If one variable is directly proportional to another variable, then a graph of these two variables will be a straight line passing through the origin of the axes. So, for example, Ohm's Law has been verified if a graph of voltage against current (for a metal conductor at constant temperature) is a straight line passing through (0,0). Similarly, when current flows through a given resistor, the power dissipated is directly proportional to the current squared. If we wanted to verify this fact we could plot a graph of power (vertical) against current squared (horizontal). This graph should also be a straight line passing through (0,0).

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