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PhysicsMatrix match+6Question 21 of 66

IIT-JEE 2007 · Paper 2, Physics question 21

Two wires each carrying a steady current II are shown in four configurations in Column I. Some of the resulting effects are described in Column II. Match the statements in Column I with the statements in Column II and indicate your answer by darkening appropriate bubbles in the 4 × 4 matrix given in the ORS.

Column I

  • A
    Point P is situated midway between the wires.
    Two long parallel straight wires, both carrying current to the right; P is midway between them.
  • B
    Point P is situated at the mid-point of the line joining the centers of the circular wires, which have same radii.
    Two equal coaxial circular loops in parallel planes, seen edge-on, with currents in the same sense; P is on the common axis midway between them.
  • C
    Point P is situated at the mid-point of the line joining the centers of the circular wires, which have same radii.
    Two equal coplanar circular loops side by side with P midway between their centres; the left loop's current goes down on its left side (anticlockwise), the right loop's goes down on its right side (clockwise).
  • D
    Point P is situated at the common center of the wires.
    Two concentric coplanar circular loops with P at the common centre; the outer loop's current goes up on its left side (clockwise) and the inner loop's goes up on its right side (anticlockwise), i.e. opposite senses.

Column II

  • P
    The magnetic fields (BB) at P due to the currents in the wires are in the same direction.
  • Q
    The magnetic fields (BB) at P due to the currents in the wires are in opposite directions.
  • R
    There is no magnetic field at P.
  • S
    The wires repel each other.
Mark every match for each entry
PQRS
A
B
C
D
Answer and worked solution

Official answer: A → Q, R; B → P; C → Q, R; D → Q

  • A: Parallel wires with currents in the same direction: at the midpoint their fields are equal and opposite, so they cancel. → Q, R. (Like currents attract, so not S.)
  • B: Coaxial loops with currents in the same sense: on the axis both fields point the same way. → P. (They attract.)
  • C: At P the left loop (anticlockwise) gives a field into the page and the right loop (clockwise) a field out of it, equal in size by symmetry: they cancel. → Q, R.
  • D: Concentric loops with opposite currents: their fields at the centre are opposite but unequal (different radii), so B ≠ 0. → Q. Opposite currents repel, so the key also accepts Q, S.

Answer: A → Q, R; B → P; C → Q, R; D → Q

Topics:Biot-Savart lawForce on a current-carrying conductor
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