In most text books and other electronics material it's said current is flowing through the circuit. Isn't this wrong? As current is flow of charge. Is there anything such as flow of flow of charge?
Asked
Active
Viewed 1,385 times
9
-
It's a common, harmless redundancy. – Alfred Centauri Mar 21 '13 at 19:07
-
"Current is flowing" does not equal "flow of flow of charge" it equals "flow of charge is flowing" – DrFriedParts Mar 21 '13 at 19:39
-
2It's like saying a river is flowing. Actually the river stays in the same place and only the water in the river flows. However, it is common to talk about "current flow", and everyone that isn't trying to be anal will know what you mean. – Olin Lathrop Mar 21 '13 at 20:03
-
Not exactly harmless. In my experience, the main source of public confusion over the nature of electricity is the wrong idea that "current" is like a substance, and that "current" can move around inside the wires. At the same time, most people aren't aware that metals are already full of mobile charges. Where did they get these misconceptions? Probably from explanations about "flow of current," where flows of charge are never mentioned. Me, I find that muddled concepts become very clear if the phrase "flow of current" is carefully avoided, and instead we teach about "flow of charges." – wbeaty Apr 01 '13 at 04:37
7 Answers
12
"Flowing" qualifies state of flow
"Current is flowing" implies current not equal to 0.
It's semantically correct. Same as, "The river is flowing."
Websters, #2: River: A large quantity of a flowing substance
It describes the degree to which the flow is occurring (e.g. not zero).
The presumption of the question is wrong
"Current is flowing" does not equal "flow of flow of charge"...
...it equals "flow of charge is flowing."
At worse, it's redundant (not wrong). Typically it's just used as I described above (to indicate a non-zero quantity).
For non-English majors (like me!), here is more detail on this specific use case from english.SE.
Essential vs. temporary attribution
Here's a highlight from the English community (user Choster, a professional writer in Washington, DC):
We would identify something as a river because we ordinarily expect it to have the characteristics of a river, even if some of those characteristics are temporarily suspended. That is, a permanent or semi-permanent stream would be a river even if it has temporarily stopped streaming, as noted in the comments, by drying up, being dammed up, or being frozen solid.
On the other hand, something which only temporarily takes on the characteristics of a river could be called so, metaphorically, but only while it possesses those characteristics. If they cease, the metaphor is no longer applicable. If I spill a jar of molasses, I may report a river of molasses running down the table leg, but once the flow stops, we would lose the river (having gained a pool).
Current is a flow and flows may be interrupted, suspended, and resumed. A charge does not lose its ordinary expectation to flow just because it is momentarily not moving.
Flow is a rate
Electric current is the rate of movement, not strictly "electric charge in motion". Because this difference is subtle and has little impact on day-to-day design and analysis work, it's largely ignored (as it should be).
A quantity "charge in motion" would have units of yes/no -- is it "charge in motion" or not? To be more useful we need quantities that describe the motion.
That is why current is defined as the rate at which charge transits a normal surface. That is, current is the flow of charges, described by how much charge moves in unit time.
Perhaps it is more illustrative to see that "velocity" is not "meters in motion," even though velocity is describing the motion in units of meters (per unit time).
Another way to intuitively grasp this idea is to work backwards. In ordinary practice we readily except the idea of "zero as a number" and in engineering (especially appertaining current) doubly so -- "The current into the lightbulb is 0-Amps."
The reason current can be 0, as a quantity, is because it is describing a rate.
DrFriedParts
- 12,482
- 36
- 54
-
The river is flowing with water; the current is flowing with electrons (but the other way). Sorry!! – Andy aka Mar 21 '13 at 18:41
-
@Andyaka but he never said in the river reference that the current was flowing, he said the river was, and in electronics we dont have different names. – Kortuk Mar 21 '13 at 19:15
-
"Current is a flow and flows may be interrupted, suspended, and resumed." Electric current is, physically, electric charge in motion. You're using the word "flow" in two different senses - that's equivocation - a logical fallacy. – Alfred Centauri Mar 22 '13 at 20:34
-
-
1
-
@alfred -- (1) Equivocation requires two applications or entities, but you provided only one instance. (2) Electric current is the rate of movement, not "electric charge in motion". You are confused about the concept of a "flow". – DrFriedParts Mar 23 '13 at 02:44
-
@DrFriedParts, I think you ought to reconsider your position that electric current is the rate of movement. – Alfred Centauri Mar 23 '13 at 03:00
5
See this website about common electricity misconceptions. The author, William Beaty, addresses your question exactly:
THE STUFF THAT FLOWS THROUGH WIRES IS CALLED 'ELECTRIC CURRENT'? Horribly misleading!
Most K-12 textbooks discuss a substance or energy called "current". They constantly talk about "flows of current." However, here's a pointed question: what flows in rivers? Is it water, or is it "current?" If I fill a bucket from the faucet, is my bucket full of "current?" No! Water moves, water flows in pipes, not "current." A flow of water is a correct phrase, while "a flow of current" is not. The same idea applies to electricity: electric current is a flow of a substance, but the name of the substance is not "current."
Since a current is a flow of charge, the common expression "flow of current" should be avoided, since literally it means "flow of flow of charge." --Modern College Physics: Sears, Wehr, & Zemanski
. . .
Go check out his website -- he talks about several similar misconceptions.
Justin
- 5,944
- 21
- 34
-
-
2"Flows of Current" is clearly wrong. But that isn't the question the OP asks. It's "Current is flowing" <-- which is actually quite correct in the English language. – DrFriedParts Mar 21 '13 at 19:38
-
Here's a pointed question: if you are running a waterwheel, do you care about water, or current? Please show me an electrical component that operates like a bucket. – Phil Frost Mar 22 '13 at 21:03
3
This is a subject matter for http://english.stackexchange.com. and perhaps http://ell.stackexchange.com
Natural language cannot be read by macro-expansion of words with their definitions. You have to process language semantically, and not in a machine-like manner.
Flowing is what a flow does. A flow of charge flows through a circuit, hence a current flows. The charge flows also.
But if we ask what is the flow of, it is a flow of charge, not a flow of the flow of charge (even though the flow of charge flows). A flow does have a flow rate, however. What is the flow rate of that flow? is a grammatical question. We can elide the first flow in that sentence, but we can also elide rate: What is the flow of that flow? The first flow refers to the quantity, and second to the phenomenon.
Do you also suspect that it's wrong to say that a river flows, because it is actually water that flows whereas the river stays where it is? The idea of such a semantic restriction runs contrary to numerous human languages from all over the globe.
Kaz
- 19,838
- 1
- 39
- 82
-
Good point. "Wind blows" and "current flows" are fine for everyday conversation. Physics explanations are different, and there we'd use strict and narrow meanings. There we'd typically *require* machine-like replacement of terms with their definitions. "Current flows" is unscientific, redundant, and perhaps even misleading to students, since current = charge flow. *Charges* flow, while currents arise and vanish, and while their charges are flowing, the currents may sit unmoving within circuitry. (So, when a pattern of current shifts position, only that would be a "flow of current?") – wbeaty Apr 01 '13 at 05:12
1
I don't see anything wrong with it. Can we talk about wind blowing or a gust of wind? Wind is, by definition, a flow of air. Wind is the word we use for air when what we care about is the air's movement, and not really the air. Yet, we wouldn't complain about gusts of wind, would we? If gusts of wind are valid, what's wrong with flows of current?
Even when there is no electromotive force, the current flow is still defined. It is zero, which is different than saying it doesn't exist, or is otherwise invalid. On a stagnant day, wind doesn't become air. The wind is still there. It is zero. Nor on a windy day does the air does become wind.
It would make sense that electrical engineers would favor current over charge, because charge that isn't moving generally doesn't do anything useful to the electrical engineer, as air that isn't moving can't do anything useful to the windmill operator.
DrFriedParts
- 12,482
- 36
- 54
Phil Frost
- 56,804
- 17
- 141
- 262
-
1I don't think "runners running" is a good example. A runner, in this context, is, e.g., *a person who runs competitively as a sport or hobby*. To say that a person is a "runner" is not to say that the person is running. However, a current is *not* a charge that sometimes moves, *a current is charge in motion*. If the runner stops running, she is still a runner. If the charge stops moving, the charge is not a current. – Alfred Centauri Mar 22 '13 at 19:22
-
@DrFriedParts I'd like to see you do anything useful with a capacitor without ever moving any charge. "Electrostatic" is just another way of saying not moving _much_ charge. It still moves, or did at one point, or will eventually. – Phil Frost Mar 22 '13 at 20:45
-
@AlfredCentauri No, in this context, a _runner_ is someone who _is running in a race_. As in, "seven runners will be participating in today's event," not, "I met this very interesting runner at the bar last night." The runner has no purpose but to run, as in electrical engineering the charge has no purpose but to be pushed around by electromotive force. – Phil Frost Mar 22 '13 at 20:57
-
@PhilFrost, your further clarification doesn't remove the essential problem with your example. This is yet another example of equivocation. If your runners are those "running in a race", they are runners before the race starts ("runners will be participating...") and after the race ends ("runners participated...") even though, before and after, they aren't necessarily physically *running*, i.e., they may be walking. Electric current is electric charge in motion and not electric charge that *will* be in motion or that *has* been in motion, but *is* in motion. – Alfred Centauri Mar 22 '13 at 22:32
-
@Alfred -- A current is not a "charge in motion". A charge in motion creates a current, but they are not identical. I don't think you correctly understand the concept of a "flow". You are welcome to your own interpretation, but you should understand that it is inconsistent with the English language and professional practice. – DrFriedParts Mar 23 '13 at 02:33
-
@DrFriedParts: "You are welcome to your own interpretation". As are you. But, evidently, "my" *interpretation* is not only mine. From, e.g., Britannica: "electric current, any movement of electric charge carriers...". Charge in motion doesn't *create* a current, it *is* as current. – Alfred Centauri Mar 23 '13 at 02:58
-
@Alfred -- Britannica is either naive or being used out-of-context. [Wikipedia](http://en.wikipedia.org/wiki/Ampere) has a long list of engineering and science textbooks with the correct definition. – DrFriedParts Mar 23 '13 at 03:01
-
-
-
@DrFriedParts, from Jackson's "Classical Electrodynamics", Third Edition, page 176: "... *realize that current is actually charge in motion*...". According to Wiley, this is "... the defining text for the graduate-level course in Electricity and Magnetism": http://www.wiley.com/WileyCDA/WileyTitle/productCd-047130932X.html Is this an adequate reference? Shall I find another? – Alfred Centauri Mar 23 '13 at 03:13
-
@AlfredCentauri "in motion" is not strictly "moving". A rest can be music. A hesitation can be dance. Zero Amperes can be a motion of charge. – Phil Frost Mar 23 '13 at 03:16
-
-
@Alfred -- This got pointless a long time ago, so this is the last I'll say on this issue. Your quotes are consistently out of context. In the introduction to chapter 6 you cite, Dr. Jackson is explaining why E and M fields are related in electrodynamics. In this context he is only concerned with the *function* of current, not the definition. His actual words (1st edition), "currents which produce magnetic fields are basically electrical in character, being charges in motion." The distinction is subtle and I doubt anyone (including me) cares, but it's there. – DrFriedParts Mar 23 '13 at 03:55
-
@DrFriedParts, just to be sure, here is the quote I gave in full context: "One apparent way out of this difficulty is to realize that current is *actually* charge in motion and to replace *I dl* by *qv* where *q* is the charge and *v* its velocity". It doesn't get much clearer than that. Remember, you made the following claim a comment above: *A current is not a "charge in motion"*. I've simply shown that your claim cannot be sustained. – Alfred Centauri Mar 23 '13 at 12:36
-
@DrFriedParts, finally, there is a footnote at the bottom of page 176. The second sentence is: "Currents are, after all, *charges in motion*". Now, I see you've edited your answer and concede that current is, *at least*, charge in motion. Progress... – Alfred Centauri Mar 23 '13 at 12:50
0
Strictly speaking, yes, current is a flow of charge. When it comes to a flow of anything, be it charge, water, or hydraulic fluid, a rate needs units attached to it. For convenience, we use the relationship \$1A=\dfrac{1C}{s}\$.
Matt Young
- 13,734
- 5
- 34
- 61
-
-
True, my point was it's a lot easier to carry through calculations and to talk about, vs. what makes physicists happy. – Matt Young Mar 21 '13 at 20:09
0
According to dictionary.com, "flowing" is an adjective and, in this context, is defined thus:
moving in or as in a stream: flowing water.
Thus, "flowing charge" or "charge is flowing" means charge (is) moving in or as in a stream.
Now, charge moving in or as in a stream is precisely what we think of as "electric current" in a circuit context.
Then, "flowing current" or "current is flowing" is, literally, charge moving in or as in a stream is moving in or as in a stream.
Thus, if "current is flowing" means something else, then either "current" or "flowing" is being used in a different sense.
DrFriedParts proposes in his answer that "current is flowing" equals "flow of charge is flowing" and that this describes the degree to which the flow is occurring (e.g. not zero).
There is, in fact, a sense in which "flowing" describes degree:
abounding; having in excess: a land flowing with milk and honey.
So, it seems, "current is flowing" isn't redundant if "flowing" is used in this or a similar sense. Although flow of charge is abounding isn't precisely the same as flow of charge is non-zero, it's certainly in the ballpark.
Alfred Centauri
- 26,502
- 1
- 25
- 63