Resources - Electricity Formulas

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Common Electronic And Electricity Formulas

Below is a reference chart giving all the equations deriving from Ohm's Law. The parameters E, I, R and P are shown in the central area, each occupying one of the four quadrants of the pie. To solve for a given parameter, find that parameter at the center of the chart and choose the equation in its quadrant that defines the quantity in terms that you have measured or know.


I = Electrical current in amperes
E = Electrical potential in volts
R = Resistance in ohms (Also sometimes "Z" or Impedance in ohms)
P = Power in watts (Also sometimes "W")

NOTE: Equations referring to power and impedance are describing a Power Factor (PF), rather than pure DC power. This quantity accounts for the reactance of the load and the AC signal.

Ohms Law (DC Current):

Current in amps = Voltage in volts / Resistance in ohms = Power in watts / Voltage in volts

Current in amps = (Power in watts / Resistance in ohms)

Voltage in volts = Current in amps × Resistance in ohms

Voltage in volts = Power in watts / Current in amps

Voltage in volts = (Power in watts × Resistance in ohms) Power in watts = (Current in amps)2 × Resistance in ohms

Power in watts = Voltage in volts × Current in amps

Power in watts = (Voltage in volts)2 / Resistance in Ohms

Resistance in ohms = Voltage in volts / Current in amps

Resistance in ohms = Power in watts / (Current in amps)2

Ohms Law (AC Current):

In the following AC Ohms Law formulas, q is the phase angle in degrees by which current lags voltage (in an inductive circuit) or by which current leads voltage (in a capacitive circuit). In a resonant circuit (such as normal household 120VAC) the phase angle is 0º and Impedance = Resistance.

Current in amps = Voltage in volts / impedance in ohms

Current in amps = (Power in Watts / Impedance in ohms × cosq)

Current in amps = Power in Watts / (Voltage in volts × cos q)

Voltage in volts = Current in amps × Impedance in ohms

Voltage in volts = Power in Watts / (current in amps × cos q)

Voltage in volts = ([Power in watts × Impedance in ohms] / cos q)

Impedance in ohms = Voltage in volts / Current in amps

Impedance in ohms = Power in watts / (Current amps2 × cos q)

Impedance in ohms =(Voltage in volts2 × cos q) / Power in watts

Power in watts = Current in amps2 × Impedance in ohms × cos q

Power in watts = Current in amps × Voltage in volts × cos q

Power in watts = ([Voltage in volts]2 × cos q) / Impedance in ohms

ELECTRONIC CIRCUIT EQUATIONS:

Resonant frequency in hertz (where XL= XC) = 1 / (2p ×[Inductance in henrys × Capacitance in farads])

Reactance in ohms of an inductance is XL

XL = 2p(frequency in hertz × Inductance in henrys)

Reactance in ohms of a capacitance is XC
XC = 1 / (2p[frequency in hertz × Capacitance in farads] )

Impedance in ohms (series) = (Resistance in ohms2 + (XL-XC)2)

Impedance in ohms (parallel) = (Resistance in ohms × Reactance) / (Resistance in ohms2 + Reactance2

Resistors in Series (values in Ohms):

Total Resistance = Resistance1 + Resistance2 + ... Resistancen

Two Resistors in Parallel (values in Ohms):

Total Resistance = Resistance1 × Resistance2 / Resistance1 + Resistance2

Multiple Resistors in Parallel (values in Ohms):

Total Resistance = 1 / (1 / Resistance1] + 1 / Resistance2 + ... 1 / Resistancen])

Capacitors in Parallel (values in microfarads):

Total Capacitance in Parallel (values in any farad) = Capacitance1 + Capacitance2 + .... Capacitancen

Capacitors in Series (values in microfarads):

Total Capacitance in Series (values in any farad) = Capacitance1 × Capacitance2 / Capacitance1 + Capacitance2

Multiple Capacitors in Series (values in farads) = 1 / ([1 / Capacitance1] + [1 / Capacitance2] + ...... [1 / Capacitancen])

LCR Series Time Circuits:

Time in seconds = Inductance in henrys / Resistance in ohms

Time in seconds = Capacitance in farads × Resistance in ohms

SINE WAVE VOLTAGE AND CURRENT:

Effective (RMS) value = 0.707 × Peak value

Effective (RMS) value = 1.11 × Average value

Average value = 0.637 × Peak value

Average value = 0.9 × Effective (RMS) value

Peak Value = 1.414 × Effective (RMS) value

Peak Value = 1.57 × Average value

DECIBELS:

db = 10 Log10 (Power in Watts #1 / Power in Watts #2)

db = 10 Log10 (Power Ratio)

db = 20 Log10 (Volts or Amps #1 / Volts or Amps #2)

db = 20 Log10 (Voltage or Current Ratio)

Power Ratio = 10(db/10)

Voltage or Current Ratio = 10(db/20) If impedances are not equal: db = 20 Log10 [(Volt1 [Z2]) / (Volt2 [Z1])]

Frequency and Wavelength

Frequency in kilohertz = (300,000) /wavelength in meters

Frequency in megahertz = (300) / wavelength in meters

Frequency in megahertz = (964) / wavelength in feet

Wavelength in meters = (300,000) / frequency in kilohertz

Wavelength in meters = (300) / frequency in megahertz

Wavelength in feet = (964) / frequency in megahertz

Wavelength = speed of sound (ft/sec or m/sec) / frequency Speed of sound = 1130 ft/sec

Antenna Length:

Quarter-wave antenna: (ordinary wire, velocity factor = 0.95)
Length in feet = 234 / frequency in megahertz

Half-wave antenna: (ordinary wire, velocity factor = 0.95)
Length in feet = 466 / frequency in megahertz

70 Volt Loudspeaker Matching Transformer Primary Impedance

70 Volt Loudspeaker Matching Transformer Primary Impedance = (Amplifier output volts)2 / Speaker Power

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