
V = R × I I = V / R R = V / I P = V × I
The formulas of Ohm's Law
Ohm's Law can be rewritten in three ways for
calculating current, resistance, and voltage.
If a
current I should flow through a resistor R, the voltage V can be calculated.
V = R × I
If there is a voltage V across a resistor R, a current I flows through it. I can be calculated.
I = V / R
If a current I flows through a resistor, and there is a voltage V across the resistor R can be calculated.
R = V / I
Quantity  Symbol  Unit  Sign 
Voltage  V or E  volt  V 
Current  I  ampere (amp)  A 
Resistance  R  ohm  Ω 
Power  P  watt  W 
Ohm's law. V = I × R, where V is the potential across a circuit element, I is the current through it, and R is its resistance. This is not a generally applicable definition of resistance. It is only applicable to ohmic resistors, those whose resistance R is constant over the range of interest and V obeys a strictly linear relation to I. Materials are said to be ohmic when V depends linearly on R. Metals are ohmic so long as one holds their temperature constant. But changing the temperature of a metal changes R slightly. When the current changes rapidly, as when turning on a lamp, or when using AC sources, slightly nonlinear and nonohmic behavior can be observed. For nonohmic resistors, R is currentdependent and the definition R = dV/dI is far more useful. This is sometimes called the dynamic resistance. Solid state devices such as thermistors are nonohmic and nonlinear. A thermistor's resistance decreases as it warms up, so its dynamic resistance is negative. Tunnel diodes and some electrochemical processes have a complicated I to V curve with a negative resistance region of operation. The dependence of resistance on current is partly due to the change in the device's temperature with increasing current, but other subtle processes also contribute to change in resistance in solid state devices.
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