Resistance and Ohm’s Law

The resistance of an electrical component tells us how many volts I need to push a current of 1A through it. It is measured in OHMS

1 Ohm = 1Volt per Amp

If we want to measure the resistance of a component, we connect up the circuit as shown.

The voltmeter is connected across the component and the ammeter can be anywhere in the loop. We take a set of values of V and plot them against I. If the V/I graph is a straight line, this means that resistance of the component doesn’t change and the component obeys Ohm’s Law. We can use any pair of values to find the resistance. You shouldn’t think of it as the slope of the line or in this case, the inverse of the slope, just select a convenient point on the line instead, read off V and I and divide one by the other.

Here’s some values to show you…The resistance = 12V / 3A = 4  ohms

I’ve put in a variable resistor so I can change the current in the circuit.

Ohm's Law is obeyed because the graph is a straight line

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