idea
The Sun heats Earth through radiation — energy travels through space and warms surfaces that absorb it.
Worked numeric note (Stefan-Boltzmann, simple)
For advanced students: total power radiated per unit area of a perfect blackbody is P/A = σ T⁴, where σ ≈ 5.67×10⁻⁸ W/m²K⁴ and T is absolute temperature (K).
Simple experiment
- Take two identical metal tins. Paint one black and the other white.
- Place them in sunlight or under a bright lamp at the same distance.
- After a few minutes, the black tin will be warmer — black surfaces absorb more radiant energy.
Do this in safe sunlight (not too hot) or under a lamp; avoid direct prolonged skin exposure to intense lamps.
Quick Comparison
| Type | Where it happens | Main features |
|---|---|---|
| Conduction | Solids (metals) | Direct contact, particle-to-particle transfer |
| Convection | Liquids & gases | Fluid motion, currents, large-scale transfer |
| Radiation | Any (including vacuum) | Energy by electromagnetic waves; no medium required |
Visual diagram (simple)
Applications & everyday importance
- Cooking: conduction in pans, convection in boiling water, and radiation from ovens.
- Weather & climate: convection drives winds and storms; radiation from the Sun warms Earth.
- Engineering: insulating materials reduce conduction; HVAC systems use convection principles.
Quick Quiz / Worksheet
- Give one everyday example each of conduction, convection, and radiation.
- Which material conducts heat better: copper or wood? Why?
- Why does a black surface warm faster in sunlight?
- Design a 2-line safety rule for each experiment you tried above.
Answers: 1) spoon in hot drink (conduction), boiling water currents (convection), sunlight warming skin (radiation). 2) Copper — it's a metal with high thermal conductivity. 3) Black absorbs more radiant energy. 4) (student writes)
Print this worksheet: Print Worksheet
Summary (in one line)
Conduction transfers heat through direct contact, convection moves heat via fluid motion, and radiation sends heat through electromagnetic waves — each plays a different role in nature and technology.
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