C
CraftExam
medium difficulty11 min

Bar Magnet and Magnetic Field Lines

A bar magnet is a magnetic dipole with a north pole and a south pole. Its magnetic field lines are imaginary curves whose tangent at any point gives the direction of the magnetic field at that point.

In one line: Outside: N to S. Inside: S to N. Together: closed loop.

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Understand it

  • Outside a bar magnet, magnetic field lines are shown from the north pole to the south pole.
  • Inside the magnet, they continue from the south pole to the north pole, so magnetic field lines form closed loops.
  • The crowding of field lines near the poles represents a stronger magnetic field there.
  • Unlike isolated electric charges, isolated magnetic poles are not observed in school-level magnetism, so field lines of a magnet do not start or stop at a single pole.

Example

If iron filings are sprinkled around a bar magnet placed under a sheet of paper, the filings arrange in curved patterns that are densest near the two poles. These curves represent the shape of the magnetic field around the magnet.

Formula and rule

Key rule: outside a bar magnet, magnetic field direction is N to S; inside the magnet, it is S to N.

The tangent to a field line gives the direction of magnetic field B at that point.

Field lines never intersect because the magnetic field at one point cannot have two directions.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Question: State two properties of magnetic field lines around a bar magnet.

  1. 1Answer: First, magnetic field lines form closed continuous loops, going N to S outside and S to N inside the magnet.
  2. 2Second, the tangent at any point gives the direction of magnetic field at that point.
  3. 3Field lines are closer near the poles, so the field is stronger there.

Method you can reuse

To answer a field-line question: identify the north and south poles; state the outside direction as N to S; state the inside direction as S to N; add that the lines are closed loops; mention non-intersection and crowding near poles if properties are asked.

Sketch it in your answer

Draw a rectangular bar magnet labelled N and S. Show symmetric curved field lines outside the magnet with arrows from N to S, and lines inside the magnet with arrows from S to N. Mark denser lines near poles to indicate stronger field.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why are magnetic field lines of a bar magnet called closed curves?

Think first, then show answer
Magnetic field lines are called closed curves because they go from the north pole to the south pole outside the magnet and continue from the south pole to the north pole inside the magnet. They do not have a starting or ending point.

Common wrong answer

Magnetic field lines start from N and end at S.

Why it loses marks: This loses the essential point that magnetic field lines form closed loops through the magnet.

Don't mix this up

Students often draw magnetic field lines as if they begin at the north pole and end at the south pole. That is incomplete because the lines continue inside the magnet and form closed loops.

Exam tip

When drawing the field pattern, show arrows from N to S outside the magnet and from S to N inside the magnet. Also mention that closer field lines mean a stronger magnetic field.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

This concept is commonly asked as a diagram-labelling question, a short answer on properties of field lines, or an assertion-reason item about why two magnetic field lines cannot intersect.
  1. 1

    Enough for 1 mark

    A bar magnet is a magnetic dipole with a north pole and a south pole. Its magnetic field lines are imaginary curves whose tangent at any point gives the direction of the magnetic field at that point.

  2. 2

    Add for 2 marks

    Example: If iron filings are sprinkled around a bar magnet placed under a sheet of paper, the filings arrange in curved patterns that are densest near the two poles. These curves represent the shape of the magnetic field around the magnet.

  3. 3

    Add for 3 marks

    The crowding of field lines near the poles represents a stronger magnetic field there.

Practice

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