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Power
Power is the time rate of transferring or transforming energy. The unit of power in the MKS system is the watt or joule per second. If w = energy in joules then by
Equation 12 can be restated in terms of power as follows
Part of the power out of the system may be useful power since the mechanical power output of an electric motor and part of the power out of the system may be in the form of heat produced by electrical and mechanical losses. Equation 11 gives the relationship between work dw, force F, and an infinitesimal displacement dx. If Eq. 11 is differentiated with respect to time, the following expression is obtained for the instantaneous power
The work dw associated with the motion of an infinitesimal charge dq, against the electrostatic force of a field, through a potential difference of v volts is expressed by
and the power is
where i = dq/dt is the electric current expressed in amperes.


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