2.1 Answer the following questions

1. What do we call energy?

2. When does a body possess energy?

3. What forms of energy do you know?

4. What do we call kinetic energy?

5. What do we call potential energy?

3 Law of conservation and transformation of energy

We know a number of cases where one kind of energy is transformed into another. Whenever energy in one form is expended, an equal amount of energy in some other form takes its place.

We have different units in which various forms of energy are measured but after the conversion of work into heat, or chemical energy into work or into electrical energy, the amount of energy is the same as before the change. In other words, when energy is spent, there is still as much energy as before. This fact which is one of the fundamentals of physics is known as the Law of the Conservation and Transformation of Energy.

According to this law energy can neither be created, nor destroyed although its form can be changed. All the transformations, the endless number of chemical changes that are always taking place, are only changing energy from one form into another without affecting the total.

We often use machines to transfer energy from one place to another and to transform energy. The dynamo, for example, changes mechanical energy into electric energy. The energy of water is also transformed into electric energy by means of hydraulic turbines.

(950)

3.1 Answer the following questions

1. Can energy be created and destroyed?

2. What machine changes mechanical energy into electric energy?

3. When is mechanical energy converted into heat?

4. Can chemical energy be converted into heat?

4 Heat is a form of energy

Heat is a form of energy. There are three different types of heat transmission, namely: conduction, convection and radiation.

Conduction. Heat may be transferred from a hotter body to a colder one by direct contact. Fast-moving molecules tend to speed up their slower neighbours on collision. We know that this method of heat transfer is called conduction.

We know that some materials are good conductors of heat and others are poor conductors. Generally speaking, metals are excellent conductors, the best conductors of electricity being also the best conductors of heat. Thus, aluminium known as a good electrical conductor is likewise a good conductor of heat. On the other hand, materials like sand, asbestos, and still air are poor conductors of heat.

Convection. In gases and liquids another process of heat transfer is very effective, namely, convection. Convection is the transfer of heat by the motion of the hot body itself carrying its heat with it. A current of liquid or gas that absorbs heat at one place and then moves to another place where it mixes with a cooler portion of the fluid and loses heat is called a convection current. Сonvection occurs only in liquids and gases.

Radiation. There is a third way by which heat travels, namely, radiation. We know the temperature of the sun is very high. We also know that, when the sun shines on the earth, the earth is warmed. The passage of heat from the sun to the earth happens by radiation. An important characteristic of radiation is that it can occur in a vacuum; it does not depend upon the presence of a substance. In addition to that, the space between the source of heat and the body to receive the heat does not rise in temperature.

(1420)