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    I got this question out of the OCR Physics A Second Edition texbook 1.

    Question: A spark travels across a gap between two spherical conductors that are 40cm apart. The current is 4mA. For electrons travelling at 3x10^7(one tenth of the speed of light), calculate the number of electrons in the spark at any instant.

    What I've tried: I attempted to use Q = It, but with no t I came up with an equation to work out t -> v/d. Then subbed in to get Q = Iv/d.

    Got the total charge to be 300,000 coulombs, but this seems high.

    then divided by e to get the total number of e's to be 1.875x10^24

    Is this correct? Or what have I done wrong?
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    From my CCEA physics knowledge I can try to help.
    You have the distance of 40cm and the speed of 1/10 speed of light. using the speed = distance/time you can find the time taken. As well as Q=it there is also Q=ne which is charge = number of electrons x the charge of electrons. The charge of an electron is constant = 1.6 x10^-19.

    using speed = dist/time, find the time.
    Then use this to find charge using Q=it
    and finally use the charge found to find the number of electrons: Q=ne

    or you can simply equate the two charge equations and do it in one step using ne=it. I hope I've done that correctly and helped you. Just remember to look at the units of speed and time and distance to make sure they're in the S.I units
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    (Original post by Davwardo)
    From my CCEA physics knowledge I can try to help.
    You have the distance of 40cm and the speed of 1/10 speed of light. using the speed = distance/time you can find the time taken. As well as Q=it there is also Q=ne which is charge = number of electrons x the charge of electrons. The charge of an electron is constant = 1.6 x10^-19.

    using speed = dist/time, find the time.
    Then use this to find charge using Q=it
    and finally use the charge found to find the number of electrons: Q=ne

    or you can simply equate the two charge equations and do it in one step using ne=it. I hope I've done that correctly and helped you. Just remember to look at the units of speed and time and distance to make sure they're in the S.I units
    I see my mistake now. I done time = speed/distance, when its the other way round!

    Derp moment indeed!
 
 
 
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