The product or quotient of the basic physical quantities, raised to the appropriate powers, in a derived physical quantity. The basic physical quantities of a mechanical system are usually taken to be mass (M), length (L), and time (T). Using these dimensions, the derived physical quantity velocity will have the dimensions L/T and acceleration will have the dimensions L/T2. As force is the product of a mass and an acceleration (see Newton's law of motion), force has the dimensions MLT−2. In electrical work in SI units, current, I, can be regarded as dimensionally independent and the dimensions of other electrical units can be found from standard relationships. Charge, for example, is measured as the product of current and time. It therefore has the dimension IT. Potential difference V is related to the current I and the power P by the relationship P = VI, where P is power. As power is force × distance ÷ time (MLT−2 × L × T−1 = ML2T3), voltage V is given by V = ML2T−3I−1.