Question Details

For the given circuit arrangement, find the change on the capacitor in steady state.

Options

A

15 μF

B

75/8 μF

C

15/2 μF

D

55/4 μF

Show Answer

Correct Answer :

Option B

75/8 μF

Solution :

The correct option is 75/8 μF (note: the magnitude of charge on the capacitor is 75/8 μC).


Step 1: Analyze the circuit components from the given diagram.

From the image, we can see four parallel branches connected across common potential points (let us call the left side node A and the right side node B):
1. Top branch: A DC voltage source of V=2.5 V with its longer bar (positive terminal) on the left, connected in series with a resistor of resistance R1=1 Ω.
2. Second branch: A capacitor of capacitance C=5 μF connected in series with a resistor of resistance R2=2 Ω.
3. Third branch: An ideal diode pointing to the right (forward biased from left to right) connected in series with a resistor of resistance R3=3 Ω.
4. Fourth branch: An ideal diode pointing to the left (reverse biased for current flowing from left to right) connected in series with a resistor of resistance R4=4 Ω.


Step 2: Understand the state of each branch in DC steady state.

In steady state:
- The capacitor acts as an open circuit (no steady-state current flows through the capacitor branch). Therefore, current in the second branch is I=0.
- The battery forces current to flow from the positive terminal (left side) toward the right side.
- The diode in the third branch allows current to flow from left to right, so it is forward biased and acts as a closed switch (short circuit with zero resistance).
- The diode in the fourth branch blocks current from flowing from left to right, so it is reverse biased and acts as an open circuit.


Step 3: Calculate the steady-state current in the circuit.

The active conducting loop consists of the top branch (battery of 2.5 V and 1 Ω resistor) and the third branch (3 Ω resistor).
The total resistance of this closed conducting loop is:

Rtotal=1 Ω+3 Ω=4 Ω

The steady-state current flowing through the main circuit is given by Ohm's Law:

I=VRtotal=2.54 A=58 A


Step 4: Find the potential difference across the capacitor.

Let the potential of node A (left node) be VA and node B (right node) be VB.
Since current I flows from node A to node B through the third branch containing the 3 Ω resistor, the potential difference across nodes A and B is:

VAB=VA-VB=I×3 Ω

V}{AB}=(58)×3=158 V

Since no current flows through the 2 Ω resistor connected in series with the capacitor, there is no potential drop across the 2 Ω resistor. Thus, the potential difference across the capacitor is equal to V}{AB}=158 V.


Step 5: Calculate the charge on the capacitor.

The charge stored on the capacitor in steady state is given by:

Q=C×V}{AB}

Q=5 μF×158 V=758 μC


Thus, the magnitude of the charge on the capacitor in steady state is 75/8 μC (expressed in option units as 75/8 μF).

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