CBSE Class 12 · 2023 · ElectrostaticsLong Answer

(b)(i) Consider two identical point charges located at points

(0, 0)

and

(a, 0)

. (1) Is there a point on the line joining them at which the electric field is zero? (2) Is there a point on the line joining them at which the electric potential is zero? Justify. (ii) State the significance of the negative value of electrostatic potential energy of a system of charges. Three charges are placed at the corners of an equilateral triangle ABC of side 2.0 m as shown in the figure (

+ 4 μ C

at A,

- 4 μ C

at B,

+ 2 μ C

at C). Calculate the electric potential energy of the system of three charges.

Solution

(i)(1) For two identical (equal, like) charges the fields are equal and opposite at the mid-point.

E = 0 at (\frac{a}{2}, 0)

(2) The potential of each like charge is positive everywhere, so their sum can never be zero on the line joining them — there is no such point.
(ii) A negative potential energy means the system is bound: positive work equal to

∣ U ∣

must be done to separate the charges to infinity.

U = \frac{1}{4 π ε _{0} r} [q_{1} q_{2} + q_{2} q_{3} + q_{3} q_{1}]

U = \frac{9 \times 1 0 ^{9}}{2.0} [(4) (- 4) + (- 4) (2) + (2) (4)] \times 1 0^{- 12}

U = \frac{9 \times 1 0 ^{9}}{2} (- 16) \times 1 0^{- 12}

U = - 72 \times 1 0^{- 3} J = - 0.072 J

CBSE Class 12 Board · 2023 · Subjective

Practise more: all CBSE board PYQs on Electrostatics →

CBSE Class 12 · 2023 · ElectrostaticsLong Answer

Solution

More board questions on Electrostatics