Английский язык. Ч. 1. Ильичева Н.А - 20 стр.

                                             20
        An object is said to have energy by virtue of its motion, which we call kinetic
energy. But it is also possible to have potential energy, which is the energy associated
with forces that depend on the position or configuration of a body (bodies) and the
surroundings. Various types of potential energy can be defined, and each type is as-
sociated with a particular force.
         A wound-up clock spring is an example of potential energy. The clock spring
acquired its potential energy because work was done on it by the person winding the
clock. As the spring unwinds, it exerts a force and does work to move the clock hands
around.
         Perhaps the most common example of potential energy is gravitational poten-
tial energy. A heavy brick held up in the air has potential energy because of its posi-
tion relative to the Earth. It has the ability to do work, for if it is released, it will fall
to the ground due to the gravitational force, and can do work on say, a stake, driving
it into the ground.

                                        Text C
                               Heat Transfer: Conduction

        Heat is transferred from one place or body to another in the three different
ways: by conduction, convention and radiation.
        When a metal poker is put in a hot fire, or a silver spoon is placed in a hot
bowl of soup, the exposed end of the poker or spoon soon becomes hot as well, even
though it is not directly in contract with the source of heat. We say that heat has been
conducted from the hot end to the cold end.
        Heat conduction in many materials can be visualized as the result of molecular
collisions. As one end of the object is heated, the molecules there move faster and
faster. As they collide with their slower-moving neighbors, they transfer some of
their energy to these molecules whose speeds thus increase. These in turn transfer
some of their energy by collision with molecules still farther along the object. Thus
energy of thermal motion is transferred by molecular collision along the object. In
metals, according to modern theory, it is collision of free electrons within the metal
with each other and with metal atoms that are visualized as being mainly responsible
for conduction.
        Heat conduction takes place only if there is a difference in temperature. In-
deed, it is found that the rate of heat flow through a substance is proportional to the
difference in temperature between its ends. The rate of heat flow also depends on the
size and shape of the object.