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Exam level9 past questions

Magnetic field due to currents

Learn how electric currents produce magnetic fields, including the use of Biot-Savart law, Ampere's law, and the calculation of fields for various conductor shapes such as straight wires, circular loops, and solenoids.

Why this shows up in the exam

NEET often tests your ability to calculate and reason about magnetic fields generated by different current-carrying conductors.

How NEET tests this

Application · 5 QsNumerical · 5 QsStatement analysis · 2 Qs

Learn the idea

A current‑carrying conductor creates a magnetic field whose magnitude is given by simple formulas that depend only on geometry and current. The key insight is that the field strength is directly proportional to the current and to any geometrical factor that counts how many times the current loops around (e.g., number of turns).

🧠 Memory hook: Think of the centre‑field formula as ‘μ₀ × (N × I) divided by 2 × radius’ – like a pizza: more slices (turns) give a larger bite (field).

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • Biot‑Savart law: B = μ₀ I dl × r̂ /(4π r²)
  • Magnetic field at centre of a circular loop (N turns): B = μ₀ N I /(2R)
  • Ampère’s law: ∮B·dl = μ₀ I_enc
  • Long straight wire: outside (r>R) B = μ₀ I /(2π r); inside (r<R) B = μ₀ I r /(2π R²)
  • Solenoid (long, tightly wound): B = μ₀ n I where n = N/ℓ
  • Direction of B given by right‑hand thumb rule

How to approach it

  1. 1Identify the conductor’s shape and symmetry (straight, loop, solenoid).
  2. 2Choose the appropriate law – Ampère’s law for high symmetry, Biot‑Savart for a single loop or segment.
  3. 3Write the standard formula for that geometry, substitute I, N, radius etc., and simplify.
  4. 4If asked for direction, apply the right‑hand rule; otherwise give the magnitude.
  5. 5Check units and that the answer varies correctly with distance or number of turns.

Worked example — watch it click

Assertion (A): The strength of the magnetic field produced at the centre of a current carrying circular coil increases on increasing the number of turns of the circular coil. Reason (R): Magnetic field strength is directly proportional to the number of turns of the circular coil.

  • ✅Both (A) and (R) are true, and (R) is the correct explanation of (A).
  • B)Both (A) and (R) are true, but (R) is not the correct explanation of (A).
  • C)(A) is true but (R) is false.
  • D)Both (A) and (R) are false.

The concept behind this problem

The worked example checks whether you recognise that the centre field of a coil contains the factor N, so increasing turns raises B linearly – exactly what the formula states.

Step by step

  1. 1The magnetic field at the center of a circular coil is B = (μ₀NI)/(2r).
  2. 2This shows B ∝ N (number of turns) when other parameters are constant.
  3. 3Assertion (A) correctly states that increasing N increases B.
  4. 4Reason (R) correctly states that B is directly proportional to N.
  5. 5Since (R) directly explains why (A) is true through the fundamental formula, (R) is the correct explanation of (A).
  6. 6Both statements are true and (R) explains (A).

Watch out

Students often assume the reason must be a new law, but the mistake is treating the proportionality as a separate statement instead of reading it directly from B = μ₀NI/(2R).

Common slip-ups that cost marks

  • •Confusing the radius of the wire (R) with the distance from the axis (r) when using the inside‑wire formula.
  • •Treating the formula B = μ₀ I /(2π r) as valid inside the conductor – it only holds for r > R.
  • •Omitting the factor N (total turns) or using N per unit length incorrectly in the loop formula.

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 9NEET 1999

Magnetic field due to 0.1 A current flowing through a circular coil of radius 0.1 m and 1000 turns at the centre of the coil is:

Push further

More challenging

6 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 6

Which of the following statements accurately describes the nature of the source and the field in Biot-Savart's Law? Statement I: The source of the magnetic field in Biot-Savart's Law is a scalar quantity, the current I. Statement II: The magnetic field produced by a current element is a vector quantity.

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