NDA (I & II) 2020 General Ability Test · Question 91 of 148

Correct — C, 'Magnetic field lines are open curves.' A 'NOT correct' question asks you to pick the false statement, and this is it. Magnetic field lines always form continuous closed loops - outside a magnet they run from the north pole to the south pole, and inside the magnet they continue from south back to north - so a field line has no free ends and is never an open curve.

• (a)Magnetic field is a quantity that has direction and magnitude — This is a true statement - the magnetic field is a vector with both magnitude and direction - so it is not the incorrect option the question wants.
• (b)Magnetic field lines are closed curves — This is true and is exactly the property that makes option (c) false; field lines close on themselves by passing through the magnet, so this cannot be the wrong statement.
• (d)No two magnetic field lines are found to cross each other — This is true - if two lines crossed, a compass at the crossing point would have to point in two directions at once, which is impossible - so it is not the answer.

Magnetic field lines are a way to picture a magnetic field. The tangent to a line gives the field's direction, and the closeness of the lines shows its strength. A key property is that they are continuous closed loops that never begin or end and never cross one another.

This is a 'NOT correct' item, and options (b) and (c) are direct opposites, so one of them must be the false statement. Because field lines are closed loops, 'open curves' is the false claim.

• Magnetic field lines emerge from the north pole and enter the south pole outside a magnet, continuing from south to north inside it, forming closed loops.
• The magnetic field is a vector; the tangent drawn to a field line at any point gives the direction of the field there.
• Field lines are crowded where the field is strong and spread apart where it is weak.
• Two magnetic field lines never intersect, because the field can point in only one direction at a given point.

• Reading past the word 'NOT' and choosing a true statement.
• Thinking field lines start and stop at the poles (open) - they actually continue through the magnet.
• Believing field lines can cross where the field is very strong.

NDA phrases it as 'which statement about the magnetic field / field lines is NOT correct', hinging on the closed-loop property.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following is a correct property of magnetic field lines?

  • (a)They intersect at the poles
  • (b)They are open curves
  • (c)They form closed continuous loops
  • (d)They always travel in straight lines

  Answer(c) They form closed continuous loops - running from N to S outside and S to N inside the magnet.

• practice — not a real PYQ

  Two magnetic field lines can never cross each other because at the crossing point

  • (a)the magnet would break
  • (b)the field would have two directions at once
  • (c)the lines are elastic
  • (d)both poles become identical

  Answer(b) the field would have two directions at once - impossible, since the field has a single direction at any point.