2022 POSTECH Phys102 Quiz 4. (December 6)
1. A circuit is composed of two resistors of resistance R1 and R2, an inductor of inductance L, and a
battery of emf  with switch S as shown in the figure. Switch S
is closed at t = 0.
(a) [4 점] Find the potential difference Vac across the resistor
R2 just after the switch is closed.
The switch is left closed a long time and then opened.
(b) [6 점] Find the potential difference Vcd across the inductor
L just after the switch is opened. Which point, c or d, is at a higher potential?
2. Consider an AC circuit shown in the figure below which is driven by a generator supplying a
voltage 𝑉 𝑡
𝑉 sin 𝜔𝑡.
(a) [5 점] Find the amplitude 𝐼
,
and phase 𝜑
of
the current 𝐼 𝑡
𝐼 , sin 𝜔𝑡 𝜑 . Also sketch a
graph of 𝐼 , as a function of 𝜔.
(b) [5 점] Find the amplitude 𝐼
,
and phase 𝜑
of
the current 𝐼 𝑡
𝐼 , sin 𝜔𝑡 𝜑 . Also sketch a
graph of 𝐼 , as a function of 𝜔.
(c) [5 점] What is the (so-called ‘crossover’) angular
frequency ω at which the average powers over one
cycle delivered to the two resistors are equal if 𝑅
𝑅 ?
𝑰𝟐 𝒕
𝑰𝟏 𝒕
𝑽 𝒕
𝑪
𝑹𝟏
𝑳
𝑹𝟐