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Heat and electromagnetic energy

Grade 8 Science Worksheets

What is Heat?

Heat is one of the most important forms of energy. We use it in almost all areas of industry – to melt metals in furnaces, to run trains, rockets and turbines, to produce glass, paper, textiles and so on. We also use heat to feel comfortable in winters, to cook our food, to provide hot water, to dry our clothes, and many more personal uses. Our bodies require a certain amount of heat – not too high, not too low – to function properly. It is called homeostasis which is the very basis of life.

If you rub your palms vigorously, they become warm. You’ve generated heat! While the sun is our main source, heat is also produced in chemical reactions, within the earth, from friction between two objects, in a nuclear reactor, and through the flow of electricity through metals.

Heat transfer

Heat passes from one object to another object or from one area in an object to another area by three methods.

When heat moves through a material it is called Conduction. When you heat a metal rod at one end, the atoms at that end get hot and vibrate faster. In doing so they strike adjacent atoms making them vibrate as well and thus the energy gets transferred from atom to atom until it reaches the other end. This is how heat is transferred by atoms or molecules vibrating in their mean position during conduction. It usually occurs in solids.

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The metal handle of a pot boiling water becomes hot at the other end due to conduction.The water is heated by convection currents set up within its molecules.The hot stove gives off radiation energy in the form of infrared rays.

Now imagine a hot stove which heats the air immediately around it. The heated air expands and becomes lighter, rising upward. It gets replaced by cold air from nearby, which in turn gets heated and rises. This sets up a continuous current of air, with cold air moving towards the hot stove and heated air moving away. This transfer of heat by the movement of a heated material itself – air in this case – is called Convection. This is the basis of plate tectonics too.

Both conduction and convection transmit heat through particles of matter. A third form,Radiation, carries heat in the form of energy waves through space, and even through vacuum where there are no particles. Electromagnetic Radiation travels from the sun. Infrared Radiation is given out by heated objects such as a heat lamp. Radiation strikes an object and warms it. The sun’s rays warm the earth’s atmosphere and the surface of the earth in this manner.

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Insulation

Insulation is a way to contain or control the movement of heat. It is useful to slow down or prevent heat from getting around. As heat travels in three forms, so also insulation can be provided by three methods.

  • Conducted heat, as in a pot’s metal handle, can be insulated with a plastic or wood cover on the handle, since plastic and wood are poor conductors of heat compared to metal.
  • On a cold, windy day, you need protective layers of woolen clothes to prevent your body from losing heat due to convection.
  • Radiation energy entering the car or house can be reduced by tinted glass windows and insulation film.

What are electromagnetic waves?

Radiation is one of the major forms of energy. It consists of Electromagnetic Waves. These waves are made up of tiny bits of matter, called Photons, which have been accelerated to great speeds to create high energy particles. They are a combination of electric and magnetic energy moving through space, hence the term.

Light from the sun, in the form of Solar Radiation, is a form of electromagnetic energy. All life on earth depends on this. Heat from the sun in this form is used by plants to make their own food. These plants give us food. Animals that eat these plants give us food too.

In addition to natural radiation from the sun, scientists produce artificial radiation for a variety of purposes. Some of these are –

  1. Radio waves used in communications
  2. Microwaves (short radio waves) used to heat food in ovens
  3. X-rays used for medical purposes
  4. Infrared rays used in TV remotes
  5. Gamma rays used to sterilize food and medical equipment

Check Point

  1. Heat moving through a material by energy transfer between vibrating atoms is called –
    1. Conduction
    2. Convection
    3. Radiation
  2. Heat that is carried by the movement of a heated material itself is called –
    1. Conduction
    2. Convection
    3. Radiation
  3. Heat that is carried in the form of energy waves through space, and even through vacuum is called –
    1. Conduction
    2. Convection
    3. Radiation
  4. We control or contain the movement of heat through –
    1. Radiation
    2. Insulation
    3. Gravitation
  5. Which of these are forms of electromagnetic energy?
    1. Solar radiation
    2. Microwave
    3. Gamma rays
    4. Infrared rays
    5. All of the above

Answer Key

  1. Conduction
  2. Convection
  3. Radiation
  4. Insulation
  5. e) All of the above

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