# PHYS 212 Study Guide - Midterm Guide: Virtual Image, Refraction, Plane Wave

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Published on 19 Apr 2015

Department

Physics

Course

PHYS 212

Professor

PHYS 212- Spring 2014

Exam #3- Review Sheet

Questions & Corrections: Hassan Shapourian (hassan88@illinois.edu)

General tip

Be careful about units. Often prefixes are used as there are small or large quantities. Always convert

prefixes back to the base unit before using them mathematically. Sometimes you can find the correct

choice by checking the signs/dimensions/limiting behavior.

LC Circuits

Oscillator circuit with energy

. Think about as “position” and as “velocity” then this expression

looks like energy of a Harmonic oscillator with mass and spring constant . Hence, the oscillation

frequency is

.

If we add a resistor to the circuit, if there is no external EMF, it will consume energy and after a long time both

capacitor and inductor will be discharged.

AC Circuits and Phasors

An RLC circuit is driven by a sinusoidal voltage source . We are interested in steady state

behavior at which the current through circuit is . Note that what matters here is the phase

difference between and ; i.e. you can put the relative phase in current: and so

.

The voltage-current relations are given by

Reactances and (in units of ohm) play the role of effective resistance very similar to the expression for

resistors . Use the mnemonic “ELI-the-ICEman” for remembering the phase rules: “EMF leads I across L,

and I leads EMF across C”.

Phasors

Each oscillating quantity is represented by a vector the projection of which on vertical

axis gives . The length of a phasor is the peak value and the angle with positive horizontal axis is the initial

phase . The phasor rotates counterclockwise with time around the origin. When quantity A has a larger angle

(phase) than quantity B, we say A leads B by .

E leads I E lags I E and I are in-phase (resonance case)