NVAEasyJEE 2023Kirchhoff's Laws & Circuits

JEE Physics 2023 Question with Solution

In the given circuit the value of I1+I3I2\left| \frac{I_1 + I_3}{I_2} \right| is _____.

Circuit diagram with two sources of $$10 \, \text{V}$$ and $$20 \, \text{V}$$ on the top branch, two parallel $$10 \, \Omega$$ resistors carrying $$I_1$$ and $$I_2$$, and one bottom $$10 \, \Omega$$ resistor carrying $$I_3$$.

Answer

Correct answer:2

Step-by-step solution

Standard Method

Given: The circuit has three resistors of 10Ω10 \, \Omega each, with currents I1I_1, I2I_2, and I3I_3 as marked.

Find: The value of I1+I3I2\left| \frac{I_1 + I_3}{I_2} \right|.

Annotated circuit showing left node at $$10 \, \text{V}$$, middle node at $$20 \, \text{V}$$, right node at $$0 \, \text{V}$$, with currents $$I_1$$, $$I_2$$, and $$I_3$$ through three $$10 \, \Omega$$ resistors.

From the extracted solution, the currents are taken as

I1=I2=102010=1AI_1 = I_2 = \frac{10}{20 - 10} = 1 \, \text{A}

and

I3=1AI_3 = 1 \, \text{A}

Now substitute these values into the required expression:

I1+I3I2=1+11=2\left| \frac{I_1 + I_3}{I_2} \right| = \left| \frac{1 + 1}{1} \right| = 2

Therefore, the value of the given expression is 22.

Using node potentials from the figure

Given: The annotated figure indicates the left side is at 10V10 \, \text{V}, the middle junction is at 20V20 \, \text{V}, and the right side is at 0V0 \, \text{V}.

Find: I1+I3I2\left| \frac{I_1 + I_3}{I_2} \right|.

Each of the two inner resistors of 10Ω10 \, \Omega is connected between the 20V20 \, \text{V} node and the 10V10 \, \text{V} node, so the potential difference across each is 10V10 \, \text{V}. Hence,

I1=I2=201010=1AI_1 = I_2 = \frac{20 - 10}{10} = 1 \, \text{A}

The bottom 10Ω10 \, \Omega resistor is connected between 10V10 \, \text{V} and 0V0 \, \text{V}, so

I3=10010=1AI_3 = \frac{10 - 0}{10} = 1 \, \text{A}

Now evaluate the ratio:

I1+I3I2=1+11=2\left| \frac{I_1 + I_3}{I_2} \right| = \left| \frac{1 + 1}{1} \right| = 2

The source HTML also contains an inconsistent intermediate value 33, but its final conclusion and the second approach both give 22, which is the accepted answer.

Therefore, the correct numerical value is 22.

Common mistakes

  • Using the battery values directly without first identifying the node potentials is incorrect. The current in each resistor depends on the potential difference across that resistor, so first read or determine the node voltages and then apply I=VRI = \frac{V}{R}.

  • Adding source voltages and resistor voltages from unrelated branches in a single KVL equation is wrong. Write loop equations only for a complete closed loop, or use node potentials when the circuit is easier to read that way.

  • Ignoring the absolute value in I1+I3I2\left| \frac{I_1 + I_3}{I_2} \right| can lead to a sign error. Even if a chosen current direction gives a negative ratio, the required final value must be non-negative.

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