Work, power and energy

The concept of energy is central to physics, as many
times the analysis of a system’s motion involves understanding how energy is
changing. The change in energy is known as work, and the work done over a given
period of time is known as power.

Energy is defined as the ability to do
work. Physicists classify energy into several types: kinetic, potential, heat,
sound, radiant energy (e.g. light) and electrical, chemical and nuclear energy.

Kinetic energy is possessed by a
moving object by virtue of its motion. It equals the work done to
accelerate the object to a particular velocity or bring a moving object to
rest. Then two principal forms of kinetic energy are known as translational and
rotational. The first is due to motion in straight line while the second is due
to motion in a circle.

Potential energy is due to the
position of an object. It is not apparent until released. Two common types are gravitational
potential energy
and elastic potential energy. An object gains
gravitational potential energy, as work is done to raise it against the force
of gravity. When the object falls, it gains kinetic energy but loses potential
energy. Elastic potential energy is gained as work is done to stretch or compress
an elastic object such as a spring.

This type converted into kinetic energy
when the spring is released and it regains its formal shape. An object
possesses heat, or thermal energy by virtue of its temperature. It is, in fact,
merely a form of kinetic energy, because the temperature of a substance depends
on the motion of its component atoms or molecules; the higher its temperature,
fester the molecules move. (Heat radiation is not, however classified as
thermal energy but as radiant energy; it comprises the infrared part of the electromagnetic
). Radiant energy consists of electromagnetic radiation and
includes radio waves, visible light, ultraviolet and infrared radiation and

Radiant energy is emitted when electrons
within atoms fall from a higher to lower energy level and release the excess
energy as radiation. Sound energy consists of moving waves of pressure
in a medium such as air, water or metal. They consist of vibrations in the
molecules of the medium, and sound can therefore be regarded as special form of
kinetic energy.

Matter that has gained or lost some
electric charge has electrical energy. It is a form of electrostatic
potential energy. The movement of charges constitutes an electric current,
which flows between two objects at different potentials when they are joined by
a conductor, because the charges move from one object to the other object until
an equal potential is restored to each. Chemical energy is possessed by
substances that undergo a chemical reaction, such as combustion. It is stored
in the chemical bonds between the atoms that make up the molecules of a

During 1 reaction, the atoms of the
reactants rearrange themselves to form different molecules of the products. If
the products have less chemical energy than the reactants, energy is released
during the reaction (in the form of heat, light or electrical energy, as in a
battery). If on the other hand, the products have more chemical energy than the
reactants, then energy is absorbed and is to be supplied to make the reaction
possible. Nuclear energy is produced when the nuclei of atoms change, either by
splitting apart or joining together.

The splitting process is known as nuclear
, the joining together as nuclear fusion. Such changes can be
accompanied by the release of enormous amount of energy in the form of heat,
light, and radioactivity. The resulting motion of the nuclei and particles also
cause an increase in the thermal and kinetic energy of its surroundings.


Work is done when a force makes an
object move. It is defined as the product of the force and the distance through
which the object moves. If the object moves in the same direction as that of
the force then work is multiple of magnitude” of the force and the
distance moved. But if the force acts in a different direction to the movement
of the object, then work done is equal to component of the force in the
direction of movement (which is less than the total force) multiplied by the
distance through which , the object moves. If a force acts in the opposite
direction to the motion (such as a force applied to slow down a moving object),
then work done is negative. Zero work is done when no motion results, as
happens, for example, when someone holds up an object without moving it. Work
is shown by the following equation. W = Fd cos0 Where W is the work done (in
Joules), F is the force applied (in N), d is the distance moved by the object
(in m) and 6 is the angle between the direction of movement and the direction
of the applied force.


In everyday language,
power means much the same thing as energy or work. However, in physics power is
the rate of doing work. Power is measured in watts, one watt being equivalent
to a rate of working of one joule per second. Thus, although it takes the same
amount of work to lift 10 kg through 10 m in 30 seconds as it does to do it in
60 seconds, it involves twice as much power to perform the task in the shorter
time. In another way, the amount of work that a machine can do depends both on
its power and on the length of time for which it operates The power of a
machine is given by: p= w/t Where P is the power (in W, watts), W is the work
done (in j), and t is the time taken (in s) to do the work.



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