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Experiment to determine the speed of light

  1. Apr 12, 2008 #1
    For my advanced physics coursework i am calculating the speed of light, one of my experiments uses Maxwell's equation for electromagnetic waves c = 1/((sqrroot of)EoxUo)

    what i had planned to do was calculate values for the magnetic permeability and the electric permittivity and plug them into that equation to find the speed of light and it's not going as i planned...

    to find the electric permittivity my experiment was using parallel plate capacitors, varying the distance between the plates and measuring the resulting charge between the plates, keeping the voltage constant, but i don't know how i use my results to find the value for Eo, i've tried plotting a graph of Q(charge) against 1/d(distance between plates), 1/d used to get a direct proportion, and finding the gradient of that line, but it's not giving me anything like the true value for Eo...please help?

    Also, for calculating Uo, i got together a helmholtz coil and a magnetic sensor(which produces a voltage and 1V = 100mT), for that experiment i varied the current through the helmholtz coil and took not of the voltage and correspoding Teslas, but i do not know what to do with my results to calculate the magnetic permeability of free space...please help?

    thanks in advance for any helpful posts.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 12, 2008 #2


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    You can't measure epsilonzero or muzero by your experiments.
    muzero is a defined number, used to define the ampere in terms of the force between two wires. If you measure amps with an ammeter, you have already chosen muzero to be
    4pi times 10^-7.
    If you use SI units for Q and V, you have already put in the connection between epsilonzero, muzero, and the now defined speed of light.
    Your results will just check your accuracy in using these already defined constants.
  4. Apr 12, 2008 #3
    by checking my accuracy in using these already defined constants, do you mean i should get a value for the constant and be able to check it's accuracy against the true value or do you mean check my accuracy in putting the constant to use?:S

    if you mean actually deriving a value for the constants that i could check the accuracy of against the true value, what would i do to determine this value?
  5. Apr 13, 2008 #4
    sorry...i get what you mean now, the voltmeter and coulometer i am using in my experiments have been calibrated using these predefined constants, so in trying to calculate them, i am in short, checking my accuracy in using them...but if i am checking my accuracy of using them, should i not get an actual value to check against the true value?
  6. Apr 15, 2008 #5


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    But you are only checking how accurately you measured.
    It is like measuring the the "fundamental constant" 2.54 cm/inches
    by measuring the length of a meter stick using a one foot ruler.
    You are just measuring your accuracy in putting the constant to use.
    If using the standard formula for capacitance, and measuring Q and V, doesn't agree with the formula, it might be that d is too large.
    You need d^2<<A for the formula to hold.
    Also, Q is difficult to measure accurately.
    In any event though, you are just testing your accuracy of measurement.
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