Electric Charges and Fields

What you'll learn

A from-scratch guide to charge, Coulomb's law, the electric field and its lines, the dipole, and Gauss's law -- with a worked numerical, a full derivation, and a quick-reference table for CBSE Class 12 (and your JEE/NEET/CUET foundation).

This chapter has

Electric Charges and Fields — solved practice questions

8 Class 12 Physics questions with step-by-step solutions. Attempt each, then reveal the answer.

1. Q1easy

   A charged body carries a charge of -1 microcoulomb. The number of excess electrons on it is (electronic charge e = 1.6 x 10^-19 C)

   • A6.25 x 10^12
   • B1.6 x 10^13
   • C6.25 x 10^18
   • D1.6 x 10^19
   Show answer & solution

   Correct answer: (A) 6.25 x 10^12

   Charge is quantised, q = n e, so n = q/e = (1 x 10^-6)/(1.6 x 10^-19) = 6.25 x 10^12 electrons.

2. Q2easy

   The electrostatic force between two point charges is F. If the distance between them is doubled while the charges are unchanged, the new force is

   • A2F
   • BF/2
   • CF/4
   • D4F
   Show answer & solution

   Correct answer: (C) F/4

   By Coulomb's law F = k q1 q2 / r^2, so F is inversely proportional to r^2. Doubling r makes the force F/4.

3. Q3easy

   Which of the following statements about electric field lines is correct?

   • ATwo field lines can intersect at a point
   • BField lines form closed loops for static charges
   • CField lines start from negative charge and end on positive charge
   • DThe tangent to a field line at any point gives the direction of the electric field there
   Show answer & solution

   Correct answer: (D) The tangent to a field line at any point gives the direction of the electric field there

   The tangent to a field line at any point gives the direction of E there. Field lines never intersect, are not closed loops for static charges, and start on positive and end on negative charge.

4. Q4easy

   The SI unit of electric flux is

   • AN C^-1
   • BN m^2 C^-1
   • CN m^-2 C
   • DC m^-2
   Show answer & solution

   Correct answer: (B) N m^2 C^-1

   Electric flux = E x area, so its unit is (N C^-1)(m^2) = N m^2 C^-1 (equivalently V m).

5. Q5medium

   Two point charges +2 microcoulomb and +3 microcoulomb are placed 30 cm apart in air. The magnitude of the force between them is (1/4 pi e0 = 9 x 10^9 N m^2 C^-2)

   • A0.6 N
   • B0.06 N
   • C6 N
   • D0.3 N
   Show answer & solution

   Correct answer: (A) 0.6 N

   F = k q1 q2 / r^2 = 9 x 10^9 x (2 x 10^-6)(3 x 10^-6) / (0.3)^2 = 0.054/0.09 = 0.6 N.

6. Q6medium

   The magnitude of the electric field at a distance of 0.3 m from a point charge of +2 microcoulomb is (1/4 pi e0 = 9 x 10^9 N m^2 C^-2)

   • A6 x 10^4 N C^-1
   • B9 x 10^4 N C^-1
   • C2 x 10^5 N C^-1
   • D2 x 10^4 N C^-1
   Show answer & solution

   Correct answer: (C) 2 x 10^5 N C^-1

   E = k q / r^2 = 9 x 10^9 x (2 x 10^-6) / (0.3)^2 = 18000/0.09 = 2 x 10^5 N C^-1.

7. Q7medium

   For a short electric dipole, the ratio of the electric field on the axial line to that on the equatorial line at the same distance from the centre is

   • A1 : 1
   • B2 : 1
   • C1 : 2
   • D4 : 1
   Show answer & solution

   Correct answer: (B) 2 : 1

   E_axial = 2kp/r^3 and E_equatorial = kp/r^3, so their ratio is 2 : 1.

8. Q8medium

   An electric dipole of moment p is placed in a uniform electric field E. The torque on it is maximum when the angle between p and E is

   • A0 degrees
   • B45 degrees
   • C180 degrees
   • D90 degrees
   Show answer & solution

   Correct answer: (D) 90 degrees

   Torque tau = pE sin(theta), which is maximum when theta = 90 degrees (sin 90 = 1).

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