Heat Summary Class 7 NCERT Summary Chapter 3

Hot and Cold

The measure of how hot or cold a substance is called its temperature. The instrument used to measure temperature is a thermometer. Temperature tells us the degree of hotness or coldness and is recorded with a unit, commonly the degree Celsius (°C).

Measuring Temperature

Thermometers are designed for different uses. The two common types are:

• Clinical thermometer: Used to measure the temperature of the human body.
• Laboratory thermometer: Used to measure the temperature of substances and objects in experiments and other non-medical situations.

Range and Scales

• The usual range of a clinical thermometer is 35°C to 42°C.
• The usual range of a laboratory thermometer is about -10°C to 110°C.
• Two widely used temperature scales are the Celsius scale and the Fahrenheit scale. The conversion between them is given by the relation F = (9/5)C + 32, where F is temperature in degrees Fahrenheit and C is temperature in degrees Celsius.
• The normal human body temperature is about 37°C.

Reading a Thermometer

To read a mercury or alcohol thermometer correctly, follow these ordered steps:

1. Observe the difference in temperature value between two successive large marks on the scale and note the number of small divisions between them.
2. Divide the large-mark interval by the number of small divisions to find the value of one small division (for example, if there are five divisions then one small division = 1/5 = 0.2°C).
3. For a clinical thermometer, first wash it with an antiseptic solution. Hold the thermometer firmly and give it a quick downward jerk so the liquid level falls below 35°C.
4. Place the bulb of the clinical thermometer under the tongue and keep it there for about one minute. Remove it and read the level of the liquid against the scale; record the reading with its unit, °C.

Note: Always state temperature with its unit (for example, 36.8°C).

Precautions while Using Thermometers

For a clinical thermometer

• Use a clinical thermometer only for measuring human body temperature; do not use it to measure temperatures of other objects or liquids.
• Avoid leaving the thermometer in direct sunlight or close to a flame since this may overheat or damage it and can cause breakage.
• Handle with care to avoid breakage and possible mercury spillage; if the thermometer breaks, follow safety procedures for mercury (use gloves, do not touch with bare hands, air the room).

For a laboratory thermometer

• Keep the laboratory thermometer upright when not in use and avoid tilting it while taking readings.
• When measuring the temperature of a liquid, ensure the bulb is completely surrounded by the liquid and does not touch the sides or bottom of the container so that it reads the substance's temperature, not the container's.
• Allow the thermometer enough time to reach thermal equilibrium with the substance before noting the reading.

Transfer of Heat

Heat energy moves from a hotter region to a colder region. There are three main methods by which heat is transferred:

Conduction

Conduction is the transfer of heat through a substance without the bulk movement of the substance itself. It is the common method of heat transfer in solids.

• Materials that allow heat to pass through them easily are called conductors of heat. Examples include iron, copper and aluminium. In metals, heat passes from particle to particle, so the warmer part of the metal transfers heat to the cooler part.
• Materials that do not allow heat to pass through them easily are called insulators or poor conductors. Examples include plastic, wood, and rubber.
• Practical examples: cooking pots are made of metal for good conduction of heat, while handles are often made of wood or have plastic covers to serve as insulators so you do not burn your hand.

Convection

Convection is the transfer of heat by the actual movement of warm fluid (liquid or gas) from one place to another. Convection currents form when part of a fluid is heated, becomes less dense, and rises while cooler, denser fluid moves down to take its place.

• Convection takes place in liquids and gases but not in solids (because particles in solids cannot move freely).
• Examples include the warming of water in a pan (warm water rises, cool water sinks) and air circulation in a room heated by a heater.
• Large-scale examples: atmospheric circulation patterns and ocean currents are driven in part by convection due to unequal heating.

Radiation

Radiation is the transfer of heat by electromagnetic waves and does not require a medium; it can occur through a vacuum.

• An everyday example is heat from the Sun reaching the Earth through space by radiation.
• Other examples include feeling warmth from a fire or from an electric heater even if the air between you and the heater is still.
• Dark-coloured bodies absorb more radiant energy, while light-coloured or shiny bodies reflect more; this affects heating by radiation.

Sea Breeze and Land Breeze (Application of Convection)

Sea Breeze

During the day, land heats up faster than the sea. The air above the land becomes warmer and rises; cooler air from above the sea blows inland to replace it. This onshore flow of cooler air from the sea towards the land is called the sea breeze.

• The rising warm air above the land moves over the sea, cools, and sinks, completing a local convection cell.
• Sea breeze is most noticeable in the afternoon along coastal areas and brings cooler air from the sea.

Land Breeze

At night the situation reverses. Land cools faster than the sea, so the air above the land becomes cooler and denser and flows out towards the sea. This offshore flow of cool air from the land to the sea is called the land breeze.

• Land breeze is usually experienced during the night and early morning over coastal regions.
• The warm air over the sea rises and the cooler air from the land moves out to replace it, forming a convection loop.

Kinds of Clothes We Wear in Winter and Summer

• Colour effect: Dark-coloured clothes absorb more radiant heat and so help us feel warmer in cold weather; light-coloured clothes reflect more sunlight and help keep us cooler in hot weather.
• Material and insulation: In winter, we wear woollen clothes because wool is a poor conductor of heat and traps air between its fibres. The trapped air acts as an insulating layer and reduces the flow of heat from our body to the cold surroundings, keeping us warm.
• Summer clothing: Loose, light-coloured clothes allow air circulation and reflect much of the sun's radiation, helping the body to remain cool.
• Practical choice: Choose clothing and colours according to climate: insulators and darker colours for cold conditions; breathable fabrics and lighter colours for warm conditions.

Additional Examples and Everyday Applications

• Touching a metal rod heated at one end demonstrates conduction-your hand at the other end will eventually feel warm as heat travels along the rod.
• Boiling water in a kettle demonstrates convection-hot water rises and cold water sinks, forming currents that mix the liquid.
• Feeling the warmth of sunlight on your skin when standing in the shade of a tree shows radiation-your skin receives radiant energy directly from the Sun unless blocked.
• Design choices in buildings (insulation, window colour, ventilation) use principles of conduction, convection, and radiation to keep interiors warm or cool efficiently.

Summary

Temperature measures the degree of hotness or coldness of a body and is read using thermometers designed for specific purposes. Heat moves from hotter to colder regions by conduction, convection, and radiation. Understanding these modes of heat transfer explains many everyday phenomena such as sea and land breezes, the choice of clothing for different seasons, and the design of cooking utensils and buildings.