Download Ohm`s Law and Electric Shock

Ohm’s Law and Electric Shock
What causes electric shock in the human body – current or voltage?
The damaging effects of electric shock are the result of current passing through the
body. From Ohm’s law, we can see that this current depends on the voltage that is
applied and also on the electrical resistance of the human body. The resistance of one’s
body depends on its condition and ranges from about 100 ohms if soaked with salt water
to about 500,000 ohms if the skin is very dry. If we touch the two electrodes of a battery
with dry fingers completing the circuit from one hand to another, we can expect to offer a
resistance of about 100,000 ohms. We usually cannot feel the current produced by 12
volts and 24 volts just barely tingles. However if your skin is moist, 24 volts can be
uncomfortable.
Effect of Electric Currents on the Body
Current (A)
Effect
0.001
Can be felt
0.005
Is painful
0.01
Causes involuntary muscle contractions (spasms)
0.015
Causes loss of muscle control
0.070
If through the heart, serious disruption; probably fatal if
current lasts for more than 1 s
Questions to Answer:
1. At a resistance of 100,000 ohms, what will be the current in your body if you
touch the terminals of a 12 V battery (what is in your car)
2. If your skin is very moist (sweaty) – and your resistance is only 1000 ohms, and
you touch the same terminals (12 V battery), how much current do you receive?
Many people are killed each year by current from common 120-volt electric circuits. If
you touch a faulty 120-volt light fixture with your hand while you are standing on the
ground, there is a 120-volt “electrical pressure” between your hand and the ground.
Given normal body moisture conditions, the current would probably not be enough to do
serious harm. But if you are standing barefoot in a wet bathtub connected through its
plumbing to the ground, the resistance between you and the ground is very small. Your
overall resistance is so low that the 120-volt potential difference may produce harmful
current in your body.
Remember that an electric shock requires a difference in electric potential – a
voltage difference between one part of your body and another part. Most of the current
will pass along the path of least electrical resistance connecting these two points.
Suppose you fell from a bridge and managed to grab onto a high voltage power line,
halting your fall. So long as you touch nothing else of different potential and even if you
hang by TWO hands, no appreciable charge will flow from one hand to the other. This is
because there is no appreciable difference in electric potential between your hands.. If
however, you reach over with one hand and grab onto a wire of different potential…zap!
Questions:
1.
1. Why can a bird perch on a high power wire and not be electrocuted? What
would happen if he put his feet on two different wires? Why?
2.
You are holding onto a high power wire. You are only 3 feet from a metal
tower, but 10 feet from the ground. Is it smarter to swing over to the metal
tower or to drop to the ground? Why?
3.
What causes electric shock, current or voltage? What does damage, current
or voltage?
Most electric plugs and sockets today are wired with three, instead of two,
connections. The principal two flat prongs on a plug are for the current-carrying double
wire. One part is “live” and the other is neutral. The round prong connects to a wire in
the electrical system that is grounded – connected directly to the ground. The electric
appliance at the other end of the plug is therefore connected to all three wires. If the live
wire in the plugged-in appliance accidentally comes in contact with the metal surface of
the appliance, and you touch the appliance, you could receive a dangerous shock. This
won’t occur when the appliance casing is grounded (via the ground wire), which ensures
that the appliance casing is always at zero ground potential.
Electric shock can overheat tissues in the body and disrupt normal nerve
functions. It can upset the nerve center that controls breathing. In rescuing shock
victims, the first thing to do is find and turn off the power source. Then do CPR until
help arrives. For hear-attack victims, on the other hand, electric shock can sometimes be
beneficial in getting the heat beat started again.
Questions:
1. Why do plugs now have three connections instead of two?
2. What is the first thing you do when rescuing persons who may have been
electrically shocked?