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Concepts Of Physics MCQ Edition [Volume 1]
Exam
Concepts Of Physics MCQ Edition [Volume 1]
Subject
Physics
Chapter
Physics and Mathematics

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Physics and Mathematics

In a discharging R – C circuit, the electric current is given by i...

In a discharging RR–CC circuit, the electric current is given by i=i0e−t/RCi = i_0 e^{-t/RC}, where i0i_0, RR and CC denote constant parameters and tt represents time. If i0=2.00 Ai_0 = 2.00 \text{ A}, R=6.00×105ΩR = 6.00 \times 10^5 \Omega and C=0.500μFC = 0.500 \mu\text{F}, calculate approximately the current at t=0.31 st = 0.31 \text{ s}.

Options

  1. A6.23e A\dfrac{6.2}{3e} \text{ A}
  2. B2.003e A\dfrac{2.00}{3e} \text{ A}
  3. C5.83e A\dfrac{5.8}{3e} \text{ A}
  4. D5.8e A\dfrac{5.8}{e} \text{ A}

Correct answer

C. 5.83e A\dfrac{5.8}{3e} \text{ A}

Step-by-step solution

The time constant of the circuit is given by τ=RC\tau = RC.

Substituting the given values:

τ=(6.00×105)×(0.500×10−6)=0.30 s\tau = (6.00 \times 10^5) \times (0.500 \times 10^{-6}) = 0.30 \text{ s}

The current at time tt is given by:

i=i0e−t/RCi = i_0 e^{-t/RC}

Substituting i0=2.00 Ai_0 = 2.00 \text{ A} and t=0.31 st = 0.31 \text{ s}:

i=2.00e−0.31/0.30=2.00e−31/30i = 2.00 e^{-0.31/0.30} = 2.00 e^{-31/30}

This can be rewritten as:

i=2.00e−1e−1/30i = 2.00 e^{-1} e^{-1/30}

Using the approximation e−x≈1−xe^{-x} \approx 1 - x for small xx, we get e−1/30≈1−130=2930e^{-1/30} \approx 1 - \dfrac{1}{30} = \dfrac{29}{30}.

Therefore, the current is approximately:

i≈2.00e(2930)=5830e=5.83e Ai \approx \dfrac{2.00}{e} \left(\dfrac{29}{30}\right) = \dfrac{58}{30e} = \dfrac{5.8}{3e} \text{ A}
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