When charge \( Q \), due to an applied voltage \( U \) is transported in the conductor, potential energy is converted into kinetic energy. The kinetic energy \( W \) that a charge gains (\(W\) positive)) or loses (\(W\) negative)) by passing through the voltage \(U\) is given by:

Formula anchor$$ \begin{align} W ~=~ Q \, U \end{align} $$

The power \(P\) is defined as the converted energy \(W\) per time period \(t\):

Definition of power quantity

Formula anchor$$ \begin{align} P ~=~ \frac{W}{t} \end{align} $$

The electric power is obtained by substituting Eq. 1 into 2:

Electric power using charge, voltage and time

Formula anchor$$ \begin{align} P ~=~ \frac{Q \, U }{t} \end{align} $$

The electric current \(I\), is the charge \(Q\) transported per time interval \(t\): \(I = Q/t \). The factor \(Q/t\) is in Eq. 3, so we replace it with \(I\) to eliminate the unknown and experimentally not easily accessible time \(t\). Thus, the electric power becomes:

Formula anchor$$ \begin{align} P ~=~ U \, I \end{align} $$

For an Ohmic conductor (these are those conductors for which Ohm's law applies), Equation 4 can be rewritten using \( U = R \, I \). The power \(P\) can therefore be expressed by the resistance \(R\) of the conductor (or a load) and the applied voltage \(U\):

+ Perfect for high school and undergraduate physics students + Contains over 500 illustrated formulas on just 140 pages + Contains tables with examples and measured constants + Easy for everyone because without vectors and integrals