NDA (I) 2018 General Ability Test · Question 104 of 150

Correct — D, There are no field lines within a bar magnet. This is the statement that is flatly false, which is what a NOT-correct item is asking for. Magnetic field lines are continuous closed loops: outside the magnet they run from the north pole round to the south pole, and inside the material they continue from south back to north, so the loop closes. The iron-filing pattern that a student sees on a sheet of paper shows only the outside half of each loop, because the filings cannot get inside the metal, and that missing half is what tempts a candidate into thinking there is nothing there. The interior field is real and measurable — it is the reason a solenoid with an iron core is a far stronger electromagnet than the same solenoid empty.

• (a)They can emanate from a point — The paper treats this as acceptable — near the tip of a long thin magnet, or at a pole, the lines crowd together and spread out from what is effectively a small region. It is the loosest of the three statements the key leaves standing, and the note below explains why.
• (b)They do not cross each other — This is correct and is one of the standard properties. If two lines crossed, the compass needle placed there would have to point in two directions at once, which is impossible — a field has one value at each point.
• (c)Field lines between two poles cannot be precisely straight lines at the ends — Also correct. The field between two facing poles is close to uniform in the middle, where the lines are straight and evenly spaced, but towards the edges the lines bulge outwards. That fringing at the ends is exactly what the statement is describing.

A field line is a drawing convention: its tangent at any point gives the direction of the field there, and the crowding of lines shows the strength. For a magnetic field the lines never begin or end, because there is no such thing as an isolated magnetic charge — break a bar magnet in two and each piece grows a fresh pair of poles instead of yielding a lone north. That is the deepest difference between magnetic and electric field lines, and it is the reason the loops must run right through the material of the magnet.

Worth stating honestly: option (a) sits uneasily with the closed-loop rule. Read strictly, saying that magnetic field lines emanate from a point is what you would say of an electric field around a point charge, and it is the very property that distinguishes electric field lines from magnetic ones. The key nevertheless marks (d), and correctly so, because a NOT-correct item asks for the statement that is wrong, not for the one that is loosely phrased — and (d) contradicts the textbook outright while (a) is defensible as a description of how lines fan out around a pole. When two options both look shaky, choose the one the syllabus explicitly denies. The strategy for the whole family is to look for the statement that would break a physical law rather than the statement that is merely inelegant.

• Magnetic field lines are continuous closed loops — from north to south outside the magnet and from south to north inside it.
• No two field lines ever intersect, because the field has a single direction at every point.
• Isolated magnetic poles have never been observed; cutting a magnet in half produces two complete magnets.
• The field between two facing poles is nearly uniform in the middle but fringes outwards at the edges.

• Assuming a bar magnet is empty of field simply because iron filings cannot show what is inside it.
• Answering a NOT-correct item by hunting for the oddly worded option instead of the false one.
• Carrying over a property of electric field lines to magnetic ones.

NDA sets a NOT-correct statement item on field lines or solenoids most years, and the false option is usually the one that denies the closed-loop property.

• practice — not a real PYQ

  Two magnetic field lines can never intersect each other because

  • (a)the field is always uniform
  • (b)the magnetic field would then have two directions at the point of intersection
  • (c)field lines carry a positive charge
  • (d)the magnet would lose its magnetism

  Answer(b) the magnetic field would then have two directions at the point of intersection — a compass needle cannot point two ways at once.

• practice — not a real PYQ

  If a bar magnet is cut into two equal pieces across its length, each piece will

  • (a)have only a north pole
  • (b)have only a south pole
  • (c)be a complete magnet with both poles
  • (d)lose its magnetism entirely

  Answer(c) be a complete magnet with both poles — isolated magnetic poles do not exist.