Hardware Reference
In-Depth Information
Electrical Circuits
Modern computers work with electricity, so to interface with them, you need to understand
the basics of how it behaves.
Electricity
is the low of tiny particles, called
electrons,
that carry a charge. Think of elec-
trons as always wanting to get home; for example, with a battery, the electrons want to get
back into the other terminal. As the electrons move through components in a circuit on their
way home they do work. This work might be to emit light in an LED or move a motor
around. If the electrons do not low, no work is done (and the LED does not shine). Figure
9-6 shows three circuits, but only one has a path for electrons to leave the power source,
pass through a component to do work and return home again!
Electronics is all about controlling electrons' journeys! In many cases it is about making or
breaking a complete path for electrons to low.
In describing circuits, there are a few terms that you may come across:
❍
Voltage - this is, in electrical terms, how “strongly” the electrons are pulled
home - that is, how much work they can do while they low through the circuit.
Think of it a bit like a water wheel and a reservoir. The greater the distance the
water falls, the more work it can do turning the water wheel as it lows past.
Voltage
describes the work that can be done and is measured in volts - for
example, an AA battery has a voltage of 1.5V between its terminals. If another
one is connected end to end, then there is greater potential to do work, a volt-
age of 3V.
❍
Ground, or 0V (or sometimes referred to as
negative
) - a reference point to
measure voltage from. If a point in a circuit is at ground, then it is at 0V, and no
work can be done. With the water example, if the water is on the ground, it
can't fall any further so can't be harnessed to do any work.
❍
Resistance - how easily the electrons can low. Different substances allow
electrons to low with different degrees of ease. Conductors, such as metals,
have a low resistance and make it easy for the electrons to low. Insulators,
such as plastics, have high resistances, which make it hard for electrons to
low. Different materials resist the low of electrons by different amounts. Even
water has a fairly low resistance, so it will allow electricity to low through it,
which is why you shouldn't use your Raspberry Pi in the bath!
You can think of these like this:
Voltage
describes the potential to do work (analogous to the
height of water), and
current
describes the rate electricity follows (the rate of low of water
passing a point).
Resistance
describes how easily electricity lows through a material - volt-
age, resistance and current are interrelated - without a voltage existing between two points,
