Electric field at a distance r from a point charge q is E = k q / r^2. What is E for q = 5 μC at r = 0.1 m? (k = 8.99×10^9)

• A. 4.5×10^3 N/C
• B. 2.25×10^6 N/C
• C. 4.5×10^6 N/C
• D. 9.0×10^6 N/C

Answer: (C)

From a point charge, the electric field strength scales as E = k q / r^2, meaning it grows with the charge and falls off with the square of the distance. Here q = 5 μC = 5 × 10^-6 C and r = 0.1 m, so r^2 = 0.01 m^2. Plugging in: E = (8.99 × 10^9) × (5 × 10^-6) / 0.01 = 4.495 × 10^6 N/C, about 4.5 × 10^6 N/C. Since the charge is positive, the field points radially outward from the charge.