Updated 1-14-11

Things to Remember
OEES 135, OEET 210

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 DC Ohm's Law and Power Formulas

I = current, V = voltage, R = resistance

I = V/R
V = IR
R = V/I
Note: Some people use E for voltage, instead of V. In fact the author of our book does so. Most people, however, use V for voltage. I recommend that you do so also. Things will be a lot less confusing for you as you use other books and work with other people.
P = IV
P = V2/R
P = I2 R

AC Ohm's Law and Power Formulas

I = V/R
I = V/XC
I = V/XL
I = V/Z

P = IV * p.f.

AC

Variable Names and Units
  • C = capacitance (Farads, F)
  • f = frequency (Hertz, Hz)
  • L = inductance (Henrys, H)
  • P = power (Watts, W). 
  • p.f. = power factor (dimensionless)
    • For pure resistances, the power factor is 1. For pure reactance (inductors or capacitors), the power factor is 0. When a circuit has both resistance and reactance, the power factor is between 0 and 1.
  • T = period (seconds, s)
  • Vp = peak voltage
  • Vp-p = peak-to-peak voltage
  • Vrms = root mean square voltage
  • X = reactance (Ohms). The term reactance is used whenever only pure inductance or pure capacitance is present, but no resistance.
  • XL = inductive reactance (Ohms). 
  • XC = capacitive reactance (Ohms)
  • Z = impedance (Ohms). The term impedance is use whenever both resistance and reactance is involved.
  • θ = angle between current and voltage (pronounced "theta" ; degrees)
Formulas
  • Vp = Vrms * 1.41
    • 1.41 is the square root of 2
  • For a symmetrical waveform (such as a sine wave centered around 0 volts), Vp-p = 2Vp
  • XL = 2πfL
  • XC = 1/2πfC
  • f = 1/T
  • P = IV * p.f.
  • p.f. = cos θ
  • Ohms Law: I = V/R, I = V/X, I = V/Z
  • High-frequency current has difficulty passing through an inductor.
  • Low-frequency current has difficulty passing through a capacitor.
  • A capacitor and an inductor can be used to create a resonant circuit . A resonant circuit can be used to either pass or block currents of a certain frequency, depending upon whether the two components are connected in series or in parallel.
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  • ELI the ICE man.
    • E means voltage (electromotive force).
    • L = inductance
    • I = current
    • C = capacitance
  • The voltage across an inductor leads the current through the inductor by 90 degrees.
  • The current through a capacitor leads the voltage across the capacitor by 90 degrees.
  • No power is used in an ideal inductor or an ideal capacitor.
  • The true power dissipated in a circuit containing resistors, inductors, and capacitors is equal to the power dissipated in the resistors.
  • True power is given by the formula P = IV * p.f.
  • Apparent power is given by the formula P = IV.
  • Thus, true power is apparent power times the power factor


Metric Prefixes

Name Abbreviation Meaning Number Power of 12
Pico p Trillionth 0.000,000,000,001 10-12
Nano n Billionth .000,000,001 10-9
Micro μ Millionth .000,001 10-6
Milli m Thousandth .001 10-3
Kilo k Thousand 1000 103
Mega M Million 1,000,000 106
Giga G Billion 1,000,000,000 109
Tera T Trillion 1,000,000,000,000 1012
  Math, Part 2
Pythagorean Theorem: c2 = a2 + b2 ,
where c = hypotenuse, and a and b are the other sides of a right triangle.		 The square root of x2 is x
sin θ = opposite/hypotenuse
cos θ = adjacent/hypotenuse
 tan θ = opposite/adjacent
 sin-1 (sin θ ) = θ
cos-1 (cos θ ) = θ
tan-1 (tan θ ) = θ
sin-1 (opposite/hypotenuse) =  θ
 cos-1 (adjacent/hypotenuse) = θ
tan-1 (opposite/adjacent) = θ
xa xb = xa+b
xa/xb = xa-b

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