Physics MCQs - "Magnetic Force on a Current Carrying Wire in Uniform Magnetic Field" for JEE Mains & NEET

Welcome to this series on Objective Physics Question Bank for NTA NEET, JEE Mains & WBJEE aspirants! The present section contains a list of the most important and conceptual physics MCQs on the topic Magnetic Force on a Current Carrying Wire/Coil in Uniform Magnetic Field of the chapter Magnetic Effect of Electric Current.


Special care has been taken in selection of problems to ensure that they fall under the  syllabus prescribed by the authorities and of the right difficulty level as demanded by the said exams. In fact a fair portion of questions are directly taken from the archives of these exams. This makes this series even more useful and relevant for the students to be used as a DPP (daily practice problems) or regular assignments.

OBJECTIVE QUESTION BANK

Out of the four different choices following each question, only one is the correct (or the most appropriate) which you have to identify and mark.


1. Force exists on a current carrying wire which is placed in external magnetic field, due to:
    A) free electrons in wire
    B) free positive ions in wire
    C) A & B both
    D) none

2. A wire of length l carries a current 'i' along x-axis. A magnetic field exists given by B = Bₒ(i + j + k). The magnitude of the magnetic force acting on the wire is
    A) Bₒil
    B) √(3)Bₒil
    C) 2Bₒil
    D) √(2)Bₒil

3. A current carrying thin conductor is placed in a uniform magnetic field such that the conductor lies parallel to the field. Magnetic force experienced by the conductor is
    A) directed perpendicular to the conductor
    B) directed parallel to the conductor
    C) zero
    D) none of these

4. A current carrying closed loop of wire is placed in a uniform magnetic field. Force exerted on the loop by the magnetic field is
    A) directed perpendicular to the plane of the loop
    B) directed in the plane of the loop
    C) dependent on the orientation of the loop
    D) zero

5. A current carrying closed loop of wire is located in a uniform magnetic field such that the field is perpendicular to the plane of the loop.
    A) Tension in the wire (due to external magnetic field) may be zero.
    B) Tension in the wire (due to external magnetic field) must be zero.
    C) Magnetic force on the wire may not be zero.
    D) None of these


6. A circular coil having mass m is kept above the ground (xz plane) at some height. The coil carries a current i in the direction indicated in the figure shown below. In which direction a uniform magnetic field B be applied so that the magnetic force balances the weight of the coil?

    A) positive x-axis
    B) negative x-axis
    C) positive z-axis
    D) none of these

7. A semi-circular wire of radius R carries a current i as shown in the figure. A uniform magnetic field of induction B exists perpendicular to the plane containing the wire. Magnetic force experienced by the wire is of magnitude

    A) 3BiR
    B) 2BiR
    C) 1.5BiR
    D) zero

8. A current carrying straight wire is oriented in a region of space where uniform magnetic field exists. Magnetic force acting on the wire is
    A) directed perpendicular to the wire
    B) directed along the wire
    C) directed along the magnetic field
    D) zero

9. A current carrying long straight wire lies in xy plane along the line y = x. A uniform magnetic field of induction 1 T exists along positive z-direction. Consider the section OA of the wire where O(0, 0) and A (√2 m, √2 m). If the wire carries a current of 1 A, force experienced by the section OA would be (in newtons):
    A) 1
    B) 2
    C) 3
    D) 4

10. A current carrying wire is arranged at any angle in an uniform magnetic field, then:
    A) only force acts on the wire
    B) only torque acts on the wire
    C) both
    D) none


ANSWERS WITH HINTS

1. A A current carrying wire has drifting free electrons inside it (opposite to the current) and that's why when placed in external magnetic field, it experiences a resultant magnetic force.

2. D Since the magnetic field is uniform, we can directly use the expression F = il × B where B = Bₒ(i + j + k) and l = li. 

3. C Apply F = il × B, or F = Bilsinθ. Since the current carrying conductor lies along the magnetic field, θ = 0° or 180°. In both of the cases, magnetic force vanishes. 

4. D Magnetic force experienced by a current carrying closed loop placed in uniform magnetic field is zero. Both the ends of the wire coincide and therefore position vector of one end with respect to another is zero i.e. I = 0 in the expression F = il × B.

5. A Depending upon the direction of current in the loop, any small section of the loop may experience a magnetic force either in radially inward or radially outward direction. Accordingly, the loop may be slacked or taut.

6. D Magnetic force experienced by the current carrying coil oriented in uniform magnetic field is always zero. So any uniform magnetic field B cannot balance the weight of the coil.

7. B Magnetic force on the wire = iIBsinθ = i(2R)Bsin90° = 2BiR.

8. A Magnetic Force F = il × B. Using the property of cross product, l and B

9. B Magnetic force on the considered section of the wire = iIBsinθ where i = 1 A, l = OA = 2 m (use distance formula), B = 1 T and θ = 90°. 

10. A Magnetic force acts on the wire given by F = il × B. The wire does not experience a magnetic torque in uniform magnetic field until it is hinged.

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