What is the time constant of a circuit having two 220-microfarad capacitors and two 1-megohm resistors, all in parallel?
The correct answer is D: 220 seconds. What is the time constant of a circuit having two 220-microfarad capacitors and two 1-megohm resistors, all in parallel is 220 seconds. For parallel: C_total = 220 + 220 = 440 μF, R_total = (1×1)/(1+1) = 0.5 MΩ. τ = RC = 0.5×10^6 × 440×10^-6 = 220 seconds. For amateur radio operators, this is important for circuit calculations. Understanding this helps when calculating time constants.
Exam Tip
Time constant for parallel RC = RC. Think 'T'ime 'C'onstant = 'R'×'C'. For parallel: C_total = 440 μF, R_total = 0.5 MΩ, τ = 0.5×10^6 × 440×10^-6 = 220 seconds. Not 55s, not 110s, not 440s - just 220 seconds.
Memory Aid
"Time constant for parallel RC = RC. Think 'T'ime 'C'onstant = '2'20 seconds. τ = 0.5 MΩ × 440 μF = 220 seconds. Important for circuit calculations."
Real-World Application
A circuit with two 220-μF capacitors and two 1-MΩ resistors, all in parallel: C_total = 220 + 220 = 440 μF, R_total = (1×1)/(1+1) = 0.5 MΩ. Time constant τ = RC = 0.5×10^6 × 440×10^-6 = 220 seconds. This is the time constant - 220 seconds.
Key Concepts
Why Other Options Are Wrong
Option A (55 seconds): Incorrect. 55 seconds is too low - the calculation gives 220 seconds. 55 seconds is wrong.
Option B (110 seconds): Incorrect. 110 seconds is too low - the calculation gives 220 seconds. 110 seconds is wrong.
Option C (440 seconds): Incorrect. 440 seconds is too high - the calculation gives 220 seconds. 440 seconds is wrong.
题目解析
The correct answer is D: 220 seconds. What is the time constant of a circuit having two 220-microfarad capacitors and two 1-megohm resistors, all in parallel is 220 seconds. For parallel: C_total = 220 + 220 = 440 μF, R_total = (1×1)/(1+1) = 0.5 MΩ. τ = RC = 0.5×10^6 × 440×10^-6 = 220 seconds. For amateur radio operators, this is important for circuit calculations. Understanding this helps when calculating time constants.
考试技巧
Time constant for parallel RC = RC. Think 'T'ime 'C'onstant = 'R'×'C'. For parallel: C_total = 440 μF, R_total = 0.5 MΩ, τ = 0.5×10^6 × 440×10^-6 = 220 seconds. Not 55s, not 110s, not 440s - just 220 seconds.
记忆口诀
Time constant for parallel RC = RC. Think 'T'ime 'C'onstant = '2'20 seconds. τ = 0.5 MΩ × 440 μF = 220 seconds. Important for circuit calculations.
实际应用示例
A circuit with two 220-μF capacitors and two 1-MΩ resistors, all in parallel: C_total = 220 + 220 = 440 μF, R_total = (1×1)/(1+1) = 0.5 MΩ. Time constant τ = RC = 0.5×10^6 × 440×10^-6 = 220 seconds. This is the time constant - 220 seconds.
错误选项分析
Option A (55 seconds): Incorrect. 55 seconds is too low - the calculation gives 220 seconds. 55 seconds is wrong. Option B (110 seconds): Incorrect. 110 seconds is too low - the calculation gives 220 seconds. 110 seconds is wrong. Option C (440 seconds): Incorrect. 440 seconds is too high - the calculation gives 220 seconds. 440 seconds is wrong.
知识点
Time constant, Two 220-microfarad capacitors, Two 1-megohm resistors, All in parallel, 220 seconds
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.