Voltage (volts) is the force that moves electrons, forcing
a current. Voltage can be compared to a tank of water elevated at a
certain height (potential). If the tank is placed low (low voltage),
water will not flow very quickly (low current). If the tank is raised to
a higher location (higher voltage), the water will flow rapidly (high
Current (Amperes) is, in simple terms a measurement of how
many electrons flow through a device. In the water tank analogy, current
would be water flow rate.
Resistance (Ohms) slows down current flow. The
higher the resistance of a circuit, the lower the current will be.
Resistance would be equivalent to pipe size. If you have the water tank
at a high level, but the pipe is very small in diameter (high resistance), not
much water will flow. If you use a big pipe (low resistance), then the
water flow rate will be larger.
Knowing the relationships between voltage, current and resistance
brings us to ohm's law: "Current is proportional to Voltage divided by
Resistance". This equation can be manipulated to obtain any value
knowing the other two. For example, by measuring the voltage and current
of a circuit, resistance can be calculated by dividing voltage by
current. When a circuit is open (disconnected), the resistance is
infinite (zero current). The formula also shows what happens when a
circuit is shorted (resistance = 0): The battery will put out as much current
as it can instantaneously (not a good sight).
DC vs. AC Circuits
On a DC circuit, current flows in one direction only.
Voltage can remain at a level or change, but it always has the same
polarity. A car's battery produces DC voltage.
AC circuits are a bit more complicated to
understand. The voltage supply reverses its polarity switching from
positive to negative. The current produced goes in one direction while
the voltage is positive and then flows in the opposite direction when voltage
is reversed. AC circuits have a frequency associated with them.
The frequency (Hertz or Hz) is how many times per second (cycles) the current
(and voltage) switch from positive to negative and back. The higher the
frequency, the faster the circuit will switch polarity. AC voltage in
the car is produced by the alternator, which is converted to DC voltage to
charge the battery (even though some of the AC energy from the alternator
remains in the electrical system, this is what causes alternator whine when
the car is running). The audio signal that comes from the head unit,
gets amplified and drives the speakers is also an AC
A Typical DC Circuit
in a circuit from the negative side of the battery to the positive side of the
battery (that is why physicists will argue with the direction of the current
in the circuit). Engineers represent current in the opposite direction
of electron flow, as in the diagram. It does not matter what convention
you follow for current direction. The important thing to keep in mind is
how much current flows through the circuit, and that you stick to only one of
the models when analyzing a circuit.
For a circuit to have current, there
has to be a path (i. e. wire) and a battery. A circuit also has a
resistance, which slows down flow of electrons.
If the path is broken,
current can not flow. The battery supplies the voltage. The top
portion of the circuit in a car is represented by the positive battery cable
going to the fuse box and to all the accessories (radio, wipers, lights,
etc). Each accessory has a resistance. As more accessories are
added, the resistance drops, and more current flows through the circuit.
To save money, car manufacturers use the car metal for the bottom part of the
circuit, instead of running a ground wire to every
Common Engineering Notations
To represent very high or low values, zeros or decimal
points are represented by letters. These are the most common used in car
| µ (micro)
|| Capacitors, which are measured in
|| 0.000001F = 1µF|
| m (mili)
|| Capacitors (F), inductors (Henries),
voltage (V), current (A)
|| 0.001Volts = 1mV|
| k (kilo)
|| Resistance (Ohms), frequency (Hertz),
|| 1000W = 1kW|
| M (mega)
|| Frequency (Hz),
|| 1,000,000 Hz =