# Power and Root-Power Quantity > [!NOTE] Power Quantity > A *power quantity* is a quantity *directly proportional* to [[power]]. Examples include energy density and luminous intensity. ^7927e3 > [!NOTE] Root-Power Quantity > A *root-power quantity* is a quantity whose *square*, in linear systems, is proportional to power. Examples include speed, voltage, and current, the latter two being *signal amplitudes*. ^7aaf14