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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.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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