Q22 of 26 Page 1

(i) Mention two properties of soft iron due to which it is preferred for making an electromagnet.
(ii) State Gausss law in magnetism. How is it different from Gausss law in electrostatics and why?

OR


Derive an expression for the axial magnetic field of a finite solenoid of length 2l and radius r carrying current I. Under what condition does the field become equivalent to that produced by a bar magnet?


To make an electromagnet, the choice of the core material is very important as it enhances the magnetic capabilities. Thus, soft iron is used as:

(i) It has high permeability. This means that it provides considerable magnetization for small magnetizing fields.


(ii) It has low retentivity. This property allows the electromagnet to lose the magnetism immediately as the magnetizing field is removed.


Gauss Law in magnetism states that the net magnetic flux through any closed surface is zero. This is because the magnetic field lines are continuous loops, hence for each magnetic field line going in we have one that’s coming out giving the net flux zero.


Its stated as,


Gauss law in electrostatics states that the net electric flux through a closed surface is proportional to the charge enclosed by the close surface. Hence, it’s zero only if the charge enclosed is zero. It’s given by,


.


In magnetism, monopoles do not exist and only dipoles exist. Hence the total flux is always zero. Whereas, charges can exists separately, thus giving a definite flux.


OR


Let there be a solenoid of radius an and of length 2l. Let the number of turns be n and the current through the solenoid be I.


Here, OP=r.


Let there be a small element dx at a distance x from O.


The number of turns in the element =



Then the magnitude of magnetic field at P is



If and



If M is the magnetic moment of the solenoid,



This will give the magnetic field as



Thus, the magnetic field due to a solenoid resembles that of a bar magnet.


More from this chapter

All 26 →
20

Mention any three applications of the internet. Explain one of these in detail.

 21

Two harmonic waves of monochromatic light

are superimposed on each other. Show that maximum intensity in interference pattern is four times the intensity due to each slit. Hence write the conditions for constructive and destructive interference in terms of the phase angle φ.

 23

During a thunderstorm the live wire of the transmission line fell down on the ground. A group of boys passing through noticed it and some of them wanted to place the wire by the side. As they were approaching the wire and trying to lift it, Hari noticed it and immediately pushed them away to prevent them from touching the wire. Two of them got hurt in the process. Hari took them to a doctor to get medical aid.

Based on the above paragraph, answer the following:


(a)Write two values which Hari displayed during the incident.
(b) Why is it that a bird can sit over a suspended live wire without any harm whereas touching it on the ground can give a fatal shock?
(c) The electric power from a power plant is set up to a very high voltage before transmitting it to distant consumers. Write the reason for it.


 24

(i) Find the magnitude and direction of current in 1 V resistor in the given circuit.


(ii)Two student’s X and Y perform an experiment on potentiometer separately using the circuit diagram shown below.



Keeping other things unchanged (a) X increases the value of resistance R, (b) Y decrease the value of resistance S in the set up. How will these changes affect the position of null point in each case and why?


OR



(a) Use Kirchhoff’s rules, calculate the current in the arm AC of the given circuit.
(b) On what principle does the meter bridge work? Why are the metal strips used in the bridge?