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E5B04

ELECTRICAL PRINCIPLES · Time constants and phase relationships: RL and RC time constants; phase angle in reactive circuits and components; admittance and susceptance

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What is the time constant of a circuit having two 220-microfarad capacitors and two 1-megohm resistors, all in parallel?

  1. A55 seconds
  2. B110 seconds
  3. C440 seconds
  4. D220 seconds
Why Time constant for an RC circuit is tau = R times C. Capacitors in parallel add, so two 220 uF caps give 440 uF. Resistors in parallel halve, so two 1 megohm resistors give 0.5 megohm. tau = 440e-6 F x 0.5e6 ohms = 220 seconds.
Watch out The choice of 440 seconds comes from adding the capacitors but forgetting that parallel resistors halve; 110 seconds comes from halving both values as if the capacitors were in series too.
Parallel: caps add, resistors divide. 440 uF x 0.5 M = 220 s.
Two 220 uF become 440, two 1 M become 0.5 M, and 440 times 0.5 lands you right back at 220. The answer is the number you started with.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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  4. E5B04 What is the time constant of a circuit having two 220-microfarad capacitors and two 1-megohm resistors, all i…
  5. E5B05 What is the effect on the magnitude of pure reactance when it is converted to susceptance?
  6. E5B06 What is susceptance?
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