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Question
Two-point charges Q1 = 400 μC and Q2 = 100 μC are kept fixed, 60 cm apart in a vacuum. Find the intensity of the electric field at the midpoint of the line joining Q1 and Q2.
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Solution
`"E"_1 = "Q"_1/(4 piin_0 "r"_1^2)` ... `["Q"_1 = 400mu"C" = 400 xx 10^-6"C"]`
`"E"_2 = "Q"_2/(4 piin_0 "r"_2^2)` ...`["Q"_2 = 100mu"C" = 100 xx 10^-6"C"]`
∴ The resultant electric field
E = E2 - E1
`= "Q"_2/(4 piin_0 "r"_2^2) - "Q"_1/(4 piin_0 "r"_1^2)`
`= 1/(4 piin_0 "r"^2) ... ["Q"_2 - "Q"_1][because "r"_1 = "r"_2 = "r"]`
`=(9 xx 10^9)/(30/100)^2 [400 - 100]xx10^-6`
`= (9xx10^11xx10^-6[3]xx100)/9`
`= 3 xx 10^7 "NC"^-1`
From Q1 to Q2
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