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I How to intuitively understand Voltage and how to measure it?

  1. Oct 30, 2018 #1
    I am trying to get a better intuitive understanding of voltage and how it is measured.

    Here is my understanding: Voltage shows the potential difference between 2 points in a circuit and a higher voltage correlates with a higher electromotive force (emf) that moves the electrons through the wire. Voltage is measured in Joules/coulomb, so electrons moving in a circuit that has higher voltage have higher energy per coulomb. How do electrons express this higher energy? By moving with greater speed? By giving off more heat? By extension, how does a voltmeter measure voltage in a circuit?

    Thanks for any insight.
     
  2. jcsd
  3. Oct 30, 2018 #2

    berkeman

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    Never anthropomorphise electrons. They hate it when you do that.
    Both. Use Wikipedia to learn about the mean free path of electrons in solid conductors.
    Wikipedia is a good place to start for that. Search on Analog to Digital Converter. I think we also had a recent thread here at the PF about the different topology ADCs that can be used in DMMs. Please have a look at all of that, and let us know if you have more questions about it all. Thanks. :smile:
     
  4. Oct 30, 2018 #3

    phinds

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    :smile: Good payback for that chuckle I gave you the other day :smile:
     
  5. Oct 30, 2018 #4

    rcgldr

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    As another example of a potential within a field would be height within a field of constant gravity (such as an imaginary infinite flat plane). The greater the height, the greater the potential. For a constant gravitational field, the potential energy would be m (mass) g (acceleration factor related to the strength of the field) h (height above the surface of the plane). Similarly imagine the field in a capacitor between a positive charge plate and a negative charged plate. The distance from the negative plate would be the equivalent of height, assuming a particle with positive charge within the field, and the voltage (potential) would be the field strength (intensity) times the distance from the negative plate. The potential energy would be the voltage (potential) times the charge of the particle in the field.
     
    Last edited: Oct 30, 2018
  6. Oct 31, 2018 #5

    sophiecentaur

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    It’s unfortunate that Electromotive Force is still a term in use. It ain’t a force; it’s Energy per Unit Charge and we have all been confused at some stage by that inappropriate term.
     
  7. Oct 31, 2018 #6
    I searched Wikipedia for the above concepts, but it was too technical for me. I just want to know something very basic: What exactly does a voltmeter measure to determine the voltage in a circuit? I don't think it measures the current flow, because that would be amperage. It must measure the energy per unit charge, but how does it do this--- does it measure heat emitted from the circuit or speed of the electrons, as both head and speed are measures of energy? I really cannot figure out what else it might measure. I doubt it can measure potential difference at 2 points, but if it does, how would it do this? Thanks for any insight.
     
  8. Oct 31, 2018 #7

    jbriggs444

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    The gradient of energy per unit charge is force per unit charge, of course. Measurable in volts per meter, Joules per coulomb-meter or Newtons per coulomb.
     
  9. Oct 31, 2018 #8

    berkeman

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    That's exactly how it does it. That potential difference can drive a current, for example, which can be sensed in a moving coil instrument as shown in the simple first example at the wikipedia page:

    https://en.wikipedia.org/wiki/Voltmeter


    220px-Voltmeter_hg.jpg
    Higher impedance voltage measurements are made by using transistor comparators. Are you familiar with how transistors work by any chance? I could post some diagrams, but if transistors are Greek to you, that may not help.
     
  10. Oct 31, 2018 #9

    sophiecentaur

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    Thee are all sorts of ways to describe quantities but, however you choose to do it, emf is not a force.
     
  11. Oct 31, 2018 #10

    jbriggs444

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    We have little problem calling gravity a force, even though it is [clasically] an energy per unit mass.
     
  12. Oct 31, 2018 #11

    sophiecentaur

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    A classical Moving Coil Voltmeter is, in fact, a sensitive microammeter in series with a very high resistance. When connected across a circuit (other than a circuit with equally high resistances in it) it relies on the fact that V = IR where R is the value of the series resistance and A is the measured (tiny) current, There is no reason for that to 'upset' you because, although it does shunt a small fraction of the current flowing and, hence, it changes things a bit, it is much the same as most instruments that measure quantities. Even a thermometer will cool the liquid it's put into and that could affect the accuracy of the measurement when the thermometer is big and beefy (robust) and you want the temperature of a thimbleful of water.
     
  13. Oct 31, 2018 #12

    sophiecentaur

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    But the weight of an object is in Newtons and 1V will never be in Newtons. There is a distinct difference. For a start, dimensional analysis of Force doesn't include Charge.
     
  14. Oct 31, 2018 #13

    Mister T

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    This is where you're confusing yourself. Voltage is the potential difference between two points in a circuit. Speaking of a circuit having a voltage doesn't make sense here. Pick any two points on any circuit. When you touch one lead of a voltmeter to one of those points, and the other lead to the other point, you are measuring the potential difference between those two points.

    Pick any two other points on this circuit or any other and repeat the above measurement. If the measurement is higher that means the potential difference is higher.

    Electrons are the charge carriers responsible for the current in these circuits, so more voltage means more current, provided the resistance between the two points is the same.

    Let's take a simple circuit consisting of a 1.5 volt battery and a bulb. Every coulomb of charge that passes through the battery picks up 1.5 joules of energy from chemical reactions in the battery. If we assume the connecting wires have negligible resistance then every time a coulomb of charge passes through the bulb it loses 1.5 joules of energy.

    The battery puts energy into the circuit, the bulb removes energy from the circuit. The end result is to convert the chemical energy in the battery to thermal energy given off by the bulb.
     
  15. Nov 1, 2018 #14

    CWatters

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    +1

    Some voltmeters use a semiconductor device called a FET to reduce the current drawn from the circuit and amplify it so it can drive the coil. These can have a resistance of 10Mohms so don't significantly affect many common circuits at all.
     
  16. Nov 1, 2018 #15

    davenn

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    or in the old days ( when Adam was a boy :wink: ) and before FET's, VTVM's Vacuum Tube Voltmeters were all the rage .... for getting that high impedance


    Dave
     
  17. Nov 2, 2018 #16

    sophiecentaur

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    +1 "FET" stands for Field Effect Transistor, which actually does work on the Voltage (The Electric Field , actually) and not the Current. Any current it happens to take is due to the imperfections in the device or the needs of the circuit it's in (same as with a VVM (Valve Voltmeter). Neither of those devices were described to students at School level and (despite the fact that they were all using DVMs in their experiments) they have 'always' been taught the old model involving a basic current measurement
     
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