# OHM'S LAW

What's going on there? To make sense of it, we need to know about a fundamental relationship between the voltage V across a resistor, the current I flowing through it, and its resistance R. This relationship is called Ohm's Law [1]:
V = I x R
For example, in the circuit at right, if the battery voltage is 5 volts and the resistor has a value of 1k ohms, the current flowing through it is 5 / 1000 = 5mA.

## Voltage Divider

Now, here's the circuit we built a moment ago. The two halves of the N-type silicon form two resistors in series. They're the same length, so they have equal resistances. Because the same current is flowing through both resistors, it follows from Ohm's Law that the voltages across them must be equal. They add up to 5 volts, there must be 2.5 volts across each resistor. Hence the purple colour of the output.
If you look carefully, you'll notice that there is a voltage gradient from one end of the N strip to the other. You may like to try moving the output point up and down to tap off different voltages and see how the LED reacts to them.
This kind of arrangement is called a "voltage divider". We won't have much use for it in digital circuits, but the principles involved are good to know about, especially if you might like to work in our analog circuit division later on.
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[1] George Ohm was a really smart guy who lived around 1800. He was so keen on electricity as a kid that his parents named him after an electrical unit.