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Fig A6
In theory pure metals contain a sea of freely moving electrons. so if
you pump electrons in at one end, electrons pop out at the same rate at the other. The
most useful analogy for resistors I've seen is to imagine them as impurities that slow down
the movement of electrons by bouncing them around (and giving off heat) [Fig A6].
If we connected the 5v to 0v with a simple bit of wire, it would have almost no resistance,
so there would be almost no limit to the rate of electrons (and current) and the power used
would be very (dangerously) high
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.
So, what a resistor does is slow down the flow of electrons to a rate in proportion to the
voltage difference between the top and bottom of the resistor:
Fig A7
Consequently, the Voltage between the top and bottom of a resistor will
be proportional to the current and the resistance (that is, if the current is limited for some
other reason, then less energy will be lost across the resistor so there will be some left at
the bottom, which means the voltage drop across the resistor will be less) [Fig A7]:
Volts = Current x Resistance
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E.g. [DON'T TRY THIS] Imagine a 1.5v battery connected by 5cm of 28SWG wire at 4.2Ω/metre. The resis-
tance would be 4.2Ω x 0.05 = 0.21Ω. Current=Volts/Resistance = 1.5/0.21 = 7.14Amps. So, power would be
about 11Watts and the battery would last about 19 minutes.
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