Question Details

Two charges Q1 = q and Q2 = mq are placed at the points P1(a, b) and P2(ma, mb), respectively, in the X Y plane, where a, b ≠ 0 and m ≠ 0, 1. If V1 is the potential at a point in the X Y plane due to charge Q1 and V2 is the potential at that point due to charge Q2. Correct statement(s) for the points at which |V1| = |V2| is/are:

Options

A

For m = -1, locus of these points is ax + by = 0.

B

For m = 2, the locus of these points is a circle of radius 23a2+b2 centered at (23a,23b).

C

For m = -2, the locus of these points is a circle of radius 2a2+b2 centered at (2a, 2b).

D

For m = -3, locus of these points is 3ax + 3by = 0.

Show Answer

Correct Answer :

Option A

For m = -1, locus of these points is ax + by = 0.

Option B

For m = 2, the locus of these points is a circle of radius 23a2+b2 centered at (23a,23b).

Solution :

The correct options are:
1. For m = -1, locus of these points is ax + by = 0.
2. For m = 2, the locus of these points is a circle of radius 23a2+b2 centered at (23a,23).

Step 1: Express the condition for electric potential magnitude.
Let any arbitrary point in the XY-plane be (x, y).
The distance of (x, y) from point P1(a, b) carrying charge Q1 = q is given by:

r1 = (x-a)2 + (y-b)2

Similarly, the distance of (x, y) from point P2(ma, mb) carrying charge Q2 = mq is given by:

r2 = (x-ma)2 + (y-mb)2

The electric potential due to a point charge Q at a distance r is V=14πε0Qr.
We are given that |V1|=|V2|, which leads to:

|q| r1 = |mq| r2 r2 = |m| r1

Squaring both sides gives the general equation for the locus:

(x-ma)2 + (y-mb)2 = m2 [ (x-a)2 + (y-b)2 ]

Step 2: Case 1: Analyze for m = -1
Substitute m = -1 into the distance condition r2=|-1|r1=r1:

(x+a)2 + (y+b)2 = (x-a)2 + (y-b)2

Expanding both sides:

x2 + 2ax + a2 + y2 + 2by + b2 = x2 - 2ax + a2 + y2 - 2by + b2

Simplifying by canceling common terms x2+y2+a2+b2:

4ax + 4by = 0 ax+by=0

Hence, for m = -1, the locus is a straight line given by ax + by = 0.

Step 3: Case 2: Analyze for m = 2
Substitute m = 2 into the squared distance equation:

(x-2a)2 + (y-2b)2 = 4 [ (x-a)2 + (y-b)2 ]

Expanding both sides:

x2 - 4ax + 4a2 + y2 - 4by + 4b2 = 4x2 - 8ax + 4a2 + 4y2 - 8by + 4b2

Rearranging all terms to one side:

3x2 - 4ax + 3y2 - 4by = 0

Dividing by 3:

x2 - 43ax + y2 - 43by = 0

Completing the squares for x and y:

(x-23a)2 + (y-23b)2 = 49a2 + 49b2 = (23a2+b2)2

This represents a circle centered at (23a,23b) with radius R=23a2+b2.

Thus, both the statement for m = -1 and the statement for m = 2 are fully verified.

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