NEET Electric Charges and Fields — Set 3

Question 1

Why do electric field lines tend to be closer together near regions of high charge density?

Accepted Answer

High charge density produces a stronger electric field. The convention of field lines dictates that their closeness (density) reflects field strength, so they cluster near regions where the charge per unit area or volume is greater.

Question 2

Why does the electric field near an infinite charged plane not depend on the nature of the material it’s made of?

Accepted Answer

The field depends only on the surface charge density ($ \sigma/2\varepsilon_0 $), as derived from Gauss’s law for an infinite plane. The material affects charge distribution, but for a given $ \sigma $, the field is universal, assuming infinite extent and uniformity.

Question 3

Two charges $ +9 \, \mu\text{C} $ and $ -3 \, \mu\text{C} $ are 70 cm apart. What is the electric field magnitude at the midpoint?

Accepted Answer

Midpoint distance = 35 cm = 0.35 m. $ E_1 = 9 \times 10^9 \times \frac{9 \times 10^{-6}}{(0.35)^2} = 6.61 \times 10^5 \, \text{N/C} $ (towards $ -3 \, \mu\text{C} $). $ E_2 = 9 \times 10^9 \times \frac{3 \times 10^{-6}}{(0.35)^2} = 2.2 \times 10^5 \, \text{N/C} $ (towards $ -3 \, \mu\text{C} $). Net $ E = 6.61 \times 10^5 + 2.2 \times 10^5 = 8.81 \times 10^5 \, \text{N/C} $.

NEET Physics: Electric Charges and Fields — Practice Set 3

Q1. Why do electric field lines tend to be closer together near regions of high charge density?

Q2. Why does the electric field near an infinite charged plane not depend on the nature of the material it’s made of?

Q3. Two charges \( +9 \, \mu\text{C} \) and \( -3 \, \mu\text{C} \) are 70 cm apart. What is the electric field magnitude at the midpoint?

Q4. Why does an electric dipole experience a torque but no net force when placed in a uniform electric field?

Q5. Why does the electric field due to an electric dipole decrease with distance as \( 1/r^3 \) rather than \( 1/r^2 \) as for a single charge?

Q6. A conducting sphere of radius 16 cm has an electric field of \( 5 \times 10^3 \, \text{N/C} \) at 40 cm from its center. What is the charge?

Q7. A conducting sphere of radius 23 cm has a surface charge density of \( 35 \, \mu\text{C/m}^2 \). What is the total charge?

Q8. Two charges \( +4 \, \mu\text{C} \) and \( -8 \, \mu\text{C} \) are 40 cm apart. What is the electric field magnitude at a point 20 cm from each charge (forming an equilateral triangle)?

Q9. A charge of \( 3 \, \mu\text{C} \) is enclosed in a cube of edge 15 cm. What is the flux through one face?

Q10. Two charges \( +10 \, \mu\text{C} \) and \( -5 \, \mu\text{C} \) are 50 cm apart. What is the distance from \( +10 \, \mu\text{C} \) where the electric field is zero?

Q11. A point charge \( -3 \, \mu\text{C} \) is at the origin. What is the electric field magnitude at \( x = 3 \, \text{m} \)?

Q12. What is the total charge of a system containing three charges: \( +3 \, \mu\text{C} \), \( -5 \, \mu\text{C} \), and \( +2 \, \mu\text{C} \)?

Q13. A closed surface has a net flux of \( 5.65 \times 10^5 \, \text{Nm}^2/\text{C} \). What is the charge enclosed?

Q14. Two charges \( +8 \, \mu\text{C} \) and \( -2 \, \mu\text{C} \) are 20 cm apart. What is the distance from \( +8 \, \mu\text{C} \) where the electric field is zero?

Q15. A dipole with charges \( +7 \, \mu\text{C} \) and \( -7 \, \mu\text{C} \) separated by 3 mm is in a field \( 4 \times 10^4 \, \text{N/C} \) at 60°. What is the torque?