FHSST Physics/Work and Energy/Definition

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The Free High School Science Texts: A Textbook for High School Students Studying Physics
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Work and Energy
Definition - Work - Energy - Mechanical Energy and Energy Conservation - Important Quantities, Equations, and Concepts - Sources

What are Work and Energy?[edit | edit source]

The energy is the ability to do a work.

Energy is measured by the result of applied force/power over a period of time to make changes/work.

Work is defined by a force applied to an object to change this object’s physical properties.

Power = Energy / Time No force used no change and none work was done.

Power and energy are closely related, although, they are not the same.

Power is the rate at which energy is delivered, not an amount of energy itself.

Electrical unit of power is Watt (named after the scientist James Watt):

1 Watt = 1 Joule / Second.

Energy = Power × Time.

Energy exist in various forms: kinetic [Ek = ½ m, v2 ], potential [Ep = m g h], thermal (heat), chemical, electrical, electrochemical, magnetic, sound, light, and nuclear.

Energy can be converted from one form into another in many ways:

Through the gravitation forces When gravity accelerates a falling object, it converts its potential energy to kinetic energy, or when an object is lifted, the gravitational field stores the energy exerted by the lifter as potential energy in the earth-object system.

Through the electric and magnetic fields forces Electrically charged particles in the presence of an electric field possess potential energy. The fields’ forces can accelerate particles and convert this particle's potential energy into kinetic energy. Charged particles can interact via electric and magnetic fields to transfer energy between them, i.e.: an electrical current in a conductor transforms electrical energy into heat.

Frictional Forces A mass object with its potential and kinetic energy associated with the position, orientation, and the object’s motion can be converted into thermal energy (heat), whenever the object slides against another object. The sliding causes the molecules on the surfaces of contact to interact via electromagnetic fields with one another and start vibrating.

Through emitting or absorbing photons of light When photons of lights fall on an object, a photon may pass through the object, be reflected by the object, or be absorbed by the atoms making up the object. Depending on the smoothness of the surface, the scale of the photon's wavelength, the reflection may be either diffuse (rough surface) or coherent (smooth surface). If the photon is absorbed, the photon's energy may also be split and converted in one of these ways: • Photo-thermal effect The energy absorbed may simply produce thermal energy, or heat in the object. In this case, the photon's energy is converted into vibrations of the molecules called phonons, which is actually heat energy. • Photoelectric effect The energy absorbed may be converted into kinetic energy of conduction electrons, and hence electrical energy. • Photochemical effect The energy may bring about chemical changes that effectively store the energy. Nuclear reactions occur when the nuclei of particles combine [fusion reaction] or when nuclei split apart [fission reaction].

In the International System of Units [SI system], the electrical energy unit is 1 Joule (named after the English physicist James Prescott Joule).

A one Joule is the amount of energy we expend as work if we exert a force of a one Newton of force over a distance of one meter.

It takes a one Joule of energy to lift 1 lb. about 9 inches.

The unit of force in the International System of Units is 1 Newton (named after the English physicist Isaac Newton).

One Newton of force is the force that can accelerate a mass of 1 kilogram (about 2.205 lbs), such that it picks up 1 meter per second of velocity during each second that the force is exerted.

Appliances rating is how much energy per unit time these appliances draw. This quantity is called the "power":

Energy = Power x Time = (100 Joules/Second) × (3600 Seconds) = 360,000 Joules