Question

The electric current in a charging R−C circuit is given by i = i₀ e^−t/RC where i₀, R and C are constant parameters of the circuit and t is time. Find the rate of change of current at (a) t = 0, (b) t = RC, (c) t = 10 RC.

Answer 1

Given: i = i₀ e^−t/RC
∴ Rate of change of current
\[= \frac{di}{dt}\]
\[= i_0 \left( \frac{- 1}{RC} \right) e^{- t/RC}\]
\[= \frac{- i_0}{RC} \times e^{- t/RC}\]
On applying the conditions given in the questions, we get:
(a) At \[t = 0, \frac{di}{dt} = \frac{- i_0}{RC} \times e^0 = \frac{- i_0}{RC}\]
(b) At  \[t = RC, \frac{di}{dt} = \frac{- i_0}{RC} \times e^{- 1} = \frac{- i_0}{RCe}\]
(c) At \[t = 10 RC, \frac{dt}{di} = \frac{- i_0}{RC} \times e^{10}\] \[= \frac{- i_0}{RC e^{10}}\]