Punjab University 2011-2nd Sem B.Sc Physics major Electricity and Magnetism - Question Paper

B.Sc. (H.S.) one Physics 2nd Semester exam 2011
PHYS 122H: Electricity & Magnetism

Date: 9-05-11 Time: 03 hrs Max. Marks: 60

Note: Attempt 5 ques. in all by answering the 1st compulsory ques. and 4 ques. out of rest 6 ques.. All ques. carry equal marks.

Q1
a) What is meant by Q value of a circuit? Show that for a series LCR circuit the value of Q is wL/R where w is the angular frequency. (3)
b) How is the electric field of a charge moving with uniform velocity various from that of a stationary charge? discuss using a diagram. (3)
c) Derive the expression for the capacitance of a dielectric filled capacitor. (3)
d) obtain an expression for admittance, impedance and resonant frequency for the series LCR circuit using complex analysis. (3)

Q2
a) A long wire is bent into a hair pin like shape. obtain an exact Expression for the magnetic field at a point P which lies at the centre of the half circle. (6)
b) Taking the case of 2 stationary parallel charged sheets of opposite charges, obtain how the electric field transforms from a stationary frame to an inertial frame which is moving with velocity v with respect to the stationary frame. (6)
Q3
a) What is mutual inductance? State and prove reciprocity theorem. (6)
b) Derive an expression for mutual inductance of a circuit containing 2 coplanar concentric rings, 1 small ring C₂ and a much larger ring C₂ through which a current I₁ is flowing. (6)
Q4
a) How large an inductance in henrys, should be connected in series with a 220 volt, 60 watt light bulb, if it is to operate normally when the combination is connected across a 440 volt, 50 cps line? (6)
b) Derive an expression for the transient voltage and current for a RL circuit. How do you describe the time constant of the circuit? (6)

Q5
a) Show using the transformation formula for Electric and Magnetic Field that E.B and E² -B² are invariant under Lorentz transformations. What is the value of E.B and E² -B² for the field of an electromagnetic wave? discuss. (6)
b) How do various kinds of magnetic materials respond to external magnetic field and modifications in magnetic field? define qualitatively the reasons for their behavior. (6)

Q6
a) State Ampere's circuital legal regulations and derive its differential form. Show how it is replaced to accommodate for non steady situations. (6)
b) Show that the subsequent electromagnetic field satisfies the Maxwell's equation:
E_x = E_z = 0; E_y = Acos(x-ct); B_x = B_y = 0; B_z = Acost(x-ct), where A is a constant. (6)

Q7
a) Find an expression for the electric potential of a change distribution at a distant point in terms of the various moments of the charge distribution. Which moment of the charge distribution of water molecules will determine the electric potential at a great distance from the source? discuss. (6)
b) How vector potential defined? Using the Differential form of Ampere's legal regulations for steady currents, derive an expression for the vector potential. (6)

Earning:   Approval pending.