APPENDIX C

In a previous experiment you discovered the attractive electrical interaction caused by static electricity acting on neutrally charged objects (e.g. the "charged" comb (with an excess of negative charges or electrons) attracting the paper bits after they developed excess positive charges at the surfaces nearest the comb). You also intercompared the strengths of gravity, the electrically charged comb and magnets. If a charged plastic comb can pick up bits of paper, do you think that an electrically charged Styrofoam plate can rotate a large 2" x 4" board?

From an early age we are all taught to have a healthy respect for a different form of electricity in the home, namely current electricity. The batteries we use in our experiments with direct current circuits create very little power, and are harmless. However, the power supplied by the electric power companies can produce electric currents that can be lethal.


Effects of Electricity on the Body

Direct Current Circuits
Sweat acts like acid in a battery (an electrolyte solution). The electrons in the copper plate are very mobile and move from the copper to your hand. On the other hand, the aluminum draws electrons from your hand. Thus by placing your hands on the two different metals your body is no longer electrically neutral, but develops a difference in the type of net electric charge (positive or negative) between your two hands (a potential difference or voltage is developed between your hands). Any time there is a potential difference in a conductor, electric charge (electrons or other charged particles) flows, which creates an electric current. Voltage is required to create a current. Although your body is a very good conductor of electricity, it does offer some resistance to the flow of current, with the greatest resistance located in the skin. Dry skin is a very poor conductor of electricity. What happened in the above experiment when your hands were wet? Did the resistance of your skin increase or decrease?

The current meter (ammeter) probably registered about 0.1 mA (0.1 milli Amperes or 0.0001 A) of current passing through your body when your hands were wet. The electric resistance of your wet hand is large (about 20,000 Ohms). This means that the voltage developed across your body when your wet hands were pressed down onto the copper and aluminum plates was about 1.5 volts (the same as a D cell, but the current your body can deliver is much less than a D cell battery). (Note: Voltage = resistance x current). If you place your wet fingers across the ends of a 1.5 volt D cell battery, your body draws only: current = voltage/resistance = 1.5 v/20,000 Ohms = 0.0001 Amps. The electric power in your body when you were a "human battery" in this experiment or if you were to hold a D cell battery with a wet hand is only about 0.0002 Watts (a 15 watt bulb generates 75,000 times more electric power) (Note: Power = current x voltage).

What current would your wet hands draw from the 9 volt battery? What electric power would pass through your body if you held a 9 volt battery with wet hands? How much electric power is needed to cause an electric "shock"?

Alternating Current Circuits
If you flipped the battery back and forth changing polarity very rapidly in the circuits you built above, you would create current in the circuit that would reverse direction each time you reversed the polarity of the battery in the battery holder. This of course is not a very practical way to create alternating current electricity. The power company supplies alternating current, because it is easier to generate large amounts of electric power of the ac variety than the dc (direct current) variety. The power company delivers electric power at 120 volts (an average of each cycle, since the voltage alternates between positive and negative voltages as the current reverses direction). The rate at which the current reverses direction is set by the power company, and in the U.S. is (unfortunately!) 60 times each second (60 cycles/second or 60 Her). Hence we refer to the electric power delivered to our homes as R120 volt, 60 Her electric power.

The amount of current an appliance draws depends on the ability of an appliance to resist electricity. Resistance depends on the material through which the electricity is passing. The human body (especially the skin) has a fairly high resistance to electricity. However, the power delivered by the electric company is sufficiently high to be very dangerous to humans. To make things worse the nerve impulses in the human body resonate (react very strongly) at a frequency of the current alternating directions at the frequency of 60 times each second (60 Her). This unfortunate biological coincidence means that nerves in the human body have maximum responsiveness to the ac electricity with a frequency of 60 times a second delivered to our homes by the electric power company.

Below is a table* which gives for two assumed resistance's of the human body (10,000 Ohms and 1000 Ohms) the current which would be drawn if you received an electric shock by making contact with an ac outlet in your home for 1 second. The resistance for the circuit that is completed when a human serves as a conduit for electricity is set by the skin. Dry skin does not conduct electricity very well, and therefore has a high resistance, about 10-100k . Moist skin is a very good electrical conductor, and therefore has a small resistance, closer to 1k. The physiological effect of a potentially fatal electric shock is to cause the heart to beat erratically (fibrillation), and unless defibrillated can cause death.

Electric Current Amperes	Voltage for Body Re- sistance 10,000 Ohms	Voltage for Body Re- sistance 1,000 Ohms	Maxi- mum Power Watts	Physiological Effect
0.001 A	10 V	1 V	0.01 W	Threshold of feeling an electric shock
0.005 A	50 V	5 V	0.25 W	Maximum current which would be harmless
0.01-0.02 A	100-200 V	10-20 V	1-4 W	Sustained muscular contraction. "Cannot let go" current.
0.050 A	500 V	50 V	25 W	Ventricular interference, pain, respiratory difficulty
0.1-0.3 A	1000- 3000 V	100-300 V	100-900 W	Ventricular fibrillation. Can be fatal.
6 A	60,000 V	6,000 V	400,000 W	Sustained ventricular contraction followed by normal heart rhythm. These are the operation parame- ters for a Defibrillator. Tempo- rary respiratory paralysis and possibly burns.

A dry finger in an electric outlet or a lamp socket will cause a "good shock", a wet finger could cause a fatal electric shock. A hair dryer or radio dropped accidentally into a bath tub can be fatal even if not switched on. Although you will not use ac electricity, and never use batteries larger than 9 v, be aware of the potential danger of household ac electricity. Do not attempt any of these experiments using ac current. The 9 v batteries are harmless and therefore useful tools with which to learn the fundamentals of electrical phenomena.

In contrast a bolt of lightning can discharge at 100,000 v and run a current of 100? amperes through a human being. About 500 people are struck by lightning in the U.S. each year. Avoid lightning by taking cover during thunderstorms, not under a tree, but in house. Even a car is good shelter during a thunderstorm not only from the rain, but it can protect you from a lethal lightning bolt. Why wouldn’t you be harmed if you were in your car and it was struck by lightning? What shouldn’t you do immediately after your car is hit by lightning?

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Last modified 9 Aug 1997
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