Bihar Board Class 12 Science Physics · Ch 1: Electric Charges and Fields

Topics in this test

Difficulty: 3 easy · 9 medium · 18 hard

Sample questions with answers

The first 5 questions of this free test.

1. Q1. The electrostatic force between two point charges is $20\ \text{N}$ when they are placed in vacuum. If the same charges are placed at the same separation in a medium of dielectric constant $K = 4$, the force becomes:

   • A. $1.25\ \text{N}$
   • B. $20\ \text{N}$
   • C. $5\ \text{N}$
   • D. $80\ \text{N}$
   Solution

   In a medium the force reduces by a factor $K$, so $F_{medium} = \frac{F_{vacuum}}{K} = \frac{20}{4} = 5\ \text{N}$.

2. Q2. Assertion (A): The electric field at every point inside a uniformly charged thin spherical shell is zero. Reason (R): A Gaussian surface drawn inside the shell encloses no net charge. Choose the correct option.

   • A. A is false but R is true
   • B. Both A and R are true, but R is not the correct explanation of A
   • C. A is true but R is false
   • D. Both A and R are true, and R is the correct explanation of A
   Solution

   All the charge lies on the outer surface, so a Gaussian surface inside encloses zero charge, giving zero field; R correctly explains A.

3. Q3. Two very large parallel plane sheets carry equal and opposite uniform surface charge densities $+\sigma$ and $-\sigma$. The magnitude of the electric field in the region between the sheets is:

   • A. $\frac{\sigma}{\varepsilon_0}$
   • B. $\frac{\sigma}{2\varepsilon_0}$
   • C. $\frac{2\sigma}{\varepsilon_0}$
   • D. zero
   Solution

   Each sheet gives $\frac{\sigma}{2\varepsilon_0}$; between the sheets the two fields add to $\frac{\sigma}{\varepsilon_0}$, while outside they cancel to zero.

4. Q4. Consider the following statements about electric field lines: (i) Two field lines can never cross each other. (ii) Field lines start from positive charges and end on negative charges. (iii) Electric field lines form closed loops. Which of the statements are correct?

   • A. (i) and (ii) only
   • B. (i), (ii) and (iii)
   • C. (i) and (iii) only
   • D. (ii) and (iii) only
   Solution

   Lines cannot cross (a point can have only one field direction) and they begin on positive and end on negative charges; electrostatic field lines never form closed loops.

5. Q5. An electric dipole of moment $2\times10^{-8}\ \text{C·m}$ is placed in a uniform electric field of $3\times10^{5}\ \text{N/C}$, making an angle of $30^\circ$ with the field. The torque acting on the dipole is:

   • A. $3\times10^{-3}\ \text{N·m}$
   • B. $6\times10^{-3}\ \text{N·m}$
   • C. $1.5\times10^{-3}\ \text{N·m}$
   • D. $5.2\times10^{-3}\ \text{N·m}$
   Solution

   Torque $\tau = pE\sin\theta = (2\times10^{-8})(3\times10^{5})(\sin 30^\circ) = 3\times10^{-3}\ \text{N·m}$.

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