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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?