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# [A2] Need help on a Point Charges / Electrical Fields question watch

1. a) A +25 microC point charge Q1 is at a distance of 60mm from a +100 microC charge Q2.

A +15pC charge q is placed at M1, 2mm from Q1, and 35mm from Q2. Calculate:

i) The resultant electric field strength at M1
ii) The magnitude and direction of force on q.

b) Show that the electric field strength due to Q1 and Q2 is zero at the point which is 20mm from Q1 and 40mm from Q2

I'm just fairly stuck badly stuck on the first two parts. If you have to find the resultant field strength, you can't use Coulomb's law of force, but rather take a E at Q1, E at Q2, and compare them, right? Exactly what do you have to do? I've tried using the field strength equation E = Q/4Pi[Epsilon]r^2, but doing this requires r, a distance: What distance is this? Do you measure from Q1 to M, Q1 to Q2? I have literally no idea where to go for this. Thanks for any help.
2. (Original post by Dggj_19)
a) A +25 microC point charge Q1 is at a distance of 60mm from a +100 microC charge Q2.

A +15pC charge q is placed at M1, 2mm from Q1, and 35mm from Q2. Calculate:
These figures can't be right. Even if q lies on the 60 mm line segment joining Q1 and Q2, you have only accounted for 37 mm.

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