1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Featured Tiny Tweak in Teaching Basic Circuits

Tags:
  1. Mar 21, 2017 #1

    anorlunda

    Staff: Mentor

    I'm sure we have all seen the frequent questions from students who don't understand the difference between potential and voltage in a circuit. They seem to think that zero voltage has physical significance. Even using the phrase "voltage drop" is not suffienct, because they say, "What happens when it drops to zero," leading to the concept that voltage is a consumable.

    Of course, EEs use the convenience of assigning some point in the circuit the arbitrary value of 0, then expressing everything relative to that. I suspect that simplification in language contributes to the students misunderstanding. In the example circuit below, I would strictly refrain from saying ##V_A##, ##V_B##, ... Instead I would only mention voltage with respect to two places in the circuit. ##V_{AG}## ##V_{BG}##. I view that as a tiny tweak in the use of language by the teachers.

    circuit2.png

    Verbally, I would never mention "voltage .. " without saying "voltage with respect to ..."

    But I have been out of school for many years. Perhaps modern teachers already do it that way. Is that so?
     
  2. jcsd
  3. Mar 21, 2017 #2

    phinds

    User Avatar
    Gold Member

    Like many threads on this forum, this simply points out one of the many unfortunate choices that have been made in using English to describe things mathematical. It isn't going to have any effect at all on how things are taught. I don't mean it's a bad idea, just that it's not going to have any effect.
     
  4. Mar 21, 2017 #3
    Most books I know define a point as ground and voltage is "understood" to be relative to the ground.
     
  5. Mar 22, 2017 #4

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Anorlunda has a point that this "understanding" isn't genetically determined and has to be taught. Lots of students think they can forget the black wire on a multimeter when they use it for a first time. And that -- to me -- is where the "solution" lies: hands on with a battery, a few resistors and a multimeter. From there to the abstraction to a mathematical problem. And not lots of lecturing from the textbook and then halfway on a Friday afternoon fit in a lab exercise
     
  6. Mar 22, 2017 #5
    I agree with this. Any decent primer on electronics is certainly going to make this distinction. So I would guess it's as much a matter of the student needing to slow down & focus on what is actually being said, and being willing to forget previous misunderstandings he/she may have picked up, as anything else.

    Maybe even a couple of batteries in series, for that matter, to further show the arbitrary nature of designating ground.
     
    Last edited: Mar 22, 2017
  7. Mar 22, 2017 #6

    robphy

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The notation ##V_{AB}## is also good because it suggests how a multimeter would be used to measure it, and why ##V_{BA}= -V_{AB}##.
    Indeed, this is just like a displacement as the difference between two position coordinates. Or relative velocities. Etc...
    Strictly speaking, one should do the same thing with time ##t## in a kinematics equation... but it's too tedious to write ##(t_-t_0)##. So, i describe ##t## as the stopwatch time.
     
  8. Mar 22, 2017 #7

    anorlunda

    Staff: Mentor

    It occurred to me that a simple homework problem could emphasise the point. Take any problem where students are asked to solve for voltages and currents. Then ask for it to be repeated shifting the ground symbol to several different points in the circuit. Students should quickly grasp the point that it makes no difference where the ground reference is.
     
  9. Mar 22, 2017 #8

    robphy

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I like the "voltage as height" rendering of schematic diagrams (see below).
    Then it's more clear that it's differences in potential [voltages] that are important.

    From OpenStax College's "College Physics" (Urone)
    https://openstax.org/details/books/college-physics
    upload_2017-3-22_15-2-31.png
    or, implemented dynamically at
    http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=30.0
    upload_2017-3-22_15-9-10.png


    Another frequent question is "how do you know which way the current flows?"
    and getting them to distinguish the "[positive] physical current" and
    the assigned coordinate variable (like a "choice of axis") used to do the analysis.
     
  10. Mar 22, 2017 #9
    I don't think the choice of 0v for a reference node is insignificant. It sure makes the math less messy.

    EDIT: If you choose the right node.
     
  11. Mar 22, 2017 #10

    anorlunda

    Staff: Mentor

    What do you do about the many students who forget that there is a reference and think voltages are absolute?
     
  12. Mar 23, 2017 #11
    Demonstrate. Assign a different node as reference, assign it some ridiculous value (e^i*h maybe), rework the problem carrying the ridiculous term through all the arithmetic, and get the exact same answers.
     
  13. Mar 23, 2017 #12

    atyy

    User Avatar
    Science Advisor

    If voltage = potential difference, isn't voltage absolute?
     
  14. Mar 23, 2017 #13

    phinds

    User Avatar
    Gold Member

    No, the DIFFERENCE is absolute, not the potential. The potential is only relative.
     
  15. Mar 23, 2017 #14

    atyy

    User Avatar
    Science Advisor

    But isn't voltage the difference?
     
  16. Mar 24, 2017 #15
    I think it is a matter of wording. I have seen some textbooks refer to the Voltage difference as voltage.
    But voltage is defined as the electric potential energy per charge relative to some point where you set it zero. Voltage difference is the change in voltage which is absolute.



    That is a good point. In our school they never mentioned anything about setting any point to zero. Which I think significantly increases your understanding of the topic
     
    Last edited: Mar 24, 2017
  17. Mar 24, 2017 #16

    atyy

    User Avatar
    Science Advisor

    Or maybe he meant that voltage is always relative to two points, and that with respect to those points, it is absolute. So it is relative and absolute.
     
  18. Mar 24, 2017 #17

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Not to mention the linguistic confusion caused by e.g. error handling: absolute errors and relative errors. Relative voltage ?
    And folks with a more general mindset who think last year's profit of 10% has doubled when the press clipping says it has increased with 10%.
     
  19. Mar 24, 2017 #18

    lewando

    User Avatar
    Homework Helper
    Gold Member

    An introduction to "across variables" and "through variables" in both electrical and mechanical domains could be helpful to the student.
     
  20. Mar 24, 2017 #19
    Just noticed this sonce it was prioritised on the page and was curious.

    I am not at all well-versed in electronics, but otherwose am confident in Physics. Our A level Physics teacher was sacked after a half-term of teaching electronics and the head of department simply rold us we were 'taught wrong', without any detail or clarification ad to what wad wrong or how - so I kind of have ring-fenced this whole subject area and had a fear or mental-block since I just felt I couldn't trust what I thought I knew.

    This meant that I avoided anything regarding electronics on a macrosxopic level, though reading Brian Cox' book telling how conductivity and transistors operate wad very accesable and informative. I am more confident now and not so afraid of electronics.

    So I would love for someone to just explain what IS the diffetence between Voltage and potential, since I am one of those who was under the impression that Coltage was a measure of localised electric potential energy.
     
  21. Mar 24, 2017 #20

    anorlunda

    Staff: Mentor

    Sure, but the one or two sentence definitions aren't adequate. The proper basic explanation of the relationship between voltage and potential (plus currents and power) takes about 1400 words plus visuals. That's too much for a simple PF post. Fortunately, it is all nicely done in the following 10 minute video from Khan Academy

    https://www.khanacademy.org/science/physics/circuits-topic/circuits-resistance/v/electric-power

    Edit: I forgot to mention @BvU 's point from #4. We measure voltage with a voltmeter with two leads. That is the real point of this thread, two leads, not one. Potential is at a point, voltage is between two points.
     
    Last edited: Mar 24, 2017
  22. Mar 24, 2017 #21
    One fine point is that "ground" is also referred to as " Common" meaning this part of the circuit has a common potential and removes from the mind a visual concept that could be confusing. The common connects many parts of a circuit together one of them usually being one of the terminals of the power source. Ground should be used for the common of a circuit that is physically connect to the earth. .
     
  23. Mar 24, 2017 #22
    As someone who is somewhat versed in Electronics, but only basic physics. I'll tell you what I was taught for what it's worth. Voltage IS the Potential. In a physics reference, it's the amount of force. Current is the amount of total volume (flow) available. The simplest idea is to always think of electricity like water flowing through a pipe. The size of the pipe, and the total flow determines the force. That being said. Potentials and Difference are ALWAYS relative, not absolute. A lot of the same math used to derive relative physics is used to derive electrical theories. Voltages can be dropped across resistive sources, but as stated. The voltage isn't "consumed" in some manner. It's still there, It's force is merely being dropped (held back if you will although somewhat inaccurate in description) by a relative resistance. The term "Common" is just that. It's a measurement point for taking readings where you connect the black probe of the DVM, scope, or whatever testing device your using. In no way should it ever be considered as a "Ground Point" or "At Ground Potential" when in fact a lot of times this isn't true. In cases where AC and DC are traveling together in a circuit (transistors and amplifiers usually). Potentials of complex circuits are usually determined by Thevenin's Theorems. Total current potentials are actually determined by Kirchoff's Law. Ohm's Laws are only used in the simplest of circuits for any analysis purposes. BTW: I left it alone for the most part, but my instructor first year summed it up quickly in class for those that "just couldn't get it". You either understand electronics concepts and theories going in, or your just another smuck trying to learn basic electrical theory so you can be a functional car mechanic.
     
  24. Mar 25, 2017 #23
    A good functional car smuck bills at $120.00 per hour around here.
     
  25. Apr 24, 2017 #24

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Good at changing parts, one at a time until the fault disappears but not at analysing at the problem, processing the evidence and changing just one part to solve the problem. That $120.00 per hour is very often a rip off but what else can one do? The tools and analysis equipment are expensive and specialised so few of us can afford to DIY any more.
    I had a wonderful guy in Hove, who mended VW vehicles. He used to say "You don't need a new part yet. We'll do it at the next service". Not many like him around.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted