CHAPTER 17. ELECTROSTATICS 17.3
Combined charge distributions ESBHI
We will now look at thefield of a positive chargeand a negative charge placed next to each other.
The net resulting field would be the addition ofthe fields from each of the charges. To start off with
let us sketch the field maps for each of the charges separately.
Electric field of a negative and a positive charge in isolation
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Notice that a test chargestarting off directly between the two would be pushed away from the positive
charge and pulled towards the negative charge in a straight line. The path it would follow wouldbe a
straight line between thecharges.
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Now let’s consider a test charge starting off a bit higher than directly between the charges. If it starts
closer to the positive charge the force it feels from the positive charge isgreater, but the negative
charge also attracts it, soit would experience a force away from the positive charge with a tiny force
attracting it towards thenegative charge. If it were a bit further from thepositive charge the forcefrom
the negative and positive charges change and infact they would be equal in magnitude if the forces
were at equal distancesfrom the charges. Afterthat point the negative charge starts to exert a stronger
force on the test charge.This means that the testcharge would move towards the negative chargewith
only a small force awayfrom the positive charge.
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Now we can fill in the other lines quite easily using the same ideas. Theresulting field map is: