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Work Definition

In physics, work is the product of force and displacement. A force is said to do work if, when acting, there is a displacement of the point of application in the direction of the force.

For Instance, when a ball is held above the ground and then dropped, the work done on the ball as it falls is equal to the weight of the ball (a force) multiplied by the distance to the ground (a displacement). When the force F is constant and the angle between the force and the displacement s is θ, then the work done is given by W = Fs cos θ.

Work transfers energy from one place to another, or one form to another. The SI unit of work is the joule (J).

Electrical Work

Electrical work is the work done on a charged particle by an electric field. The equation for electrical work is equivalent to that of mechanical work:

Where Q is the charge of the particle, q the unit charge, E the electric field, which at a location is the force at that location divided by a unit ('test') charge, FE is the Coulomb (electric) force, r is the displacement and ∙ is the dot product.

The electrical work per unit of charge, when moving a negligible test charge between two points, is defined as the voltage between those points. The work can be done by electrochemical devices (electrochemical cells) or different metals junctions generating an electromotive force.

Thermodynamic Work

In thermodynamics, work performed by a system is energy transferred by the system to its surroundings, by a mechanism through which the system can spontaneously exert macroscopic forces on its surroundings, where those forces, and their external effects, can be measured. In the surroundings, through suitable passive linkages, the whole of the work done by such forces can lift a weight. Also, just through such mechanisms, energy can transfer from the surroundings to the system; in a sign convention used in physics (though chemistry uses the opposite sign convention), such energy transfer is counted as a negative amount of work done by the system on its surroundings.

The externally measured forces and external effects may be electromagnetic, gravitational, or pressure/volume or other macroscopically mechanical variables. For thermodynamic work, these externally measured quantities are exactly matched by values of or contributions to changes in macroscopic internal state variables of the system, which always occur in conjugate pairs, for example pressure and volume or magnetic flux density and magnetization.

By an external system that lies in the surroundings, not necessarily a thermodynamic system as strictly defined by the usual thermodynamic state variables, otherwise than by transfer of matter, work can be said to be done on a thermodynamic system. Part of such surroundings-defined work can have a mechanism just as for system-defined thermodynamic work done by the system, while the rest of such surroundings-defined work appears, to the thermodynamic system, not as a negative amount of thermodynamic work done by it, but, rather, as heat transferred to it. The paddle stirring experiments of Joule provide an example, illustrating the concept of isochoric (or constant volume) mechanical work, in this case sometimes called shaft work. Such work is not thermodynamic work as defined here, because it acts through friction, within, and on the surface of, the thermodynamic system, and does not act through macroscopic forces that the system can spontaneously exert on its surroundings, describable by its state variables. Surroundings-defined work can also be non-mechanical. An example is Joule heating, because it occurs through friction as the electric current passes through the thermodynamic system. When it is done isochorically, and no matter is transferred, such an energy transfer is regarded as a heat transfer into the system of interest.

Thermodynamic work is a specialized version of the concept of work in physics. In the SI system of measurement, work is measured in joules (J). The rate at which work is performed is power.

Sources

“Work (Physics).” Wikipedia, Wikimedia Foundation, 13 Apr. 2020, en.wikipedia.org/wiki/Work_(physics).

“Work (Electrical).” Wikipedia, Wikimedia Foundation, 19 Feb. 2020, en.wikipedia.org/wiki/Work_(electrical).

“Work (Thermodynamics).” Wikipedia, Wikimedia Foundation, 23 Mar. 2020, en.wikipedia.org/wiki/Work_(thermodynamics).

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