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# electric fields watch

1. i keep getting a different answer to the book when i do this question and cant see where im goin wrong

a metal sphere of radius 20cm carries a positive charge of 20*10^-6 C
an identical sphere carrying a negative charge of 10*10^-1 C is placed next to the first sphere
there is a gap of 10cm between them

calculate the electric force that each sphere exerts on the other

thanks
2. Which equation are you using? I dont what to go through the whole thing incase I'm wrong... I've used:

E = "1 over (4 pi epsilon-nought)" x "Qq/r²"

where epsilon nought = permittivity of free space
Q = charge on first sphere
q = charge on second sphere

That's the only method I can think of, unless I've misunderstood and somebody wants to prove me wrong?
3. yeah thats what i used i get 7.2 but thats different to the book

I'll go through method should the answer be right or else it'd just baffle :X
5. F = kQ1xQ2/r^2

= 8.99x10^9 x 20x10^-6x10^10^-1 / (0.2+0.2+0.1)^2

= 719200N (attractive)
6. (Original post by Economist)
F = kQ1xQ2/r^2

= 8.99x10^9 x 20x10^-6x10^10^-1 / (0.2+0.2+0.1)^2

= 719200N (attractive)
Yeah I make that too...

Edit: 719344N, but I used the full thing instead of 8.99x109
7. ^^

what i used too =)

That epilson thing, its in the formula sheet if your doing Edexcel so don't worry about remembering its value but as long as you remember where it came from that 1over4pie..... i just remember it comes from the back of the phy paper
8. ok im not crazy then cos i got 7.2
the book says its 5N maybe its just the book bein silly
9. -720000N aswell
10. ~720000N too..
you need to get this sort of stuff correct. Its about as easy as they come

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