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Thursday, October 28, 2010

Ohm's Law

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

Name Formula sign Unit Symbol
voltage V or E volt V

current I ampere (amp) A

resistance R ohm Ω

power P watt W

Eddy Current Inspection Formula - Ohm's Law
I = Current (amp)

V = Voltage (volt)

Z = Impedance (ohm)

Ohm's Law is a mathematical equation that shows the relationship between electric voltage (V), current (I) and impedance (Z). Ohm's Law is often seen with impedance replaced by resistance, which is fine for electricians wiring a house with only resistance in the circuits. However, in nondestructive testing the circuits also have inductive reactance and capacitance which also impede the flow of electricity. Therefore, impedance is used in this version of Ohm's Law. Ohm's Law can be expressed in the following three ways.

V = I x Z

Z = V / I

I = V / Z

Ohm's law is used in eddy current testing to determine the change in impedance caused by some change in the material or loading of the test coil. Eddy current instruments typically apply a constant AC current and monitor the voltage or visa versa. A change in the voltage (or current) in an eddy current test system can be directly related to a change in impedance.