JAMB 2025 Physics · Question 39
In an A.C circuit, the instantaneous current is 7A. What is the root mean square(r.m.s) value of the current I
Options
- A.5A
- B.6A
- C.3A
- D.4A
Answer: A. 5A
Explanation written with AI and checked against the answer key
Analyze the question The question gives an "instantaneous current" of 7 A and asks for the r.m.s. value. The instantaneous current alone is not enough to find the r.m.s. value. We need to know whether 7 A is the peak value or an instantaneous value at some particular time. Concept For a sinusoidal A.C.: I_rms = I₀ / √2 ≈ 0.707 × I₀ where I₀ is the peak (maximum) current. Solution If we treat the 7 A as the peak current (the most likely intention of the question): I_rms = 7 / √2 = 7 × 0.707 ≈ 4.95 A ≈ 5 A This matches option A. Note on the question The wording is flawed. An instantaneous current is generally not the peak current, so strictly the r.m.s. value cannot be found from it alone. Still, working from 7 A as the peak value gives about 4.95 A, which is closest to 5 A. 5A. **Tip Whenever you see "peak" or "maximum" current/voltage and need r.m.s., divide by √2 (about 1.414), or multiply by 0.707. Quick check: 7 ÷ 1.414 ≈ 4.95, which is nearly 5. Also remember that the r.m.s. value is always smaller than the peak value, which rules out options like 6 A once you calculate.AI can make mistakes. Report anything that looks wrong.
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