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Foundation2 past questions

Magnetic flux and units

Study the concept of magnetic flux through surfaces and the units used to measure magnetic field strength.

Why this shows up in the exam

NEET expects you to calculate magnetic flux and be familiar with the correct SI units for magnetic quantities.

How NEET tests this

NumericalDirect recall

Learn the idea

Magnetic flux is the amount of magnetic field passing through a surface (Φ = B·A·cosθ). The key insight is that the unit of magnetic flux density B can be written in three equivalent SI forms, all representing the same physical dimension.

🧠 Memory hook: Think of a ‘T‑shirt’: a Tesla (T) covers a square‑meter area just like a T‑shirt covers a body – so T = Wb per m².

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

Formulas & facts to keep ready

  • Φ = B·A·cosθ (NCERT)
  • Magnetic flux Φ is measured in Weber (Wb)
  • Magnetic flux density B is measured in Tesla (T)
  • 1 T = 1 Wb / m² = 1 N / (A·m)
  • When the surface is perpendicular to B, cosθ = 1
  • Resistance of the loop does not affect the flux value

How to approach it

  1. 1Identify the area of the surface and its orientation to the field
  2. 2Write Φ = B·A·cosθ and substitute the given values
  3. 3If the question asks for the unit of B, recall the three equivalent forms: T, Wb/m², N/(A·m)
  4. 4Choose the option that matches the equivalence
  5. 5Check that no extra factors (e.g., resistance) are needed for pure flux calculations

Worked example — watch it click

Unit of magnetic flux density (or magnetic induction) is:

  • A)Tesla
  • B)Weber/metre²
  • C)Newton/ampere-metre
  • ✅All of the above

The concept behind this problem

The example tests whether you know that the SI unit of magnetic flux density can be expressed in three different but equivalent ways, all of which appear in the options.

Step by step

  1. 1Magnetic flux density B has SI unit Tesla (T).
  2. 2From Φ = BA, we get [B] = Wb/m².
  3. 3From F = BIl, we get [B] = N/(A·m).
  4. 4These are all equivalent: 1 T = 1 Wb/m² = 1 N/(A·m).
  5. 5All three options represent the same unit.

Watch out

Choosing only ‘Tesla’ and ignoring the equivalent forms Wb/m² and N/(A·m).

Common slip-ups that cost marks

  • •Confusing magnetic flux (Wb) with flux density (T)
  • •Omitting the cosine factor when the surface is not perpendicular
  • •Assuming only one form of the unit is acceptable – the exam may list any equivalent expression

🌟 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 2NEET 2022

A square loop of side 1 m and resistance 1 Ω is placed in a magnetic field of 0.5 T. If the plane of loop is perpendicular to the direction of magnetic field, the magnetic flux through the loop is:

Push further

More challenging

1 harder question 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 1

A rectangular loop of dimensions 20 cm by 10 cm is oriented such that its plane is parallel to a uniform magnetic field of 0.8 T. What is the magnetic flux passing through the loop?

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