In illustration 1.1 i didn't understand why they have written final value of Q as negative when value of Q was found to be positive

# Doubt from electrostatics

Using calculation they only found out the magnitude of Q

For sign they simply analysed that if the new charge is positive then system can’t be in equilibrium, it must be a -ve charge .

To get magnitudes as well as sign of Q through calculations you’ll have to alter the equation a little bit

There in equation for equilibrium of 1 they have equated force due to Q= force due to q , i.e they have assumed sign of Q is opposite to that of q . And hence only magnitude came up

In illustration 1.21 i have a doubt in final result in value of Q 4k should be in denominator not numerator

Yes , it should be in denominator

Please help anyone

- Simply the field due to only one point charge at centre.

But answer to this is zero given in answer key

@abhijit_2020 there i missed one thing

For semi infinite wire - it’s kλ/r towards left also , i wrote vertical component as kλ/r but horizontal component kλ/r will also be there

One more doubt in question 1 that i am uploading i am getting sin@tan@ both in expression of lamda in final answer but one teacher told sin@ will not be there who is correct