# 7th Class Science Heat NCERT Summary - Heat Energy

NCERT Summary - Heat Energy

Category : 7th Class

Heat Energy

• Energy is the ability to do work. When an object has the ability to do work, it is said that it has energy.
• Heat is a form of energy and has the ability to do work.
• Heat energy can also be converted to other forms. For example:
• When charcoal (coal of wood) is burnt, it emits light. In this way, heat energy produces light.
• The heat energy in a firecracker produces both sound and light.
• In a hot air balloon, the hot gases are lighter than the surrounding air. They rise and are capable of lifting heavy masses. Here heat energy is used to produce mechanical energy.
• A steam engine converts heat energy into mechanical energy and powers the train.
• Other forms of energy can also be converted to heat energy. For example,
• The heat (heat energy) produced from the mechanical energy by rubbing palms vigorously against each other can be felt easily.
• When a candle bums in air, chemical energy is converted into heat energy.
• In an electric bulb, electrical energy is converted into light and heat.

EFFECTS OF HEAT

• When an object is heated, many changes take place. For example,
• The object may expand in size.
• It may change its state (ice changes into water on heating).
• Heat can also speed up chemical reactions.
• Heat can even kill.
• In fact, we boil milk and drinking water to kill harmful bacteria. So, heat may kill germs.

HOTNESS AND TEMPERATURE

• The hotness or coldness of an object can be measured from the temperature of that object, i.e., the degree of hotness of an object is called its temperature.
• We can compare the temperature of two objects and decide which is higher by using our sense of touch. But we do this only if their heat is bearable to us.
• Even if the heat of the two objects is bearable, touching them might not give a very reliable estimate of their temperatures. Similarly, we cannot rely on our sense of touch to measure the temperature of a sick man.

AMOUNT OF HEAT

• We can measure the amount of heat from the change in temperature that it produces in an object.
• The unit of heat is called Celsius.
• One calorie is the amount of heat that can raise the temperature of 1 gram of water by$1{}^\circ C$ . This means that when a calorie of heat is supplied to 1 gram of water, its temperature will rise by$1{}^\circ C$.
• Sometimes, we also use kilocalorie to measure the amount of heat. One kilocalorie is equal to 1000 calories. That is, 1 kilocalorie can raise the temperature of 1 kilogram of water by$1{}^\circ C$ .
• The energy content of food is measured in kilocalories.
• The modem and generally accepted unit of heat energy is no longer calorie, but the joule. The joule is accepted internationally and is abbreviated as "J".

Note: One calorie is equal to 4.184 Joules.

MELTING POINT

• Substances exist in three forms or states—solid, liquid and gas. When a solid substance like ice or wax is heated, it melts and becomes liquid. This change of state occurs at a definite temperature. For example, ice melts at temperatures above $0{}^\circ C$and wax melts at about$63{}^\circ C$.
• The temperature at which a substance changes its state from solid to liquid is called its melting point.

BOILING POINT

• The temperature at which a substance changes its state from liquid to gas is called its boiling point.

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