JEE Advanced · Physics · 17. Electrostatics

Four charges $Q_{1}, Q_{2}, Q_{3}$ and $Q_{4}$ of same magnitude are fixed along the x axis at $x = - 2 a, - a, + a$ and $+ 2 a$ , respectively. A positive charge q is placed on the positive y axis at a distance $b > 0$ . Four options of the signs of these charges are given in List I. The direction of the forces on the charge q is given in List II. Match List I with List II and select the correct answer using the code given below the lists.

List I List II
A. $Q_{1}, Q_{2}, Q_{3} Q_{4}$ all positive $P .$ $+ x$
B. $Q_{1}, Q_{2}$ positive; $Q_{3}, Q_{4}$ negative Q. $- x$
C. $Q_{1}, Q_{4}$ positive; $Q_{2}, Q_{3}$ negative $R .$ $+ y$
D. $Q_{1}, Q_{3}$ positive; $Q_{2}, Q_{4}$ negative $S .$ $- y$

A a-r;b-p;c-q;d-s;
B a-q;b-s;c-r;d-p;
C a-s;b-q;c-r;d-p;
D a-r;b-p;c-s;d-q;

Verified Solution

Answer & Solution

Correct Answer

(D) a-r;b-p;c-s;d-q;

Step-by-step Solution

Detailed explanation

P: By

Q_{1}

and

Q_{4}, Q_{3}

and

Q_{2}

F is in +y
Q: By

Q_{1}

and

Q_{4}, Q_{2}

and

Q_{3}

F is in +ve x.
R: By

Q_{1}

and

Q_{4}

, F is in +ve y
By

Q_{2}

and

Q_{3}

, F is in -ve y
But later has more magnitude, since its closer to (0, b). Therefore net force is in - y
S: By

Q_{1}

and

Q_{4}

, F is in +ve x and by

Q_{2}

and

Q_{3}

, F is in -x, but later is more in magnitude, since its closer to (0, b). Therefore net force is in -ve x.

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

Same subject

Explore more questions on app

From JEE Advanced

Explore more questions on app

	List I		List II
A.	$Q_{1}, Q_{2}, Q_{3} Q_{4}$ all positive	$P .$	$+ x$
B.	$Q_{1}, Q_{2}$ positive; $Q_{3}, Q_{4}$ negative	Q.	$- x$
C.	$Q_{1}, Q_{4}$ positive; $Q_{2}, Q_{3}$ negative	$R .$	$+ y$
D.	$Q_{1}, Q_{3}$ positive; $Q_{2}, Q_{4}$ negative	$S .$	$- y$