What is a Circuit?
Ok, time to get back to work. Thus far we have discussed voltage and current. This time we are going to talk about another electrical concept, the “CIRCUIT”. Have you ever heard someone talking about an electrical problem and heard them say, “must have been a short”?? This is the short version (no pun intended) of the phrase, “short circuit”, and it is one of the most used and perhaps mis-used phrases in electricity. In order to understand what a “short” or a short circuit is, we must first understand what a circuit is.
In a previous discussion we talked about current flow. In any electrical system, voltage (the pressure) causes current to flow in a CIRCUIT. In a direct current system, the current flows in only one direction, while in an Alternating Current system, the current flows in both directions. The current flows in a CIRCUIT.
What is a circuit? It is the complete path through which electrical current flows.
In a simple direct current (DC) circuit, such as a battery, a switch, and a light, the current leaves the battery, flows through wire to the switch, through more wire to the lamp, then through more wire back to the battery, a complete circuit.
An alternating current (AC) circuit works in a similar manner except the electrons which make up the current flow in the circuit, move back and forth in the circuit, moving first in one direction, then reversing and moving in the other direction. As far as the operation of the circuit goes, it is the same in that there is a beginning, and end, and no interruptions between.
In our simple circuit example above, the current (either DC or AC) flowed through all the components of the circuit, the battery, the wire, the switch, and the lamp. In a short circuit, the electrical current does not pass through one or more of the components of the circuit. Normally this will cause problems in the operation of the circuit.
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Larry Cad
Jon, excellent point, thus the comment about the electrons vibrating!!
Jon Vermilye
Actually, the electrons don’t actually move all that much. Although electricity moves through a wire at the speed of light, the electrons don’t. If all it took was a couple of volts pushing electrons to move them at the speed of light we wouldn’t need to build cyclotrons and other particle accelerators to get them moving. The best comparison I’ve seen is to think of the effect of a cue ball hitting a row of billiard balls in a line. The energy from the cue ball hitting the first one is transfered to each in line & the final one moves.
larrycad
Mac, do you REALLY want to go there?????
Mac McClellan
Yes, but are the electrons flowing, or is it the holes? 😀
(Rhetorical question strictly for the purpose of humor. No warranties express or implied, void where prohibited, etc.)
larrycad
Price, thanks for that. I was going to calculate it out but you saved me the trouble.
Price Martin
And during that 1/60 of a second an electron will move 3,100 miles.
larrycad
In theory, AC electrons move back and forth an equal distance with each cycle. This is particularly true if both “half cycles” of the AC sine wave are of equal amplitude. The electrons actually vibrate in place and don’t typically move along in the wire. Much of this discussion is theoretical as it is almost impossible to actually “see” electrons. The time base for a single AC sine wave is 16 milliseconds, or 1/60th of a second. As the half cycle begins (at zero volts) and climbs to its peak voltage, a particular electron is displaced from its beginning point to a point some distance from the beginning point. As the wave voltage increases the displacement speed increases, then as the wave voltage decreases back to zero the displacement speed decreases to zero. Then the whole thing starts again, except in the opposite direction with the electron returning to its original location. Thus, it appears to vibrate.
2oldman
Thanks.. it helps a little.
Does Ac move back and forth equal distances in a vibration (nothing actually moves), or does the vibration ‘march’ in a direction?
larrycad
Mr. Oldman, your observation that the analogy breaks down when discussing AC current is correct. This is a common weakness with most analogies. In reality, the analogy when applied to DC current breaks down also as DC current doesn’t actually “run” out of the wire as the water does with the hose. A more accurate analogy would be to visualize the water contained within the hose, and to be able to observe the water as it moves within the hose. If the hose is a complete circuit as discussed above, the water would always return to it’s source (i.e. a pump?). The difference between AC and DC then would be that with DC, the water would always move in on direction, while with AC, the water would move first in one direction, and then reverse and move in the opposite direction. If the water was “60 cycle or 60 hertz” water, it would change direction 60 times a second, or would appear to vibrate within the hose. This is a much closer description of what goes on in a wire which is connected into a circuit. The electrons move along the length of the wire in one direction with DC, or vibrate within the wire with AC.
For this main reason, most electrical training courses have a whole seperate section for teaching DC and AC. Although we talk about them as being similar, they work and act differently in circuits.
Hope this helps a little.
Larry
2oldman
I apologize if I’ve missed something in this or your previous posts.
I understand the water hose analogy for DC, but after all these years I’ve yet to get my brain around AC amperage. How is that analogous to water in a hose? Or is it? I mean, water doesn’t come out of the hose and then jump back in.
Please explain! thanks