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

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.