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JEE Main2013PhysicsMagnetic Effects of CurrentActual

One of the two small circular coils, (none of them having any self - inductance) is suspended with a V-shaped copper wire, with plane horizontal. The other coil is placed just below the first one with plane horizontal. Both the coils are connected in series with a dc supply. The coils are found to attract each other with a force. Which one of the following statements is incorrect?

Options

  1. ABoth the coils carry currents in the same direction.
  2. BCoils will attract each other, even if the supply is an ac source.
  3. CForce is proportional to d ⁻¹
  4. DForce is proportional to d ⁻²

Correct answer

D. Force is proportional to d ⁻²

Step-by-step solution

As the force between the two coils is attractive hence currents in the coil must be in the same direction and the force, F I ₁ I ₂ 2 d where I₁ and I₂ are the currents flowing through the coils.

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