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    Ohm's law for a full circuit

    Consider a basic electric circuit with a real voltage source and load, which is represented with resistor R. The real voltage source has also its own inner resistance R0.

    full circuit scheme.PNG

    The current I is:

    I=ER+R0I = \frac{E}{R + R_{0}}

    The voltage U is not the same as electromotive force E, because some portion of voltage drops at R0:

    U=E−U0U = E - U_{0}

    Because U is also I*R, we can compute it also this way:

    U=E⋅RR+R0U = \frac{E\cdot R}{R + R_{0}}

    The power consumed by load is:

    P=U⋅IP = U\cdot I

    or, expressed through the consumer's resistance R:

    P=(ER+R0)2⋅RP = \left ( \frac{E}{R + R_{0}} \right )^{2}\cdot R

    Consider E = 12 V, and R0 = 1.5 Ω. Let's graph the power dependence on consumer resistance R: