In-text Questions — Force on a Current-Carrying Conductor — Magnetic Effects of Electric Current

1. 13 marksNCERT Cl-10 Science, In-text Qs after §12.3's worked example, Q1

   Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.) (a) mass (b) speed (c) velocity (d) momentum

   Hint. The magnetic force is always perpendicular to the particle's velocity — think about what that does and doesn't let it change.

   Step 1 — Recall the direction of the magnetic force on a moving charge. The magnetic force on a moving charged particle is always perpendicular to its velocity.

   Step 2 — Work out the consequence for speed. Since a force perpendicular to motion does no work on the particle, its kinetic energy — and hence its speed — stays constant.

   Step 3 — Work out the consequence for velocity and momentum. Even though speed doesn't change, the force continuously changes the direction of motion, so both velocity and momentum (vector quantities depending on direction) do change.

   Step 4 — Mass. Mass is unaffected by any of this.

   ✦ Answer: (c) velocity and (d) momentum can change (in direction); (b) speed and (a) mass stay the same.

   Where students slip. Including speed as something that changes — the magnetic force changes only the direction of motion, never the speed, since it does no work on the particle.

2. 23 marksNCERT Cl-10 Science, In-text Qs after §12.3's worked example, Q2

   In Activity 12.7, how do we think the displacement of rod AB will be affected if (i) current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased?

   Hint. All three changes push the same underlying quantity (the force on the rod) in the same direction.

   Step 1 — Recall what the force on the rod depends on. The force on a current-carrying conductor in a magnetic field increases with the current, the strength of the magnetic field, and the length of the conductor within the field.

   Step 2 — Apply each change in turn. (i) Increasing the current increases the force, so the displacement increases. (ii) A stronger magnet means a stronger field, again increasing the force and the displacement. (iii) A longer rod also increases the force, so the displacement increases too.

   ✦ Answer: In all three cases — more current, a stronger magnet, or a longer rod — the displacement of rod AB increases.

   Where students slip. Assuming only the current affects the displacement — the field strength and the conductor's length are just as much a part of what determines the force (and hence displacement) on the rod.

3. 33 marksNCERT Cl-10 Science, In-text Qs after §12.3's worked example, Q3

   A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is (a) towards south (b) towards east (c) downward (d) upward

   Hint. Since the charge is positive, its direction of motion is also the direction of conventional current — apply Fleming's left-hand rule directly.

   Step 1 — Set up the known directions. The particle (positive charge, so current direction = motion direction) moves west; the force/deflection is north.

   Step 2 — Apply Fleming's left-hand rule. With the second finger (current) pointing west and the thumb (force) pointing north, the first finger (field) must point vertically upward for the three directions to stay mutually perpendicular and consistent.

   Step 3 — Confirm this is the only fit. Any horizontal (north-south or east-west) field component either contributes nothing to this particular force or points the force the wrong way, so a purely vertical, upward field is what fits.

   ✦ Answer: (d) upward.

   Where students slip. Picking a horizontal direction like east or south — working through Fleming's left-hand rule carefully with west (current) and north (force) fixed shows the third, mutually perpendicular direction must be vertical, not horizontal.

Solutions written by the tuition.in editorial team and checked against the NCERT Class 10 Science textbook, Reprint 2026-27 (jesc112.pdf) — five in-text question sets (12 questions total) plus one end-of-chapter Exercise (9 questions, not 18 as some older manifests claim). The electric motor, electromagnetic induction and electric generator have been removed from this chapter entirely (confirmed by full-text search) — they are named once as example devices, with no construction/working explained, and don't appear in the chapter's own 'What you have learnt' summary. One exercise question (Q7(iii)) still references induced current despite this; it is answered using Fleming's right-hand rule, flagged accordingly.. Questions are referenced from the NCERT textbook for identification.