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How to measure power

  1. Nov 5, 2012 #1
    In a simple circuit with a power source, electric conductors and a load, you can measure the power being dissipated by measuring the voltage across the load and multiplying by the current.

    If you only have access to one conductor, for example, an electrical distribution cable, how can you measure the amount of electrical energy passing through the conductor?
  2. jcsd
  3. Nov 5, 2012 #2
    Based on the strict reading of your post - it you ONLY have access to a single conductor, you can not measure the energy flowing through it. If you only have access to one conductor - how do you know what the circuit is?

    However it sounds like you are talking about AC - if you know the system is a single conductor and the system is grounded you can use ground as a reference, but you will NEED to be able to measure phase angle or have a AC Power meter. But you will also need to know what the system is - not just have access to it.

    If you do not know enough about the whole circuit to know the return path - then you can not measure or estimate the power / energy.
  4. Nov 5, 2012 #3
    I asked myself this question when reading about using high voltages to reduce losses in electric power transmission lines. I tried to calculate the ratio of power dissipated on a length of power line to the amount of power transmitted through the same length of power line.

    Of course, I can construct an entire circuit and make the calculations but I am still wondering why that is necessary.

    I have a vague idea that the question "How much electrical energy is flowing through this conductor" does not make sense but I cannot work out exactly why.
  5. Nov 5, 2012 #4

    No you can't. The total energy is the voltage multiplied by the in phase current to get the instantaneous power, and further multiplied by a small interval of time to get a small bit of energy. Then the small bits of energy are summed, and divided by the total time to get the average power. You must know the value of current at each value of voltage before you can calculate the instantaneous power. That means voltage and current have to be measured concurently, and not sequentially.

  6. Nov 6, 2012 #5
    "voltage multiplied by the in phase current to get the instantaneous power"

    I didn't really specify whether it was average power or instantaneous but to simplify things, lets say it is a constant DC voltage source and a purely resistive load. My question is:

    Does the question "How much electrical energy is flowing though this conductor?" actually make sense and if it does, how would you measure it?
  7. Nov 6, 2012 #6


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    You can measure current through, from access to a single conductor, by using a clamp meter ..... I have one in my tool kit as do most electricians :)

  8. Nov 6, 2012 #7

    That's right, you did not. Therefore, I was correct in assuming an arbitrary voltage and current. In that case, you have to integrate the instantaneous power with respect to time over a time period to get the total energy, and then divide by the time period to get the average power.

    That certainly does simplify things. Now we have a constant voltage and current. In addition, they are both in phase with each other.

    Your question is incomplete. You asked how much energy, but you did not specify over what period of time. Is it a second, minute, or hour? In other words, any amount of energy can be transmitted if you wait long enough. Did you mean power instead?

  9. Nov 6, 2012 #8
    Your question of How Much Energy is Flowing though a SINGLE conductor - Really can not be answered. Your DC Voltage source question can not be related to a single conductor (point)- the voltage applied MUST be between two points - not one. Then for current you need a circuit, a complete loop and you are transferring power from one system to another - through at least 2 conductors.
    You can make a valid circuit using ground as one of the conductors - but then you are measuring voltage relative to ground ( Ground it technically a conductor in this case - but it is not a copper wire)
  10. Nov 7, 2012 #9
    It is clear that you need a complete circuit for energy to be transferred from the source to the load. It looks like there is no practical or theoretical method of measuring that flow of energy if you only have access to one conductor.

    Intuitively, I would have though that if energy is flowing through the conductor, it must be possible to measure it (by only making measurements of the conductor itself). Something is wrong with my intuitive understanding of electricity.
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