Heat Transfer
1. What is Heat Transfer?
Heat is a form of ENERGY that flows from a HIGHER temperature body to a LOWER temperature body.
Temperature vs Heat:
| Temperature | Heat |
|---|---|
| Measures HOW HOT something is | Total THERMAL energy |
| NOT a form of energy | IS a form of energy |
| Unit: °C, K, °F | Unit: Joule (J) |
| Does NOT depend on mass | DEPENDS on mass |
2. Three Modes of Heat Transfer
| Mode | Medium Required | How it Works | Speed |
|---|---|---|---|
| Conduction | YES (solid) | Particle VIBRATION passes energy | SLOW |
| Convection | YES (fluid — liquid/gas) | Movement of MASS — hot fluid rises | SLOWER |
| Radiation | NO (can travel through vacuum) | Electromagnetic WAVES | FASTEST (speed of light) |
3. Conduction
Heat transfer through a SOLID without the particles themselves moving.
Mechanism: Particles at the HOT end VIBRATE more vigorously → collisions pass energy to neighbouring particles.
Conductors and Insulators
| Good Conductors | Poor Conductors (Insulators) |
|---|---|
| Metals (copper, aluminium, iron, silver) | Wood, plastic, rubber |
| Heat flows EASILY through them | Heat flow is RESTRICTED |
Worked Example: Why does a metal spoon feel COLDER than a wooden spoon at the same temperature?
Metals are GOOD conductors — they RAPIDLY conduct heat AWAY from your hand (your hand feels cold). Wood is a POOR conductor — it takes heat from your hand SLOWLY.
4. Convection
Heat transfer through FLUIDS (liquids and gases) by the ACTUAL movement of particles.
Process:
- Fluid near the heat source EXPANDS → becomes LESS dense
- WARMER fluid RISES
- COOLER fluid SINKS to take its place
- This creates a CONVECTION CURRENT
Applications of Convection
Sea Breeze (DAY): Land heats up FASTER than sea. Air over land RISES. Cooler air from the sea FLOWS IN to replace it.
Land Breeze (NIGHT): Land cools down FASTER than sea. Air over sea RISES. Cooler air from the land FLOWS towards the sea.
AC placement: Air conditioners are placed HIGH on walls because cool air SINKS.
5. Radiation
Heat transfer through ELECTROMAGNETIC waves (infrared radiation). Does NOT require a medium.
Properties:
- Travels at the SPEED of light (3 × 10⁸ m/s)
- Travels through VACUUM (reaches us from the Sun)
- Can be REFLECTED (shiny surfaces) or ABSORBED (dark surfaces)
- DARK, rough surfaces are GOOD absorbers and emitters
- SHINY, white surfaces are POOR absorbers and emitters (good reflectors)
6. Thermos Flask (Vacuum Flask)
A thermos flask MINIMISES all THREE modes of heat transfer.
| Part | Prevents |
|---|---|
| VACUUM between double walls | NO medium for conduction or convection |
| SILVERED inner and outer surfaces | REFLECTS radiation back |
| CORK or plastic STOPPER | Poor conductor — prevents conduction |
| CORK SUPPORT at bottom | Prevents conduction through the base |
'The thermos flask keeps HOT liquids hot and COLD liquids cold by SLOWING DOWN heat transfer in all three modes.'
7. Global Warming and the Greenhouse Effect
The EARTH's atmosphere acts like a greenhouse — it traps some heat from the Sun.
Greenhouse gases: CO₂, methane, water vapour Greenhouse effect: Necessary for life — keeps Earth warm Global warming: EXCESSIVE greenhouse gases → temperature rises
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Heat and temperature are the same' | Heat is TOTAL energy. Temperature is AVERAGE kinetic energy of particles |
| 'Conduction occurs in liquids' | Conduction is PREDOMINANT in solids. Liquids and gases transfer heat mainly by CONVECTION |
| 'Radiation needs a medium' | Radiation does NOT need a medium. Heat from the Sun reaches us through the VACUUM of space |
| 'Silvered surfaces absorb more heat' | Silvered REFLECT heat (reflect radiation). DARK surfaces ABSORB more heat |
ICSE Exam Focus (5–7 marks)
- 2-mark questions: Define conduction, convection, and radiation
- 3-mark questions: Distinguish between conductors and insulators with examples
- 4-mark questions: Explain sea breeze / land breeze or thermos flask
- 5-mark questions: Why questions — practical applications of heat transfer
Self-Test
Q1. What is the mode of heat transfer in solids? A1. CONDUCTION. Heat is transferred by particle VIBRATIONS without the particles moving.
Q2. Why are cooking utensils made of metal and their handles of plastic? A2. Metals are GOOD conductors of heat — they conduct heat quickly to cook food. Plastics are POOR conductors (insulators) — they prevent heat from reaching your hand.
Q3. Explain the formation of a sea breeze. A3. During the day, land heats up FASTER than the sea. Air over the land expands, becomes LESS dense, and RISES. Cooler, denser air from the sea FLOWS IN to replace it, creating a sea breeze.
Q4. How does a thermos flask prevent heat loss by conduction, convection, and radiation? A4. Conduction: VACUUM between walls (no medium) and cork stopper. Convection: VACUUM prevents convection currents. Radiation: SILVERED walls reflect radiation back.
Q5. Why are solar water heaters painted black? A5. Black surfaces are GOOD absorbers of radiation. They ABSORB maximum heat from the Sun.
Q6. What is the difference between heat and temperature? A6. Heat is the TOTAL energy of molecular motion in a body (depends on mass). Temperature is the AVERAGE kinetic energy of particles (does NOT depend on mass).
