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Old Feb 6th 2018, 07:59 AM   #1
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Charge Problem

I had the problem:

An iron arrowhead has an initial charge of 3.35e−6 C. How many electrons are required to give it a charge of −2.82 μC?

This was my attempt-
1) -2.82μC= -2.82e-6 C

2) q=+/- Ne
3.35e-6= N(1.6e-19)

3) N= 3.35e-6/1.6e-19 = 2.09e13

4) -2.82e-6= N(-1.6e-19)

5) N= -2.82e-6/-1.6e-19= 1.76e13

6) 2.09e13- 1.76e13 = 3.3e12 electrons.

The answer key says it should be 3.86e+13 electrons, so I'm close at least but I can't figure out what I did wrong, I checked my division and conversions several times. Any help would be great!

Thanks!!
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Old Feb 6th 2018, 11:15 AM   #2
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nearly, but...

I think you have missed a subtle key point of the question:

The arrow is initially positively charged, thus has a deficit of electrons,

you need to add 2.09e13 electrons just to get to zero charge,
then add a further 1.76e13 electrons to get to the charge you want.
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Old Feb 9th 2018, 10:38 AM   #3
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Originally Posted by Woody View Post
I think you have missed a subtle key point of the question:

The arrow is initially positively charged, thus has a deficit of electrons,

you need to add 2.09e13 electrons just to get to zero charge,
then add a further 1.76e13 electrons to get to the charge you want.

Thank you!!!
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