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.