Heat is a form of energy that flows from a hotter body to a cooler body. Understanding how heat moves through different media is essential to explain everyday phenomena like why metals feel colder than wood at the same temperature, or how a thermos flask keeps drinks hot.
Key Concepts and Definitions
Temperature is the measure of the degree of hotness or coldness of a body. It is measured in degrees Celsius (°C) using a thermometer.
Heat is thermal energy that transfers from a region of higher temperature to a region of lower temperature.
Conduction is the transfer of heat through a solid material without the movement of the material itself. Heat passes from particle to particle by direct contact.
Convection is the transfer of heat through fluids (liquids and gases) by the actual movement of the fluid particles. Warmer fluid rises and cooler fluid sinks, setting up convection currents.
Radiation is the transfer of heat energy as electromagnetic waves without any medium. It can travel through a vacuum.
Conductors are materials that allow heat to pass through them easily (e.g., metals like iron, copper, aluminium).
Insulators are materials that do not allow heat to pass through them easily (e.g., wood, plastic, rubber, wool).
Good absorbers of heat are dark, rough surfaces. Good reflectors are shiny, light-coloured surfaces.
Worked Examples
Why does a metal spoon become hot when placed in a cup of hot tea?
The metal spoon is in direct contact with the hot liquid. Heat travels from the hot tea through the metal by conduction — the rapidly vibrating particles of the tea transfer energy to the particles of the metal, which then vibrate faster and pass energy along the spoon.
How does a room heater warm up an entire room even though only air near it is heated first?
Air near the heater warms up, expands, and becomes less dense, so it rises. Cooler, denser air from the upper parts of the room flows down to replace it. This circular movement of air is a convection current, which gradually warms the whole room.
How does heat from the Sun reach the Earth?
There is no medium (matter) between the Sun and Earth for conduction or convection. Heat travels as infrared radiation — electromagnetic waves that need no medium — across space and warms Earth's surface.
Why do we wear dark-coloured clothes in winter and light-coloured clothes in summer?
Dark colours are good absorbers of heat radiation. In winter, dark clothes absorb more heat from sunlight, keeping us warm. Light colours reflect most of the radiation; in summer they keep us cool.
Why is the bottom of a cooking vessel made of metal while the handle is made of plastic or wood?
The metal bottom conducts heat quickly from the flame to the food. Plastic or wood handles are poor conductors (insulators), so they stay cool and safe to touch — heat is not conducted along them to the hand.
How does land breeze and sea breeze occur?
During the day, land heats up faster than the sea. Air over land rises (convection), and cooler sea air moves inland — this is a sea breeze. At night the reverse happens: the sea retains heat longer, air rises over the sea, and a land breeze blows out to sea.
Why does a thermos flask keep liquids hot or cold for a long time?
A thermos flask has double walls with a vacuum between them. Conduction is prevented because the walls barely touch; convection is prevented because there is no medium in the vacuum; radiation is minimised by silvering the inner walls (silver reflects radiation).
Key Formula
There is no numeric formula at this NCERT level, but remember: Heat flows from higher temperature to lower temperature — never in the opposite direction on its own.
Common mistakes
Students often confuse heat and temperature. Temperature is a measure of how hot something is; heat is the energy that moves between objects. A large tub of warm water has more heat energy than a small cup of boiling water, even though the cup has a higher temperature.
Summary
Heat transfers by three methods: conduction (through solids by particle vibration), convection (through fluids by particle movement), and radiation (through any medium or vacuum as waves). Good conductors transfer heat quickly; insulators resist heat flow. Dark surfaces absorb and emit radiation well; shiny surfaces reflect it. These principles explain natural phenomena like sea breezes, thermos flasks, and seasonal clothing choices.