NVAEasyJEE 2023Biot–Savart Law

JEE Physics 2023 Question with Solution

Two identical circular wires of radius 20cm20 \, \text{cm} and carrying current 2A\sqrt{2} \, \text{A} are placed in perpendicular planes as shown in the figure. The net magnetic field at the center of the circular wires is _____ × 108T10^{-8} \, \text{T}.

Two identical circular current-carrying circular loops placed in mutually perpendicular planes, intersecting at the common center, with x-axis horizontal and y-axis vertical, and current directions marked on both loops.

Answer

Correct answer:628

Step-by-step solution

Standard Method

Given: Two identical circular wires of radius R=20cm=0.2mR = 20 \, \text{cm} = 0.2 \, \text{m} carry current I=2AI = \sqrt{2} \, \text{A} and are placed in perpendicular planes.

Find: The net magnetic field at the common center in the form x×108Tx \times 10^{-8} \, \text{T}.

Magnetic field at the center of one circular loop is

B=μ0I2RB = \frac{\mu_0 I}{2R}

Since the planes of the two loops are perpendicular, the magnetic fields B1B_1 and B2B_2 at the center are also perpendicular. Therefore,

B0=B12+B22B_0 = \sqrt{B_1^2 + B_2^2}

For identical loops, B1=B2=BB_1 = B_2 = B, so

B0=2BB_0 = \sqrt{2}B

Substituting B=μ0I2RB = \frac{\mu_0 I}{2R},

B0=2μ0I2RB_0 = \sqrt{2}\frac{\mu_0 I}{2R}

Now put I=2AI = \sqrt{2} \, \text{A} and R=0.2mR = 0.2 \, \text{m}:

B0=2μ0×22×0.2B_0 = \sqrt{2}\frac{\mu_0 \times \sqrt{2}}{2 \times 0.2} B0=2μ00.4×2=μ00.2=5μ0B_0 = \frac{2\mu_0}{0.4 \times 2} = \frac{\mu_0}{0.2} = 5\mu_0

Using μ0=4π×107T m/A\mu_0 = 4\pi \times 10^{-7} \, \text{T m/A},

B0=5×4π×107=20π×107TB_0 = 5 \times 4\pi \times 10^{-7} = 20\pi \times 10^{-7} \, \text{T}

Taking π=3.14\pi = 3.14,

B0=62.8×107=628×108TB_0 = 62.8 \times 10^{-7} = 628 \times 10^{-8} \, \text{T}

Therefore, the required numerical value is 628.

Common mistakes

  • Using scalar addition B0=B1+B2B_0 = B_1 + B_2 is incorrect because the two magnetic fields are mutually perpendicular. Use vector addition: B0=B12+B22B_0 = \sqrt{B_1^2 + B_2^2}.

  • Taking the radius as 20m20 \, \text{m} or 2020 instead of converting 20cm20 \, \text{cm} to 0.2m0.2 \, \text{m} gives a wrong magnitude. Always convert SI units before substitution.

  • Missing the factor 2\sqrt{2} in the current I=2AI = \sqrt{2} \, \text{A} leads to an incorrect field for each loop. Substitute the given current exactly as written.

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