## Very Important Questions Collection

HSEB | Science Faculty

Magnetic Effect Of Current – Physics Grade XII

Short Questions with Answer (Solution)

**1. Distinguish between electric and magnetic fields.**

Ans. The following are the main points of differences between electric and magnetic fields :

Electric Field | Magnetic Field |

a. The source of electric field is electric charge. | a. The source of magnetic field is the current element. |

b. It is associated with stationary as well charge as moving charge. | b. It is associated with a moving charge. |

**2. Define the term cross field.**

Ans. If a uniform electric field and a uniform magnetic field are perpendicular to each other in such a way that they produce deflections of a charged particle in the opposite directions, they are known as cross fields. The uniform electric field is produced by a parallel plate capacitor while a uniform magnetic field is produced by a current carrying coil. If the two fields are so adjusted that force exerted by each field on the electron beam is same, then the beam goes undeleted.In this case,

`F`

_{m} = F_{e} => eE = Bev => v = E/B.

i.e. the velocity of beam of electron can be measured by using cross fields.

**3. What is Lorentz force ?**

Ans. The total force experienced by a moving charged particle when both electric and magnetic fields are present is called Lorentz force. The value of Lorentz force experienced by a charge q moving with velocity v in the presence of both an electric field vector E and a magnetic field vector B is:

vector F = q(vectorE) + q (vectorv x vectorB)

= q[vectorE + (vectorv x vectorB)]

This equation is also called as Lorentz equation.

**4. Can a uniform magnet field be used to speed up a charged particle? Explain.**

Ans. The force acting on the moving charged particle q with velocity V in the uniform magnetic field B is:

vectorF = q(vectorv x vectorB)

This force is always perpendicular to the motion of the particle and hence no work is done on the charged particle. Now work done is equal to change in K.E. As no work is done so change in K.E is zero and hence the charged particle cannot speed up although its direction of motion changes by the uniform magnetic field.

**5. A proton moving in a straight line enters a strong magnetic field along the field direction. How will its path and velocity change?**

Ans. When a proton enters to a strong magnetic field along the field direction, the force experienced by a proton is given by F = Bqvsino . In this case, proton is moving along the field, i.e. o = 0 degree, the F = Bqvsino = 0. Here, force experienced by the charge is zero. so, its path and velocity remains unchanged.

**6. Why does the kinetic energy of a charged particle moving in a magnetic field remain constant?**

Ans. The force experienced by a charge q in magnetic field vectorB moving with velocity vector vectorV is:

` vectorF = q(vectorV x vectorB)`

Hence vectorF ia always perpendicular to vectorv.

so, power = rate of work done = vector F . vectorV = 0.

It means the work done by magnetic field on the charged particle moving in it zero.Since increase in kinetic energy of the charged particle moving in a magnetic field is constant.

**7. State Biot and Savart law (Laplace’s law).**

Ans. The mathematical form of Biot and Savart law is given by:

Where, B is the flux density at any point due to the flow of current I through a conductor. This law gives the magnitude of magnetic field due to current flowing through a conductor.

**8. Can you prove Biot and Savart law directly ?**

Ans. No, Biot and Savart law gives the magnetic field at a point due to a very small current element. It is not possible to set up experimental arrangement for this small current element.

**9. State Ampere’s law (or Ampere’s circuital low).**

Ans. The Ampere’s law states that the line integral of magnetic flux density vector B for a closed curve is equal to the uo times the net current bounded by the area of closed surface (in SI system and in air medium ).

**10. Define an Ampere in terms of the force between current carrying conductors.**

Ans. An Ampere current in terms of the force between current carrying conductors can be defined as the current flowing in each of two infinity long parallel wires of negligibly small area of cross section separated by a distance of one meter in vacuum, produces of a force between the wires of 2 x 10^{-7} Newton per meter.

**11. What do mean by Hall Effect ?**

Ans. When a magnetic field is applied to a current carrying conductor, a voltage is developed across the specimen in the direction perpendicular to both current and the magnetic field. This effect is called Hall Effect. The Hall Effect or Hall coefficient RH is defined as the ratio of the transverse magnetic field.

`i.e. R`

_{H} =E_{H}/J_{X} BZ

Also, this coefficient can be used to find the nature and number of charge carriers per unit volume by relation, R_{H} = – 1/ne ,where e is the charge of an electron.