Q.

Two point charges Q and –Q/4 are separated by a distance x. Then
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a

Electric field is zero at a point on the axis which is at a distance x on the right side of the charge –Q/4

b

Potential is zero at a point on the axis which is x/3 on the right side of the charge –Q/4

c

There exist two points on the axis where electric field is zero. 

d

Potential is zero at a point on the axis which is x/5 on the left side of the charge –Q/4

answer is A, B, C.

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Detailed Solution

Let r be the distance on the right of −Q/4 where the potential is zero. 

So,

kQ(r+x)kQ4r=04rrx=0r=x/3

When r be the distance on the left of −Q/4,

kQ(xr)kQ4r=04rx+r=0r=x/5

Let electric field is zero at a distance r on the right side of the charge Q/4, so

kQ(r+x)2kQ4r2=04r2(r+x)2=0(2r+r+x)(2rrx)=0

∴2r+r+x=0⇒r=−x/3

2r−r−x=0⇒r=x

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