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Faraday Cage and Circuit Disconnect
Name: Don
Status: Student
Grade: 9-12
Location: CT
Country: United States
Date: Fall 2009
Question:
If an automobile has been struck by a hot electrical wire
and then proceeds to roll away clear from the wire, is it possible
that the car body continues to hold a charge and needs to be
discharged before someone can actually touch it without fear of
getting shocked?
Replies:
Hi Don
I believe the answer is a definite maybe. There are a couple of things
going on here. First, the power line is carrying an alternating
potential that peaks 120 times a second (60 Hz), so it depends when
the wire disconnects. Secondly, assuming there is no direct discharge
from the car to the earth, the car will act as one side of a
capacitor with respect to the earth, so the car will hold a charge
proportional to the potential on the car and its capacitance. Because
there are so many environmental factors that are also present that
determine conductivity to the ground, it is likely that any charge
will bleed off by itself.
Hope this helps.
Bob Froehlich
Certainly the car body would continue to hold a charge for
some time after being charged up, but the capacitance of a car
is very small -- maybe a few hundred picofarads. You could safely
discharge it by touching it with your finger.
Tim Mooney
Hi Don
Intuitive question.
I think it is possible for a car to hold a Faraday Cage effect charge after
contact with a hot electric wire,
But I have never heard of it happening to any serious extent. Overall, it
depends on how well the tires insulate the charged car from the ground. You
do experience that effect on cold, dry winter days when you stick your key
in the door lock of a car and a small arc of electricity crosses between the
key and the car.
Here is another more common example.
First, Potential difference (voltage) is not always measured from earth
ground. In aircraft, the fuselage is used as the common voltage ground and
is referenced as 0 Volts. Then any circuits that need voltage take the
voltage from the aircraft's generators referenced to the fuselage,
regardless of the actual voltage of the fuselage above earth ground.
When a Navy helicopter delivers cargo to a ship by hanging a cable over the
ship, the ship's crewman wears a special glove that has a cable in it that
runs to the deck in case the helicopter's fuselage is a couple of volts
above earth ground, represented at the ship's deck. If the crewman does not
wear the glove, the crewman is in danger of receiving a shock.
On the trailing edge of the wings of fixed wing aircraft you will find a
black cable with a frayed end at the very tip of the wing. The purpose of
those frayed ends are to discharge the voltage potential difference between
the aircraft and earth ground so the shock when the aircraft lands is not too
large.
Sincere regards,
Mike Stewart
CDR USNR (Ret)
That is an interesting question. We all have heard that a steel car
body provides protection to the occupants if an electric wire falls
onto it, or even (perhaps) if lightning strikes it. That is because
the car body will conduct electricity through the body and frame,
around the outside of the occupants, and the occupants will not be
harmed. That seems to be true. The question is whether the car
body is able to hold some electric charge after being hit from a
falling wire. If an object has some capacitance, and is
electrically insulated, then it has the ability to hold an electric
charge. The car's body can indeed be a capacitor and can hold an
electric charge on its surface, much like a human can. In fact, when
getting out of a car, it is sometimes possible for a person to
receive a startling shock when the person touches a metal part of
the door. So both the human body and the car have
capacitance! Thus a car that is charged by an electric wire and
rolls away may also be able to give a shock, but it would not be
dangerous. One thing to consider is that the electricity in utility
power is alternating, and reverses polarity 120 times each second.
So it is not clear what polarity the car would have after it rolled
away, or if the polarity would actually be near zero.
Robert Erck
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Update: June 2012
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