| MadSci Network: Physics |
Hi Chun Ming!
Actually, even
if DC was used instead of AC, one of the wires would still be "live" and the other "dead". Electrostatic surface charge or "electricity" exists
on both of the wires, yet the wires don't behave identically.
Briefly, the difference exists because the electric companies intentionally connect
one of the pair of AC wires to the earth. If you stand on the earth,
then you are connected to the "neutral" wire, and you can safely touch that wire, but you might be
electrocuted if you touch the "hot" wire. If you don't stand on the earth, and if you are totally insulated from the ground, then you won't be
shocked when you touch the "hot" wire.
Power lines behave differently than batteries because batteries are
not connected to the earth. Imagine that we have a 200-volt battery on the table. 200 volts is more than enough to give you a shock. If we
reach out and touch the negative battery terminal... nothing happens. We feel no
shock because there is no complete circuit, there is no circular path for
electric current. Reach out and touch the positive battery terminal, and
the story is the same: no shock. But if we touch BOTH the negative and
the positive terminals at the same time, YOW! We feel intense pain, and
we let go quickly. For electric current to be created in your flesh,
there must be a complete circuit; there must be a circular path
which allows the charges within the conductors to circulate. Two
wires can impose a potential-difference across your flesh, and this
acts like electrical pressure and causes the charges in your body to
flow. A single wire cannot do this.
Suppose we connect one terminal of our 200V battery to the earth. Suppose
we stand on the damp ground with bare feet. Now if we touch the
"grounded" battery terminal, nothing will happen (our feet are already
connected to that terminal electrically.) But if we touch the other
terminal, we get zapped. The positive or negative polarity doesn't matter. The grounded terminal is the "neutral wire", while the
other terminal is the "hot wire." If neither terminal was connected to the earth,
then both terminals would behave the same, and we would only receive a
shock if we touched both terminals at the same time. And if neither side of the AC power line was grounded, it
would be far safer to stand in a pool of water while using electrical
appliances.
WHY does the electric company one side of the AC lines to the ground?
See: WHY THREE PRONGS?
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