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Challenges and Evaluation in Physics . N.K. Sehgal November 2013. Physics.
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Challenges and Evaluation in Physics N.K. Sehgal November 2013
Physics The subject of Physics is a beautiful amalgamation of philosophical thinking, reality and practical utility. It aims to understand and explain a vast variety of natural phenomenon and observations, in the physical world, through a relatively small number of principals, rules and laws. Nature and physical universe do not appear to yield their secrets readily. However, the human mind has always struggled and tried to gain an understanding, and perhaps even a mastery, over nature. The curiosity to understand nature and natural phenomenon, and to seek explanations, of the vast variety of everyday observations and happenings, has been the guiding and motivating force behind all sciences in general and physics in particular.
Physics In broad terms, one may say that the role of physics is to provide a logically consistent picture of universe that is in agreement with experience, i.e., a picture that, in a way,’ saves the phenomenon’. We may consider Physics as the branch of science concerned with the discovery, and logical characterization, of universal laws which govern matter, energy, space and time.
Physics The teaching of physics is both a challenge and an opportunity. The challenge lies in dispelling the common notion, and fear, that ‘Physics is perhaps the most difficult of all sciences’. The opportunity lies in making the students realize that the basic principles of physics have helped mankind to develop ,and use, the vast variety of devices - heat engines, electricity generators, wireless communication, lasers, television, air conditioners, computers, robots, ultrasound, MIR and PET imaging, mobile phones nuclear reactors and artificial satellites, to name but a few - that have changed the culture and functioning of the human society as a whole.
Physics Physics is thus very much a part of the world around us and is relevant and important for all of us. It has perhaps been rightly said, “A basic, and almost fundamental requirement, for an informed and enlightened citizenship of the 21st century, is to have a basic understanding of the principles by which the physical world operates.” As teachers, and more importantly, as students of physics, we all have to constantly put in our intellect, ingenuity, dedication, hard work and perseverance, together to have more and more of citizens meeting the above basic requirement. We now have a vast, complex and exciting picture of the universe----from its atomic to extragalactic range. However the words of Sir Issac Newton, put forward almost three hundred years ago, are very much relevant even now and perhaps forever.
Physics Newton is reported to have said “ I do not know what I may appear to the world but, to myself, I seem to have been like a little boy ,playing on the sea shore and diverting myself, now and then, in finding a smoother pebble, or a prettier than ordinary shell, while the great ocean of truth lay all undiscovered before me.” The challenge of finding ‘smoother pebbles’, ‘prettier shells’ and ‘exploring undiscovered oceans of truth’ is still there for all of us. And we need to continue to encourage the ”STUDENT” in all of us to complete as much as possible of this ‘intellectually satisfying’ and ‘practically useful’ never ending task.
Conceptual Clarity – ‘Points’ To Be Highlighted • Coulomb’s law – The direction of the force on charge 2 due to charge 1 is along the position vector ( ) of charge 2 w.r.t. charge 1. • Calculation of the relevant unit vector; permittivity . • Quantized nature of charge significant only at the microscopic level; ‘Quark’ dilemma? • Superposition Principle – significant principle : mutual inter action between two charges is unaffected by the presence or otherwise of other charges.
Electric Field – defined mathematically, can be associated with a physical meaning and provides an elegant way of understanding the ‘electrical characteristics’ of a system of charges. • Accelerated charges produce e.m. waves/electro magnetic fields which are regarded as ‘physical quantities’ : speed of propagation of e.m. waves = c • Electrical field line – an institutive non mathematical way of visualizing electric field; ‘properties’ are associated with them. • Electrical Flux - concept ‘carried over’ from hydrodynamics; associated with ‘rate of flow’. Flux is ‘non-zero’ through a closed surface only if it contains a ‘source’ or a ‘sink’.
Electric Dipole – A very useful ‘entity’ for understanding behavior of matter at molecular/atomic level. Field varies as ; faster than that of a point charge. • A dipole experiences only a torque in a uniform external electrical field ; however, there is both a torque and a force in a (highly) non-uniform field. • Gauss’s law : An alternative way of stating Coulomb’s law. A consequence of the ‘inverse square nature’ of Coulomb’s law; we can have a Gauss’s law for other inverse square fields’ also. • We use Gauss’s law to calculate electric field, form electrical flux. Hence its application gets limited to special, symmetric charge distributions.
Electrical Potential : A useful scalar quantity for describing / mapping an electric field. • Not defined uniquely; choice of ‘zero’ is arbitrary. Can be viewed as a measure of the P. E. per unit charge. • Relation between potential and electrical field need to be viewed carefully. Component of electrical field, in any direction, equals the negative of the rate of change of potential in that direction. • Comparison of the variation of potential (as ) with that of the electric field (as ). • P.E. of a system of charges equal the work done in assembling the configuration and would depend upon whether, or not, an external electrical field is also present.
Question No.2 : A parallel plate capacitor has the space between its plates filled with a dielectric. Whose dielectric constant varies with distance (from the negative plate) as per the relation? K(r)= Ko+ α r Show that the capacitance, C, of the capacitor, is given by
Question No.4: • The x- component of the electric field , of a charge distribution varies with distance, along the x-axis in the manner shown. • a) What is the potential differences between the points x = 2m and x = 10m? • (b) Find the change of potential energy of a charge of 10C as it is moved from x = 4m to x = 8m.
Conceptual Clarity – ‘Points’ To Be Highlighted • Definition of Electric current • Concepts of ‘drift velocity’ and ‘random thermal velocity’. • Mobility equals drift velocity / charge. • The establishment of current in a conductor is caused by ‘local electron drifts’ resulting form an almost instantaneous establishment of ‘electric field’ in the conductor. • Current density vector and its significance • Understanding Ohm’s law – concept of relaxation time.
Understanding the difference in the resistivity change behavior of conductors and semi conductors. • Electrical Energy & Power; how does transmission at high voltages help in reducing transmission power losses. • Special properties of resistor combinations in series and in parallel. • Cells; their combinations in series & parallel; internal resistance ; difference between terminal p.d and emf. • For the parallel combination : • Physical significance of Kirchoff’s rules; their use in practical situations and adoption of an appropriate ‘sign convention’.
Wheat stone Bridge; meter bridge; relevant problems. • For a ‘balanced Wheat stone bridge’ there is no current in the detector arm. • Principle of a Potentiometer; two simple practical applications. • Two causes for ‘one sided deflection in a potentiometer and understanding the practical difference in the two cases. • Using potentiometer (along with Ohm’s law) to measure ‘currents’ and resistances. • Increasing the sensitivity of a potentiometer.
Conceptual Clarity – ‘Points’ To Be Highlighted • Basic source of magnetic field – moving charges or currents. • Charges – static or moving – always produce an Electric field. However, it is only moving charges that, in addition, also produce a magnetic field. • Charges – static or moving – always experience a force in an external electric field. However, it is only moving charges, that, experience a magnetic force, i.e. a force due to a magnetic field. • Magnetic force in given by the expression Its direction can be worked out by using the standard coordinate axis system and the well known relations for the cross products amongst the unit vectors
We can use the above force expression to define (i) the direction of the magnetic field at a point and (ii) the unit of the magnetic field. • Magnetic force on a current carrying conductor. . • General helical nature of the motion of a charged particle in a magnetic field; radius of helix = ; • pitch of helix . • Time period / frequency are both independent of the velocity / energy as well as the radius. • Concept of cyclotron frequency ; principle of a cyclotron. • Understanding how an electron field can be used to cancel the force on a moving charge due to a magnetic field and how this idea can be used to design a ‘velocity selector’.
Ampere’s Circuital law; realizing it as an alternative form ‘of the Biot-Sarvart law. Its ‘similarity’ with Gauss’s law in Electrostatics. • Understanding the reasons for the limited possible use of Ampere’s circuital law. Understanding the significance of the term “Enclosed current’. • Solenoid and Toroid ; Using Ampere’s law for both. • Uniform nature of the ‘near-axis’, ‘near centre’ field of a long thin solenoid. • Understanding the similarity in the expressions for the magnetic fields of a long solenoid and a toroid.
Combining the results for the (i) magnetic field due to a long straight current carrying wire and (ii) force experienced by a current carrying wire in a magnetic field. • Force between two long parallel current carrying wires. Understanding clearly why ‘like currents attract ‘ and ‘unlike currents repel’. Arriving at the definition of ‘ampere’. • Torque on a current loop in a magnetic field; it use in the design of a moving coil galvanometer. • Basic design and construction of the moving coil galvanometer ; significance of having a ‘radial field’ in it. • Ammeter and Voltmeter; Understanding why the resistance of an ideal ammeter should be zero while that of an ideal voltmeter should be infinite.
Errors introduced by finite values of ammeter / voltmeter resistances. • How to ‘convert’ a given moving coil galvanometer into an ammeter or voltmeter of desired range? • Equivalence of a ‘current loop’ to a ‘magnetic dipole’ – using the analogy with an electric dipole’s electric filed. • Associating an equivalent ‘orbital’ magnetic moment with an electron orbiting around the nucleus. • Understanding the difference between the current sensitivity and voltage sensitivity of a moving coil galvanometer.
Writing expressions for the axial and equatorial fields of a magnetic dipole – from the corresponding expression for the electric field of an electric dipole. at a point in the plane of the loop, at a distance x from the centre(x >> R, the radius of loop) equal nearly . • Any planar current loop, of area A, is equivalent to a magnetic dipole of magnetic moment . Looking at the possible applications of this result.
Magnetism • Magnetism was initially studied as a subject in its own right. It was only in the early part of the 19th century that Oersted, Ampere, Biot and Savart correlated it with moving charges or currents. • The ‘bar magnet’, is in many ways, considered similar to an electric dipole. • Results for a bar magnet are not ‘derived’; they are simply ‘arrived at’ by comparison and co-relation with an electric dipole. • Magnetic field lines help us to have a visual and intitutive ‘picture’ of a given magnetic field. • The ‘closed curve’ nature of magnetic field lines is a significant difference from the ‘start from positive charge and end at a negative charge; nature of electric filed lines.
