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# Wiring Fundamentals: Ohm's Law

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## Ohm's Law

### 01.44

 00:00 - As we have now discussed three of the fundamental electrical concepts, and used an analogy to the physical world to understand better how they relate to one another, it's time for some of that math I warned you about earlier to formalise that relationship, and show how it can be used to ensure our wiring harness design is correct. 00:17 There is a simple formula that relates voltage, current, and resistance, known as ohm's law. 00:23 This formula states that between any two points in an electrical circuit, the voltage difference is equal to the current flowing, multiplied by the resistance that current encounters. 00:33 This is commonly stated as V=IR The beauty of this equation is that it can be easily rearranged to calculate any of the three fundamental values in an electronic circuit. 00:44 As long as we know the other two. 00:47 Current for example is equal to the voltage divided by the resistance, I=V/R And resistance is equal to the voltage divided by the current, R=V/I In the automotive electrical world, a most common use of ohm's law, is to calculate the current that will flow in the circuit when we apply a certain voltage to it. 01:08 For example if we know a particular fuel injector has a resistance of 6.9 ohms and we're going to apply an electrical pressure of 13.8 volts across it, we can calculate the resulting current that will flow by dividing the voltage by the resistance. 01:22 13.8 volts divided by 6.9 ohms, gives us a current of two amps. 01:27 This allows us to determine the smallest size of wire we can safely use to run this injector.