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Can electric energy be directly measured?

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  1. Jul 30, 2017 #1
    I expect that power monitors directly measure the current, voltage change, and maybe phase to directly calculate current power use.

    Can electric energy directly be measured, or do meters just calculate power, and integrate it to obtain energy?
     
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  3. Jul 30, 2017 #2

    russ_watters

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    I guess it may depend on what you mean by "directly", but old style meters spun like clocks.
     
  4. Jul 30, 2017 #3
    Thanks Russ, Sounds like the old meters just measured power, and used this spinner as an integrator. I suppose someone could make some direct monitoring energy meter by comparing the energy to a known energy value (which doesn't sound like a good idea), but I am really interested in "typical" meters. I will conclude that power is measured and energy (change) is just the integral of power.
     
  5. Jul 30, 2017 #4

    sophiecentaur

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    Energy is not transferred instantly so a conventional spinning disc will give the Energy supplied in a given time. (Integrated, as you say) Isn't that what you need?
    Electricity companies swear by them. They have a very good range of measured power, from much less than 1W to tens of kW.
    Edit: These days you can get AC power meters that measure V and I and do the calculation V.I, which is power, of course.
     
  6. Jul 30, 2017 #5

    Merlin3189

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    Counting the rotations integrates the speed of rotation of the disc, which is proportional to power derived from current and voltage coils..

    But the original question seems odd to me. What does he want the meter to show? And what is it he's objecting to in the measurement - is it the integral or the fact that power is found by measuring current and voltage?

    If he wants it to show energy transferred along the wires, then that has to be over a period of time (maybe, though not necessarily, from switch on up to the current time.) But that is implicitly an integral, no way round it. (If power is constant or varying linearly with time, you can do the maths by multiplication, but that's just because it's a trivial integral.)

    If he wants to measure energy directly*, not via voltage and current vectors, then can he show me some energy and tell me about its measurable properties? What would it mean to measure energy directly? Or momentum or velocity or resistance ...? Even a fundamental quantity like current is measured in terms of a force. Very few things we can actually measure directly - only mass and length spring to mind - and that's because there's an actual lump of mass we can pick up and compare: even length is dubious, because I can't imagine how we can compare lengths with the modern standard metre.

    * I see OP now suggests "comparing the energy to a known energy value" The question is not hether it is a good idea, but what is energy? How can you have a standard amount? How can you compare energy. And remember, in your context, the energy that went down the wires may have been rotational KE when it started and may now be something else, heat, chemical, KE, gravitational PE, light, probably some mixture of them. So what are you comparing to what?

    I can't think of any situation where you can measure energy or even power directly. You measure other quantities and infer energy or power.
     
  7. Jul 30, 2017 #6

    russ_watters

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    That seems arbitrary and argumentative at best. Since power and energy are so related, you could take any meter that measures one and argue it is "really" measuring the other, using the same logic. But really, I think it is wrong:
    By what means is it measuring time?
     
  8. Jul 30, 2017 #7

    russ_watters

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    Here's a similar device:

    Counter-font-b-Distance-b-font-font-b-Measuring-b-font-font-b-Wheel-b-font.jpg

    Can someone explain to me how this device, with no electronics, measures speed and time and then performs multiplication or integration?
     
  9. Jul 30, 2017 #8

    Merlin3189

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    Sorry, don't follow that one?
    I just thought that number of rotations = ∫ ω dt so the spinning disc rotates the reducing gears to count turns, 10x turns, 100xturns, 1000x turns, etc for the dials
    Since ω ∝ power, energy used = ∫ ω dt
    We get the definite integral by subtracting the previous reading from the present reading.

    Why do I need to measure time? The energy meter is just recording the amount of energy that has passed through. It starts at an arbitrary value, which is the amount of energy passing through during adjustment and acceptance testing before installation. (or much higher if it is a reconditioned unit.)
    I guess that modern digital meters actually can be reset to zero at the start of a measurement period.
     
  10. Jul 30, 2017 #9

    Merlin3189

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    I've no idea how it measures time.

    Like the disc in the meter, some external agency makes it rotate. In the watt meter it is electricity. Here it is someone pushing it along the road.
    The little counter attached is made of reducing gears which the wheel makes to rotate.
    The speed of the rotating wheel ω(t) is integrated by counting the rotations. So the counter tells you ## \int_{t=0}^{now} ω(t) \, dt ##
    You do need to read the counter at the start of your trip, to find ## \int_{t=0}^{start} ω(t) \, dt ##
    Then ## \int_{t=start}^{now} ω(t) \, dt = \int_{t=0}^{now} ω(t) \, dt - \int_{t=0}^{start} ω(t) \, dt ## = distance travelled, which was what was wanted.

    It doesn't tell you the speed (well, any speed) but if you had a watch and recorded the time at the start and end, you could divide the distance by the difference in time to find your average (mean) speed.

    If you wanted it to multiply, the only way I can think of off hand, would be to mark out a length representing one number, then walk the device along it the other number of times. Of course you want the latter number to be an integer, so you have to scale it as we had to on computers when we had only integer arithmetic available.

    Edit 1: corrected error in w5
    Edit 2: I forgot to put constants in my integrals to allow for conversion from radians to turns (2π) and from turns to metres (circumference of wheel.)
     
    Last edited: Jul 30, 2017
  11. Jul 30, 2017 #10
    Hi All!

    It has been a while since I studied this this subject. A real long time! My purpose of this question was just to reafirm my understanding.

    @sophiecentaur, Yes, integrated is what I need. Actually, just confirmation that such is what I need.

    @Merlin3189. Even mass and length are just viewed under our perceptive viewpoints, so they too are relative. But if we take typical normal assumptions of our view point, I agree energy can't be directly measured, but why not power?

    @russ_watters. Do you know of a practical direct energy meter? It is measuring time applied by given energy. And given average power and agreed upon units, we can determine time.

    @russ_watters Because of the diameter of the circle and assigned units.

    @Merlin3189. No comment.

    Thanks!
     
  12. Jul 30, 2017 #11
    I am wondering in what case can any form of energy be measured in a single parameter, i.e. directly.

    Kwh meters have a voltage and current coil, and use a magnetic circuit to multiply them.
     
  13. Jul 30, 2017 #12
    Thanks Windadct, i misspoke about my perceived definition of "directly". I acknowledge that I am measuring current and voltage but see that as directly. I feel when I ride my bike, I can measure distance and measure time and measure speed directly. But not the same with energy, and can only monitor current and voltage (or heat created) but need to apply time to get energy (unlike my bike distance analogy).
     
  14. Jul 30, 2017 #13

    russ_watters

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    The answer is that it doesn't measure time.
    Not by that device it isn't. The fact that the mathematical relation between speed and distance exists does not imply that any particular device is using it.
    Right. It measures distance and in order to find speed, you need to separately measure time and divide.
    The OP claimed that an energy meter measures power and integrates over time (which it also must measure in order to do the integration) to get energy. I disagree: I think it measures energy directly. It appears you agree with me...
     
  15. Jul 31, 2017 #14

    russ_watters

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    Certainly:
    http://www.gordonelectricsupply.com...h0EmsOjF-DzgfM7GexAKDJ-CerZQ5K6AaAmVDEALw_wcB
    I'm afraid that makes no sense.
    Since it only measures energy, not power, you can't use it to measure time in that way. If you connect a load with a known constant power, then you could.
    Right: so it doesn't measure speed and time to find distance, it measures distance directly. Just like an old-style electric meter.
     
  16. Jul 31, 2017 #15

    russ_watters

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    If you are measuring distance and time and then calculating speed, doesn't that violate your criteria for "directly" measuring speed? It's time and distance that you are measuring "directly" and speed "indirectly" (by calculation), isn't it?

    Old-style rotating meters don't measure current, voltage or time.
     
  17. Jul 31, 2017 #16

    sophiecentaur

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    I think the OP is on a hopeless mission here. He hasn't really defined what he wants and is trying to scratch an intuitive itch. We have all been there at one time or another, when we think we have hit on something fundamental. That itch will go away if he reads all the above posts carefully and with an open mind. :smile:
     
  18. Jul 31, 2017 #17

    Merlin3189

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    I apologise to Russ for not taking him seriously. I thought #7 was intended to be facetious and replied in similar vein.

    Two things made me just take for granted the role of a such a device as an integrator. One was using a planimeter back in the 60's to measure the area under a graph. It resembled Russ' device - at least it had a wheel you slid across the paper, which made a dial go round - and the result was what we would have obtained had there been a known function for the graph and we'd integrated it.
    The other thing was the descriptions of energy meters, which all seem to work by adding up the sequence of power values.

    To me, both devices seem to be doing integration, or at least summation of the products of rates and a very small interval. But I can see that I need to give it a bit more thought to either come up with some more substantial argument, or realise my mistake.
     
  19. Jul 31, 2017 #18

    russ_watters

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    Fair enough....since I am often sarcastic, it sometimes gets missed when I'm not.
    Without looking too much into how they work, it does suggest to me something like a slide rule, which does indeed perform calculations. The distance wheel, though, doesn't; it just counts.
     
  20. Aug 1, 2017 #19
    While I don't agree this device measures the absolute energy value, I agree it measures change in energy. I believe my source of confusion related to thinking change of time was the x axis which isn't the case thus it isn't displaying power. But it still seems to me that it must be monitoring power internally, and not energy.

    I've never built an energy (oops, I almost said power) meter, but if I did, I suppose I would measure the current by inducing some secondary loop, measure the voltage drop, use a chip or circuit to multiple the two values to determine power, and then integrate power over time using a chip, spinning dial, etc.

    Where am I going astray?
     
  21. Aug 1, 2017 #20

    russ_watters

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    What is an "absolute energy value" and what does it have to do with your question.
    I think you keep moving the goalposts and are using a meaningless/reversible definition in order to keep getting the answer you want?
     
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