Heat

What you'll learn

• Difference between heat and temperature.
• How a thermometer works; types of thermometers.
• Three modes of heat transfer: conduction, convection, radiation.
• Real-world applications of each mode.

Key concepts

Heat vs Temperature

Temperature scales:

• Celsius (°C): water freezes at 0°C, boils at 100°C.
• Fahrenheit (°F): F = (9/5 × C) + 32; body temperature = 98.6°F = 37°C.
• Kelvin (K): K = C + 273; used in science.

Thermometers

• Thermometer: instrument used to measure temperature.
• Uses expansion of liquid (mercury or alcohol) with temperature.
• Mercury thermometer: silver-coloured liquid; range −39°C to 357°C. (Being phased out — mercury is toxic.)
• Alcohol thermometer: coloured liquid; range −115°C to 78°C; safer.

Types:

Using a clinical thermometer:

• Normal human body temperature: 37°C (98.6°F).
• Wash before/after use; kink stops mercury falling back; hold for 1–2 min in mouth/armpit.
• Read before shaking down.

Modes of Heat Transfer

1. Conduction

• Heat transfer through a solid by particle-to-particle collision.
• Particles vibrate → transfer energy to neighbours → heat moves from hot end to cold end.
• No actual movement of particles; only energy passes.
• Best conductors: metals — iron, copper, aluminium, silver (silver best; copper second best).
• Poor conductors (insulators): wood, plastic, rubber, glass, air.

Examples:

• Metal spoon gets hot when left in hot tea.
• Cooking pan handle made of plastic — poor conductor → doesn't get as hot.
• Ice packs wrapped in cloth — cloth slows conduction → stays cold longer.
• Wearing layers in winter — air trapped between layers = good insulator.
• Birds puff up feathers in cold — traps more air.

2. Convection

• Heat transfer through fluids (liquids and gases) by actual movement of particles.
• Hot fluid rises (less dense); cold fluid sinks (denser) → convection current.

Examples:

• Heating a pot of water: water at bottom heats → rises → cooler water sinks → circular current → all water heats.
• Sea breeze: daytime — land heats faster → hot air over land rises → cool air from sea blows in (sea breeze). At night, land cools faster → land breeze.
• Monsoons in India: heating of land vs ocean creates large-scale convection.
• Hot air balloons: hot air inside is less dense → balloon rises.
• Ventilators placed near ceiling — hot air rises and escapes through ventilator.
• Room heater placed at floor level — heats air → rises → sets up convection current → heats whole room.

3. Radiation

• Heat transfer by electromagnetic waves (infrared radiation) without needing any medium.
• Can travel through vacuum; does NOT require particles.
• All objects emit radiation; hotter objects emit more.

Factors affecting radiation:

• Colour and surface:
  • Black/dark, rough surfaces absorb and emit radiation BEST.
  • White/shiny, smooth surfaces absorb and emit radiation LEAST (reflect most).

Examples:

• Earth receives heat from Sun (through vacuum of space) — only by radiation.
• Sitting near a campfire — feel warm even without touching (radiation, not conduction/convection).
• Solar cookers — black painted inner surface absorbs maximum radiation.
• Wearing white/light clothes in summer — reflects radiation, stays cooler.
• Thermos flask (vacuum flask): vacuum stops conduction and convection; silvered walls reduce radiation.
• Greenhouses: glass allows visible light in but traps infrared (radiation) — keeps plants warm.

Comparison of modes

Quick check

• What is the difference between heat and temperature?
• Normal body temperature is 37°C. Convert to Fahrenheit.
• Explain why a metal rod feels hotter to touch than a wooden table at the same temperature.
• Why are ventilators placed near the ceiling of rooms?
• Explain how a thermos flask reduces heat loss by all three modes of heat transfer.

Open the Practice tab for graded questions on Heat.