# PHE-07 Solved Assignment For IGNOU B.Sc 2018 Session

Course Name: ELECTRIC AND MAGNETIC PHENOMENA
Course Code: PHE-07
Assignment Code: PHE-07/TMA/2018
Medium: English
Format: PDF
Valid Till: 31st December 2018

Questions Solved In This Solution Guide:

1. a) Calculate the ratio of the electrostatic force and the gravitational force exerted by two protons on each other.
b) The line density of charge in a wire is 0.2 Cm’l. If this wire is symmetrically enclosed, along its length, by a cylindrical surface of radius 1 m and length 1.5 m, calculate i) the electric ﬂux through the cylindrical surface, and ii) the electric field at the curved surface of the cylinder.
2. a) Show that, if there are any unbalanced, static charges on an isolated conductor, they must reside on its surface.
b) The magnitude of work done in taking a unit positive charge in electric field E from point A to point B is given by:
W=-∫BA E.dr
Show that the value of the line integral of the electric field (right hand side of the above equation) does not depend on the path taken to move the unit positive charge from point A to B.
3. a) A positive 20μC charge is placed at the centre of a circle of radius 20 cm. If we move a positive 20μC charge once along the circumference of the circle, will any work be done in the process? Justify your answer.
b) Two concentric thin metallic spheres having radii 30 cm and 20 cm carry 10 μC and 5 μC, charges respectively. Calculate the electric potential at a distance of 25 cm from the centre of the spheres.
4. a) Explain the terms polarisation surface charges and molecular polarisability for a dielectric material.
b) State and explain the boundary condition for the displacement vector D at the boundary separating two dielectric media.
c) If the space between the plates of a capacitor is filled will a dielectric material, does it affect the value of capacitance? Justify your answer.
5. a) The radius of the wire in a coaxial cable is 0.65 mm and the inner radius of the coaxial conducting cylinder is 1.45 mm. Assuming that there is vacuum between the wire and the cylinder, calculate the capacitance of a 1.5 m length of the cable.
b) A parallel plate capacitor is made of two rectangular aluminium sheets of lateral dimensions 500 mm and 20 mm. The space between the plates is filled with polystyrene and the separation between plates is 0.15 mm. Calculate the capacitance of the capacitor and the value of the maximum potential difference that can be applied across the capacitor without dielectric breakdown. Take the dielectric constant of polystyrene as 2.6 and the value of the maximum electric field that polystyrene can withstand as 25×106 Vm^-1.
6. a) What is the difference between ohmic conductor and non—ohmic conductor? Give one example each and draw their IV characteristic curves.
b) The mass density of copper is 8.95><103 kg m_3. If one charge carrier is contributed by each copper atom, calculate the number density of charge carriers in copper. If 5A current is ﬂowing in a copper wire of cross—sectional area 4><10_6 m2, calculate the drift velocity of electrons.
c) An electron and a proton, moving with equal velocity, enter a region of uniform, perpendicular magnetic field. Calculate the ratio of the radii of their circular paths in the field.
7. a) Two long, straight, parallel wires A and B, separated by a distance of 30 cm, carry currents IA 2 10A and IB = 30A. The currents in both the wires ﬂow in the same direction. Calculate the net magnetic field at the midpoint of the line joining the two wires and perpendicular to them.
b) A long, straight wire of radius 5.0 mm carries a current of 20A. i) Calculate the magnetic field at the surface of the wire, and ii) calculate the perpendicular distance, from the axis of the wire, at which the magnitude of magnetic field will be half of its value at the wire surface.
c) At the centre of an air core solenoid, the value of the magnetic field B is 0.40 mT. If the current ﬂowing in the solenoid is 0.4 A, calculate the number of turns per cm.
8. a) Using Maxwell’s equations in free space, derive the wave equation for the electric field vector. Obtain the conditions under which the following time varying electric and magnetic fields satisfy Maxwell’s equations in vacuum with no source charges or currents:
E=k̂ E0 sin y—vt
B =Î B0 sin y—vt
where E0 and B0 are constant.
b) A uniform plane wave of 100 kHz travelling in free space strikes a large block of a material having 8 = 480, u = 9110 and 6: 0 normal to the surface. If the incident magnetic field vector is given by IE 210—6 cos (mt — By) 2 tesla write the complete expressions for the incident, reﬂected, and transmitted field vectors.

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