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.