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Old Mar 21st 2018, 06:39 AM   #1
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Net Electric Fields

I keep getting this problem wrong-

Two charges, q1 = −3.00 mC and q2 = 4.50 mC, are located at −2.00 cm and 3.00 cm on the x axis, respectively. What are the magnitude and direction of the resulting field at (a) x = −3.00 cm? (b) x = 0 cm? (c) x = 5.00 cm?

My physics book doesn't bother explaining how to work field problems so I'm not sure what I'm doing wrong. I've been struggling with charge direction in electric and magnetic fields so I suppose I don't have a very good understanding of that. Anyway, in my latest attempt to solve this problem I used Coulomb's law to find individual fields and then add them.

For the first one I came up with:

E=k|q|/r^2
E1=k|-3e-3C|/1^2
E1=k(3e-3)/1
E1=2.697e7N/C

E2=k(4.5e-3C)/6^2
E2=1.12e6 N/C

2.697e7+1.12e6= 2.8e7N/C, which the answer says is wrong.

I don't understand this at all
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Old Mar 21st 2018, 04:03 PM   #2
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E=k|q|/r^2
E1=k|-3e-3C|/1^2
E1=k(3e-3)/1
E1=2.697e7N/C
Why 1 squared? distance should be in metres, not centimetres.

Note also the E is a vector and points to the right for q1 at a distance of -3cm from the origin. (That is it opposes the contribution from q2 at this point)
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Old Mar 26th 2018, 04:34 PM   #3
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Originally Posted by studiot View Post
Why 1 squared? distance should be in metres, not centimetres.

Note also the E is a vector and points to the right for q1 at a distance of -3cm from the origin. (That is it opposes the contribution from q2 at this point)
Thank you!! So the charge would point to the right then?
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