Physics 24: General Physics Winter 2022
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Physics 24: General Physics
Winter 2022
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1. (AC Currents) Given a switching circuit consisting of a direct-current-voltage source U0, an ohmic resistance R and a capacity C as shown in this figure:
(A) At the time t = t0 a direct voltage U0 is switched on (i.e. the switch is in position (1)).
Calculate the voltages UC across the capacitor and UR across the ohmic resistance as well as the current I as a function of the time t.
(B) At the time t = t1 the voltage at the capacitor is UC(t1 ) = U0, the voltage source is
disconnected, and the loop short-circuited (i.e. the switch is then in position (2)). Calculate again UC (t), UR(t), I(t).
2. (Maxwell equations and waves) Consider a linear, homogeneous, uncharged insulator. (A) Formulate the Maxwell equations for the electromagnetic fields and . (B) Show that fulfills the homogeneous wave equation.
(C) The electric field strength is given as a plane wave
(r-, t) = (eˆx − 2eˆy)ei(.r-一ωt)
(1)
where = keˆz . The symbol eˆz denotes the unit vector in z direction. Similar for the other two directions. Calculate the magnetic induction (r-, t), and determine its polarization.
(D) The magnetic induction is given as a plane wave of the type
(r-, t) = B0 cos(kz − ωt)eˆx + B0 sin(kz − ωt)eˆy
Calculate the electric field strength (r-, t) and determine its polarization.
(2)
3. (Maxwell equations in matter) A plane wave of frequency ω is incident on the plane interface between two media of different index of refraction n and n\ respectively. The figure below shows a visualization.
The plane wave has the same phase at the points A and B . Derive only by the requirement that the phase is also the same at the points A\ and B\ the law of refraction:
sin ϑ n\
=
sin ϑ\ n
(3)
4. (Mutual Induction) A rectangular conductor loop of length a and width b carries a current I2 . It is located within the magnetic field of a thin wire which carries a current I1 . The figure below shows a. visualization:
(A) Calculate the coefficient of the mutual inductance L12 .
(B) Which force is exerted by the current I1 on the conductor loop?
(C) The current I1 in the wire is switched on at the time t = 0 according to
I1(t) = I0(1 − e一αt) (4)
Calculate the voltage induced in the conductor loop. The self-inductance of the circuit is assumed to be negligible.
2022-12-07