# Magnetic Field HC Verma Solutions of Que for Short Ans Ch-34 Vol-2

**Magnetic Field HC Verma Solutions of Que for Short Ans Ch-34 Vol-2** Concept of Physics. Step by Step Solution of Questions for short answer of Ch-34 **Magnetic Field ****HC Verma **Question of Bharti Bhawan Publishers . Visit official Website CISCE for detail information about ISC Board Class-12 Physics.

**Magnetic Field HC Verma Solutions of Que for Short Ans Ch-34 Vol-2**

Board | ISC and other board |

Publications | Bharti Bhawan Publishers |

Ch-34 | Magnetic Field |

Class | 12 |

Vol | 2nd |

writer | H C Verma |

Book Name | Concept of Physics |

Topics | Solutions of Question for Short Answer |

Page-Number | 228, 229 |

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Que for Short Ans

Obj-1

Obj-2

Exercise

**Magnetic Field H C Verma Que for Short Ans**

### Solutions of Ch-34 Vol-2 Concept of Physics for Class-12

**Question 1 :-**

Suppose a charged particle moves with a velocity *v* near a wire carrying an electric current. So, a magnetic force acts on it. If the same particle is seen from a frame moving with velocity *v* in the same direction, the charge will be found to be at rest. Will the magnetic force become zero in this frame? Will the magnetic field become zero in this frame?

**Answer 1 :-**

Magnetic force becomes zero as the particle is at rest in this frame of reference and we know that force on a particle,

** F = qv B sin (θ”>)**,

where *v* is the velocity of the particle.

So, when *v *= 0,* F* = 0.

Magnetic field on the particles still exists because the current is independent of the frame of reference. For any reason, if the electrons of the wire seem to be at rest in a frame of reference, the protons are still flowing opposite to the frame of reference. Due to this, the current and the magnetic fields still exist.

**Magnetic Field H C Verma Que for Short Ans**

Solutions of Ch-34 Vol-2 Concept of Physics for Class-12

**Question 2 :-**

Can a charged particle be accelerated by a magnetic field? Can its speed be increased?

**Answer 2 :-**

Yes, a charged particle can be accelerated by a magnetic field. A magnetic field exerts force on the charged particle, which is perpendicular to both the magnetic field and velocity. If initially the charged particle is moving at right angle to the magnetic field, then the resultant trajectory of the particle is circular motion. In circular motion, the magnitude of the velocity remains constant but direction changes continuously. So, the motion is accelerated but speed remains constant.

**Question 3 :-**

Will a current loop placed in a magnetic field always experience a zero force?

**Answer 3 :-**

No, it depend on the magnetic field, i.e. whether the field is a uniform or a non-uniform magnetic field and also on the orientation of the current loop. In case of a uniform magnetic field, the force on the circular loop is zero if the magnetic field is parallel to the plane of the loop and in case of a non-uniform magnetic field, the force may or may not be zero.

**Question 4 :-**

The free electrons in a conducting wire are in constant thermal motion. If such a wire, carrying no current, is placed in a magnetic field, is there a magnetic force on each free electron? Is there a magnetic force on the wire?

**Answer 4 :-**

Also, F = LLB sin (θ)

So, if the current in the wire is zero, then the force experienced by the wire will also be zero.

**Question 5 :-**

Assume that the magnetic field is uniform in a cubical region and zero outside. Can you project a charged particle from outside into the field, so that the particle describes a complete circle in the field?

**Answer 5 :-**

Let us assume that the magnetic field is uniform and is acting along positive x axis in the cubical region.

Now if we project a charged particle inside this cube along positive y axis then as the direction of velocity and magnetic field is perpendicular to each other so the resultant trajectory of the particle will be a circle.

**Question 6 :-**

An electron beam projected along the positive *x*-axis deflects along the positive *y*-axis. If this deflection is caused by a magnetic field, what is the direction of the field? Can we conclude that the field is parallel to the *z*-axis?

**Answer 6 :-**

As the particle gets deflected towards the positive y-axis, we can conclude that force is acting on the particle along the positive y-axis. Now, as the electron is moving along the positive x-axis, the current can be assumed to be flowing along the negative x-axis. Applying Fleming’s left-hand rule, we find that the thumb points in the direction of force, i.e. the positive y-axis and the middle finger points in the direction of current, i.e. negative x-axis. Consequently, the forefinger gives us the direction of magnetic field, i.e. out of the plane of the paper or in the positive z-direction. So, we can conclude that the magnetic field is pointing along the positive z-axis.

**Question 7 :-**

Is it possible for a current loop to stay without rotating in a uniform magnetic field? If yes, what should be the orientation of the loop?

**Answer 7 :-**

It follows from the fact that torque acting on the loop is directly proportional to si θθ, where θθ is the angle made by the area vector with the direction of the magnetic field. So,

**we can see from this correlation that torque is zero if**

**θ = 0 or θ = 180°.**

**ττ=m‾×B‾**

**= mB sin (θ)**

**⇒ For θ = 0 or integral multiple of π ,**

**τ = 0**

Which implies that the coil will not rotate.

**Question 8 :-**

The net charge in a current-carrying wire is zero. Then, why does a magnetic field exert a force on it?

**Answer 8 :-**

The net charge in a current- carrying wire is zero. Yet, negative charge, i.e. electrons are moving in the wire towards the positive terminal. It is this motion of electrons in the conductor which produces the current in the wire and is also responsible for the magnetic force acting on the wire.

*F = **qV B *sin(), where *F* is the force, *q* is the charge of electrons, *V *is the velocity of electrons and *B* is the magnetic field.

Moreover, the positive charges on the wire are due to nucleus containing proton. As they are not moving so there is no force on them, so the force is only due to the moving electrons in the wire.

**Question 9 :-**

The torque on a current loop is zero if the angle between the positive normal and the magnetic field is either θ = 0 or θ = 180°. In which of the two orientations, the equilibrium is stable?

**Answer 9 :-**

If the angle between the positive normal and the magnetic field is 0, then the equilibrium is stable. It follows directly from the fact that *U* = – *mB c*os θ where *m* is the magnetic moment. So, when θ is 0, Potential energy, i.e. *U *of the system is negative, the system is more stable. But if θ is 180°, *U* is positive or the system is unstable.

Stability of a system depends on its energy and every system tries to minimise its energy.

**Question 10 :-**

Verify that the units weber and volt second are the same.

**Answer 10 :-**

Force experienced by the charge q moving with velocity v in a magnetic field B is given by

**F = q VB**

Hence, **B = F/qV**

Also, weber/m^{2} is the unit for magnetic field B.

Now, equating both the units of the magnetic field B, we get :

Thus, the units weber and volt second are same.

— : End of **Magnetic Field**** **HC Verma vol -2 Que for Short Ans :–

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