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Physical Effects of Electricity
11 questions By Tony R. Kuphaldt
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Question 4 of 11
A 22-gauge metal wire three feet in length contains approximately 28.96 ×1021 “free” electrons within its volume. Suppose this wire is placed in an electric circuit conducting a current equal to 6.25 ×1018 electrons per second. That is, if you were able to choose a spot along the length of this wire and were able to count electrons as they drifted by that spot, you would tally 6,250,000,000,000,000,000 electrons passing by each second. (This is a reasonable rate for electric current in a wire of this size.)
Calculate the average velocity of electrons through this wire.
Reveal answerAverage electron velocity = 0.000647 feet per second, or 6.47 ×10−4 ft/s. This is very slow: only 0.00777 inches per second, or 0.197 millimeters per second!
Notes:Despite the rapid progression of the effects of electron motion throughout a circuit (i.e. approximately the speed of light), the actual electron velocity is extremely slow by comparison.
Base figures used in this calculation are as follows:
- • Number of free electrons per cubic meter of metal (an example taken from Encyclopedia Brittanica 15th edition, 1983, volume 6, page 551) = 1029 electrons per m3. The metal type was not specified.
- • 22 gauge wire has a diameter of 0.025 inches.
Questions like this may be challenging to students without a strong math or science background. One problem-solving strategy I have found very useful is to simplify the terms of a problem until a solution becomes obvious, then use that simplified example to establish a pattern (equation) for obtaining a solution given any initial parameters. For instance, what would be the average electron velocity if the current were 28.96 ×1021 electrons per second, the same figure as the number of free electrons residing in the wire? Obviously, the flow velocity would be one wire length per second, or 3 feet per second. Now, alter the current rate so that it is something closer to the one given in the problem (6.25 ×1018), but yet still simple enough to calculate mentally. Say, half the first rate: 14.48 ×1021 electrons per second. Obviously, with a flow rate half as much, the velocity will be half as well: 1.5 feet per second instead of 3 feet per second. A few iterations of this technique should reveal a pattern for solution:
v = 3 I QWhere,
v = Average electron velocity (feet per second)
I = Electric current (electrons per second)
Q = Number of electrons contained in wire
It is also very helpful to have knowledgeable students demonstrate their solution techniques in front of the class so that others may learn novel methods of problem-solving.
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Question 5 of 11
In the late 1700’s, an Italian professor of anatomy, Luigi Galvani, discovered that the leg muscles of a recently deceased frog could be made to twitch when subjected to an electric current. What phenomenon is suggested by Galvani’s discovery? In other words, what does this tell us about the operation of muscle fibers in living creatures? More importantly, what practical importance does this have for people working near electric circuits?
Reveal answerEssentially, muscle fibers are “activated” by electrical signals. I’ll let you figure out what practical importance this effect has for you!
Notes:This question presents an excellent opportunity to discuss one of the important aspects of electrical safety: involuntary muscle contraction.
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Question 6 of 11
Paramedics and other emergency medical personnel sometimes use a machine called a defibrillator to re-start the regular heart-beat of a person whose heart has stopped or is spastically convulsing (fibrillating). Explain what this machine does to “restart” a person’s heart.
Reveal answerA defibrillator machine sends a powerful surge of electric current through the victim’s chest to force his or her heart muscles to contract.
Follow-up question: explain how this principle is similar to that of an electric stun-gun (“TASER”) or other high-voltage, non-lethal weapon to immobilize a person. How does a defibrillator’s function differ from that of a stun-gun?
Notes:This question is intended to start a lively and active discussion on the effects of electric current on human muscle and nerve tissue. Students are likely to come up with all sorts of interesting questions about defibrillators and stun-guns, so be sure to do your own research in preparation for posing this question to them?
