Magnetic properties of matter


  • All substances possess magnetic properties and most general definition of magnetism defines it as a particular form of interactions originating between moving electrically charged particles
  • Magnetic interaction relates spatially separate material objects and it is transmitted by means of magnetic field about which we have already studied .This magnetic field is important characteristics of EM form of matter
  • We already know that source of magnetic field is a moving electric charge i.e. an electric current. On atomic scale, there are two types of macroscopic current associated with electrons
    a) Orbital current is which electron in an atom moves about the nucleus in closed paths constituting electric currents loops
    b) Spin currents related to the internal degrees of freedom of the motion of electrons and this can only be understood through quantum mechanics

  • Like electrons in an atom, atomic nucleus may also have magnetic properties like magnetic moment but it is fairly smaller then that of electrons
  • Magnetic moment m is nothing but the quantitative measure of the magnetism of a particle
  • For an elementary closed loop with a current I in it, the magnitude |m| of a magnetic moment vector equals the current times the loop area S i.e.
    |m|=IS
    and direction of m can be determined using right hand rule
  • All micro structural elements of matter electrons, protons and neutrons are elementary carriers of magnetic moment and combination of these can be principle sources of magnetism
  • Thus magnetic properties are inherent to all the substances i.e. they are all magnets
  • An external magnetic field has an influence on these atomic orbital and spin currents and two basic effects of an external field are observed
    i) First is diamagnetic effect which is consequences of faraday's law of induction. According to the Lenz law’s, a magnetic field always sets up an induced current with its magnetic field direction opposite to an initial field .Therefore diamagnetic moment created by the external field is always negative related to this field
    ii) Second effect occurs if there is a resultant non zero magnetic moment in the atom i.e. there is a spin magnetic moment and orbital magnetic moment .In this case external field will attempt to orient the intrinsic atomic magnetic moment in its own direction .As a result a positive moment parallel to the field is created and this is called paramagnetic moment
  • Because of the universality of the diamagnetic effect, all substances possess diamagnetism
  • However, diamagnetism is by no means actually observed in all matter. This is because in many instances the diamagnetic effect is masked by the more powerful paramagnetic effect
  • Thus in paramagnetic substances we actually always observe a difference effect produced by the prominent Para magnetism and weaker diamagnetism
  • mechanics test questions

    Question 1
    A ball is thrown vertically upwards with a velocity u from top of a tower. It strikes the ground with a velocity 3u. The time taken by the ball to reach the ground is given by
    (a) u/g
    (b) 2u/g
    (c) 3u/g
    (d) 4u/g

    Question 2
    A body slides down an inclined plane of inclination . The coefficient of friction down the plane varies in the direct proportion to the distance moved down the plane(=kx). The body will move down the plane with
    (a) constant acceleration =gsin
    (b) constant acceleration =(gsin-gcos)
    (c) constant retardation =(gcos-gsin)
    (d) variable acceleration that first decreases from gsin to zero and that becomes negative.

    Question 3
    A escalator is moving downwards with a uniform speed u. A man of mass m is running upwards on it at a uniform speed v. If the height of escalator is h, the work done by man in going up the escalator is
    (a) zero
    (b) mgh
    (c) mghu/(v-u)
    (d) mghv/(v-u)

    Question 4
    A particle moves in a circular orbit with a uniform angular speed. However , the plane of the circular orbit is itself rotating at a constant angular speed. We may then say that
    (a) the angular velocity as well as angular acceleration of particle are both constant.
    (b) neither the angular velocity nor the angular acceleration of the particle are constant.
    (c) the angular velocity of the particle varies but its angular acceleration is constant.
    (d) the angular velocity of the particle remains constant but the angular acceleration varies.
     
    Question 5
    Two objects of mass m and 4m are at rest at infinite separation. They move towards each other under mutual gravitational attraction. Then at separation r, which one of the following is true.
    (a) the total energy of the system is not zero
    (b) the force between them is not zero
    (c) the center of mass of the system is at rest
    (d) all the above are true

    Answer
    1. b
    2. d
    3. d
    4. c
    5. d

    AIEEE test questions


    Question 1

    A body of density enters a tank of water of density ' after falling through height h. The maximum depth to which it sinks in water is









    Question 2.
    A charged particle q is shot towards another charged particle Q which is fixed, with speed v. It approaches Q up to a closest distance r and then returns. If q were given a speed 2v , the distance of approach would be
    (a) r
    (b) 2r
    (c) r/2
    (d) r/4

    Question 3.
    The Young’s modulus of a perfectly rigid body is
    (a)  zero
    (b)  unity
    (c)  infinity
    (d)  may have any finite non zero value

    Question 4.
    When an EM wave enters an ionised layer of earth atmosphere present in ionosphere
    (a)  the electron cloud will not oscillate in the electric field of the wave
    (b) the electron cloud will oscillate in the electric field of wave in  phase of sinusoidal EM wave
    (c) the electron cloud will oscillate in the electric field of wave in the opposite phase of sinusoidal EM wave
    (d) the electron cloud will oscillate in the electric field of wave with a phase retardation of 90 degree for a sinusoidal EM wave

    Question 5.
    Two bodies are charged by rubbing one against the other. During the process one becomes positively charged while other becomes negatively charged. then
    (a) mass of each body remains unchanged
    (b) mass of each body changes marginally
    (c) mass of each body changes slightly and hence the total mass
    (d) mass of each body changes slightly but total mass remains the same

    Answers
    1. (a)
    2. (d)
    3. (c)
    4. (d)



    For solutions visit link

    Elasticity question

    Question:
    A load of 31.4 kg is suspended from a wire of radius 10-3m and density 9103Kg/m3. Calculate the change in the temperature of the wire if 75% of the work done is converted into heat. The Young’s modulus and the heat capacity of the material of the wire are 9.81010Nm-1and 490 J/KgK respectively.
    Solution
    For solution visit this Link

    How to solve problems involving time dependent acceleration

    Here i tried to sum up the steps to solve problems involving time dependent acceleration. We can easily calculate the position of the particle/object
    (1) Consider that acceleration a is some function of time such that
    a=f(t)              
    this equation can also be written in terms of velocity i.e.
    a=dv/dt=f(t)                                        .....1
    (2) Now we integrate equation 1 as a function of time
    dv=f(t)dt
    v=∫dv=∫f(t)dt + C                ......2
    here C is some constant of integration and can be calculated from the initial conditions.
    (3) Now we know that v=dx/dt , using this in the solution of equation 2 we can easily find position of the particle.

    Derivation of Coulumb's Law from Gauss's Law



    • Coulumb's law can be derived from Gauss's law.
    • Consider electric field of a single isolated positive charge of magnitude q as shown below in the figure.



    • Field of a positive charge is in radially outward direction everywhere and magnitude of electric field intensity is same for all points at a distance r from the charge.
    • We can assume Gaussian surface to be a sphere of radius r enclosing the charge q.
    • From Gauss's law

      since E is constant at all points on the surface therefore,

      surface area of the sphere is A=4πr2
      thus,
       
    • Now force acting on point charge q' at distance r from point charge q is

      This is nothing but the mathematical statement of Coulumb's law.
    For full notes visit physicscatalyst.com

    How to choose a physics book for competitive exams

    When you start preparing for competitive exams like PMT, IITJEE, AIEEEi am sure most of you face this question  "From which book should I study Physics?". There are lots of books available in the market in Physics for preparation of exams like  IITJEE/AIEEE/PMT and it could be quite confusing choosing from numerous available options.
    I would advise you all to choose one good book for example NCERT book or concepts of physics by HC Verma for reading text and clearing your concepts in physics. Use only one book for studying a given topic as different books have different approach towards the topic and you could easily get confused. Start a topic or a chapter by reading it first from NCERT book and if you fail to understand then refer another help book. Solve all problems given in NCERT as they cover range of problems from easy to tough. After that try and solve problems  asked in entrance exams from that topic. After this, you will be able to judge for yourself if reading that topic/chapter from some advanced book is necessary or not. If you feel that what you learnt from NCERT is not enough to solve questions in competitive exams then (and only then !) read that topic from some other advanced book.For this you can use "Resnik and Halliday's book" , HC Verms's Concept in physics etc. . Other books can also be used according to your personal preference.
    It's high time you start preparing for your 2012 entrance exams so all the best for your preparations. 

    Physics for ITTJEE/AIEEE : Radioactivity

    Law of radioactive decay: for full notes visit Radioactivity

    Introduction to radioactivity


    • Phenomenon of radioactivity was first discovered by A.H.Bacquerel in 1896 while studying fluorescence and phosphorence of compounds irradiated by visible light
    • these phosphorescent materials glow in dark after being exposed to visible light
    • while conducting experiment on uranium salts, he found that uranium salts has a capability to blacken the photographic plate kept in a dark place wrapped through a paper
    • Subsequent experiments showed that radioactivity is a nuclear phenomenon in which an unstable nucleus under goes a decay process referred as radioactive decay
    • There are three types of radioactivity decays that occur in nature .These are α decay ,β decay and γ decay.
    • We now define radioactive decay as the process by which unstable atomic nucleus looses energy by emitting ionizing particles or radiations ( α,β and γ rays) 
    • Radioactive decay of an atomic nucleus is a spontaneous process and can occur without any interaction of other particles outside the atom
    • This process of radioactive decay is random and we can not predict whether a given radioactive atom will emit radiations at a particular instant of time or not
    • Phenomenon of radioactivity is observed in heavy elements like uranium and unstable isotopes like carbon 14
    For mor information visit physicscatalyst.com

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