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Heat
ICSE Class 7 Physics • Chapter 5
Chapter Overview
Heat is a form of energy that flows from a hotter body to a colder body. In this chapter, we explore how Heat causes temperature changes and expansion, and we study the three primary modes of heat transfer: Conduction, Convection, and Radiation.
5.1 Heat and Temperature
- Heat: A form of energy that produces the sensation of warmth. Its S.I. unit is the Joule ($J$). It can also be measured in calories ($1\text{ calorie} \approx 4.18\text{ J}$).
- Temperature: A measurable physical quantity that indicates the exact degree of hotness or coldness of a body. Its S.I. unit is the Kelvin ($K$), though Celsius ($^\circ C$) is commonly used.
Key Difference: Heat refers to the total thermal energy in a body, while Temperature measures the average kinetic energy of its particles.
5.2 Effects of Heat
When an object absorbs heat, three major physical changes can occur:
- Rise in Temperature: The object simply becomes hotter.
- Change of State: Solid ice melts into liquid water, or water boils into steam. During a change of state, the temperature remains totally constant.
- Thermal Expansion: Almost all solids, liquids, and gases expand (increase in volume) when heated and contract when cooled.
Practical Applications of Thermal Expansion
- Railway Tracks: Small gaps are intentionally left between steel railway tracks to allow for expansion during hot summer days. If gaps are missing, the tracks will bend and derail trains.
- Opening Tight Lids: If a metal lid on a glass jar is extremely tight, running hot water over the metal lid causes it to expand slightly more than the glass, making it easy to open.
5.3 Transfer of Heat
Heat always flows from a region of higher temperature to a region of lower temperature until thermal equilibrium is reached. There are three modes of transfer:
1. Conduction
The process of heat transfer in solids without any actual bodily movement of the vibrating particles.
- When you hold an iron rod in a fire, heat travels from the hot end to your hand through conduction.
- Good Conductors: Metals like copper, aluminum, and silver. They are used for making cooking utensils.
- Insulators (Bad Conductors): Wood, plastic, air, and wool. Plastic handles on pans prevent our hands from getting burned.
2. Convection
The process of heat transfer in liquids and gases by the actual physical movement of the heated matter itself.
- When boiling water in a pan, the hot water at the bottom becomes lighter and physically rises up, while the cold dense water from the top moves down to take its place. This creates a continuous convection current.
- Sea Breeze and Land Breeze: These coastal winds primarily occur due to huge convection currents in the atmosphere resulting from the uneven heating of land and sea.
3. Radiation
The process of heat transfer that remarkably requires no material medium whatsoever. Heat travels as electromagnetic waves at the speed of light.
- The immense heat from the Sun reaches the Earth exclusively through the vast vacuum of space via radiation.
- Dark colors are excellent absorbers and radiators of heat, whereas light and shiny surfaces are excellent reflectors. (This is why we wear light-colored cotton clothes in hot summers).
AI Image Prompt: A vibrant 3D diagram showing a campfire heating a metal pot of water. Label "Conduction" on the metal handle being held by a hand. Label "Convection" with circular arrows inside the boiling water. Label "Radiation" showing squiggly heat waves traveling directly sideways from the roaring fire to warm a person's outstretched hands nearby.
Q1. Why are ventilators (exhaust fans) usually located near the ceiling of a room?
Ans: Due to respiration and bodily heat, the stale air inside a room gets warm. Warm air is lighter and physically rises upward by convection. The ventilators near the top allow this warm, stale air to escape, while fresh, cooler air enters through windows.