Equivalent current from rotating charges

Steps:

• The disc completes n revolutions per unit time, so each charge passes a fixed point on the edge n times per unit time.
• In one revolution, all four charges pass any fixed point on the circumference, contributing total charge 4Q.
• Electric current is the rate of charge flow past a point, so equivalent current I equals total charge passing per unit time.
• Thus, I = 4Q × n = 4Qn.

Why A is correct:

• Current I is defined as charge per unit time; 4Q flows past a point n times per unit time, yielding I = 4Qn.

Why the others are wrong:

• B: Divides by n, reversing the flow rate and yielding charge accumulation instead.
• C: Introduces extraneous 8π factor, possibly from magnetic field formulas involving 2πr.
• D: Includes 2π but halves the charge count, incorrectly treating it as two charges or partial flow.

Final answer: A