Ohm’s Law

Ohm's law is probably the most fundamental equation used by electrical engineers. It states that the voltage (V - volts) across a resistor is given by the current (I - amperes) flowing through it, multiplied by the resistance (R - ohms).

Equation: V = I R
The direction of the voltage (V) arrow across a resistor is always opposite to the flow of conventional current through the resistor.

Worked Example 1

Find the voltage across a 6.8 Ω resistor if the current flowing through it is 0.2 A.
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V = I × R

  = 0.2 × 6.8

  = 1.36 V

Alternative Forms

The above equation may easily be transposed to make current, I or resistance, R the subject:

Equation: I = V / R


Equation: R = V / I

However, a possibly easier alternative to transposition is to remember the Ohm's law triangle, using this to visually derive the three forms of Ohm's law.

First, draw the triangle and then cover the variable on the left of the equals sign. The arrangement of the remaining two variables gives the right-hand side.

You can use the triangle method to visually transpose any equation with this same general form.

Worked Example 2

Find the current flowing through a 120 Ω resistor if the applied voltage is 30 V.
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I = V ÷ R

  = 30 / 120

  = 0.25 A (or 250 mA)

Worked Example 3

Calculate the resistance of a wirewound resistor if a current of 0.5 A flows when the applied voltage is 20 V.
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R = V ÷ I

  = 20 / 0.5

  = 40 Ω

It is important to avoid exceeding the power rating of the resistor, which may cause it to overheat. You should always calculate the maximum power consumed and confirm this is less than the power rating of the resistor.


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