DELHI SCHOOL TEACHERS FORUM

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CLASS -12 PHYSICS Topics : Motion of a charged particle in a magnetic field, Magnetic force on a current carrying conductor placed in magnetic field, Biot Savart's Law based on the class held on 17/07/2020.

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1. Two charged particles traverse identical helical paths in a completely opposite sense in a uniform magnetic field B = B0\hat{k}. *

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(a) They have equal z-components of momenta.

(b) They must have equal charges.

(d) The charge to mass ratio satisfy:

2. Biot-Savart law indicates that the moving electrons (velocity v) produce a magnetic field B such that *

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(a) B ⊥ v.

(b) B || v.

(d) it is along the line joining the electron and point of observation.

3. A current carrying circular loop of radius R is placed in the x-y plane with centre at the origin. Half of the loop with x > 0 is now bent so that it now lies in the y – z plane. *

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(a) The magnitude of magnetic moment now diminishes.

(b) The magnetic moment does not change.

(d) The magnitude of B at (0, 0, z), z » R is unchanged.

4. An electron is projected with uniform velocity along the axis of a current carrying long solenoid. Which of the following is true? *

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(a) The electron will be accelerated along the axis.

(b) The electron path will be circular about the axis.

(d) The electron will continue to move with uniform velocity along the axis of the solenoid.

5. In a cyclotron, a charged particle *

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(a) undergoes acceleration all the time.

(b) speeds up between the dees because of the magnetic field.

(d) slows down within a dee and speeds up between dees.

6. A circular current loop of magnetic moment Mis in an arbitrary orientation in an external magnetic field B. The work done to rotate the loop by 30° about an axis perpendicular to its plane is *

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(a) MB

(b)

(c)

(d) zero

7. A rectangular loop carrying a current i is situated near a long straight wire such that the wire is parallel to the one of the sides of the loop and is in the plane of the loop. If a steady current I is established in wire as shown in figure, the loop will *

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(a) rotate about an axis parallel to the wire.

(b) move away from the wire or towards right.

(d) remain stationary.

8. A circular coil of radius 4 cm and of 20 turns carries a current of 3 amperes. It is placed in a magnetic field of intensity of 0.5 weber/m². The magnetic dipole moment of the coil is *

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(a) 0.15 ampere-m²

(b) 0.3 ampere-m²

(d) 0.6 ampere-m²

9. A cubical region of space is filled with some uniform electric and magnetic fields. An electron enters the cube across one of its faces with velocity v and a positron enters via opposite face with velocity -v. At this instant, *

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(a) the electric forces on both the particles cause identical accelerations.

(b) the magnetic forces on both the particles cause equal accelerations.

(d) the motion of the centre of mass (CM) is determined by E alone.

10. Consider a wire carrying a steady current, I placed in a uniform magnetic field B perpendicular to its length. Consider the charges inside the wire. It is known that magnetic forces do not work. This implies that, *

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(a) motion of charges inside the conductor is unaffected by B, since they do not absorb energy.

(b) Some charges inside the wire move to the surface as a result of B.

(d) If the wire moves under the influence of B, no work is done by the electric force on the ions, assumed fixed within the wire.

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