In an electrical circuit, ground is where electrical current returns to
the source. As discussed in Resistors above, when there is opposition
to the flow of electrical current, there is a backup of voltage. This
is much like a clogged sink where water will back up behind the clog.
In most circuits, the negative side of the battery is much like the
drain of a sink. It must have an unrestricted ability to suck in any
current that comes to it. Let's say you have a point that you think is
ground, but there is some resistance between that point and the actual
ground. Once current flows, voltage will back up behind the resistance.
In fact, the voltage can become dangerously high. Therefore, ground must
have two characteristics. First, it is the point that we compare all
other voltages to and is considered zero volts. Second, it must be able
to absorb whatever current is necessary and yet not change its voltage.
It must have essentially no resistance.
Sourcing and sinking current
We talk about current as if it always flows to ground from some higher
voltage. Current can flow from ground to a lower voltage just as
easily. When conventional current flows into a point, we call that
sinking current (like water going into a sink and down the
drain). When conventional current flows out of a point, we call
that sourcing current. Regardless of whether a ground is sinking
current or sourcing current, its voltage must not change. Therefore,
there must be no significant resistance involved with a ground.
The inherent resistance of the earth is very low. Therefore, the Earth
can absorb or deliver (sink or source) vast amounts of electrical
current. This is why a true Earth ground is vital to many systems.
Let's say you are touching the chassis of a piece of electrical
equipment and that chassis somehow gets connected to the mains voltage
(i.e., a short circuit). If your body is connected to the earth that
will put a considerable voltage difference across you, a dangerous situation to
say the least. However, let's say that chassis is connected to a true
earth ground through a suitably thick wire. Now, even though there may
be a high current traveling through that wire to ground, there is not
enough resistance to cause a significant backup of voltage. Like a
clear drain, nothing backs up; therefore there is little danger to you.
Electricity takes the path of least resistance to ground.
You may have already figured out from
what little you know that electricity doesn't work that way. Let's say
my ground wire has 100 ohms of resistance. If you push 1 ampere of
current through that 100 ohms, you will get a backup of 100 volts. A
hundred volts is a hundred volts. Your body has hundreds of thousands
of ohms of resistance, yet if you touch that 100 volts, you are going to
get dangerously zapped. The electrical current will be proportional to
the resistance. Most will go through the lower resistance, but the
higher resistance carries its allotted share. Under the right
conditions, 100 volts can drive enough current through your body to
cause cardiac arrest. Now let's make the ground wire resistance 1/100
of an ohm. That same ampere of current will only cause a backup of
1/100 of a volt, a much safer level. The reason a true earth ground
protects you is that the tiny resistance of the ground wire will not
allow enough voltage to back up to be significantly dangerous.
What is Ground
Grounded Confusion Mark II - Answers to Questions
Ground and Series-Parallel Confusion - Answers to Questions