Capacitors and Dielectrics - MCAT Chemical and Physical Foundations of Biological Systems

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Question

Capacitors with capacitances of 3 μF, 7 μF and 10 μF are wired in parallel. What is the capacitance of the circuit?

  1. it cannot be determined without knowing the resistance of the circuit
  2. it cannot be determined without knowing the time constant in the circuit
  3. 20 μF
  4. approximately 1.75 μF
  5. none of these is correct

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Answer

Response 3 is the correct choice. Electrons will space themselves as far apart as possible, because of charge repulsion; therefore, in a parallel arrangement, they will jump onto each capacitor and “load it up.” The parallel capacitance is calculated by simply adding the individual values. The value would be about 1.75 μF if the three were connected in series, where the formula is \dpi{100} \small \frac{1}{C_{total}}=\frac{1}{C_{1}}+\frac{1}{C_{2}}+\frac{1}{C_{3}} (reciprocal of total equals sum of reciprocal of each). The time constant of a circuit is given by the formula τ = RC, and it relates to how fast a capacitor can charge to full capacitance.

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