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# A level physics electricity question... watch

1. 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?
2. 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
3. (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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