• Basic electricity formulas

    Although it not specific to sound, we include this document with some basic electricity formulas. They can be found in any electricity textbook, but we have added them to the DoPA Library for reference.

    Ohm's law

    The most basic formulas derive from Ohm's law, which specifies that the electric current between two points is proportional to the potential difference (voltage) between them and inversely proportional to the resistance between them. The formula is:
    I = ———

    where I is the current (intensity) in amps and V is the voltage in volts. Since we use alternating current in audio, we have replaced resistance with impedance (Z, and this could also be resistance R), measured in ohms. Clearing Z and V we have these other two formulas:

    Z = ———

    V = I*Z

    Joule's law

    Electrical power is the rate at which an electrical circuit transfers electrical energy. It is measured in watts, after the engineer James Watt famous for his advances in steam engines.

    This formula is erroneously assigned to Ohm's law even though the original law did not include it. Joule's law determines that electrical power is given by the product of voltage and current:
    P = V*I

    Combinations of both laws

    When we combine both laws (Ohm's law which determines the current and Joule's law which determines the power) we obtain numerous derived formulas which are added to the original ones. A usual way of bringing them all together is by means of this circle of formulas: