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Two point charges sepearated by 1.5cm, have charges +2 and -4 microcoul. Suppose that it is determined that 10 field lines radiate out from 2.0 microcoul charge. If so, what is inferred about the -4 microcoul charge with respect to field lines?

a. 20 radiate in  b. 10 radiate out  c.10 radiate in  d. 20 radiate out.

I know that it has to radiate in since it is a negative charge, but how do I differentiate between a and c?

Hello Sona,

If the 2 charges were isolated from each other (and from other charges), the -4 uC charge would have 20 field lines radiating into it. When the charges are brought within 1.5 cm, it is clear that the 10 lines from the +2 uC charge would merge with 10 of the 20 lines radiating from the -4 uC charge.

And if you have studied Gauss's Law, you will see that a Gaussian surface that encloses both charges will have the same field lines penetrating the surface as you would find if there was only one -2 uC charge inside.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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