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Electric Current, Electric Power
 
Electricity and Electric Charge

● Formulas and Calculations ●
 
The relationship between electrical parameters 
Electric voltage V, amperage I, resistivity R, impedance Z, wattage or power P
 

The nominal impedance Z = 4, 8, and 16 ohms (loudspeakers) is often assumed as resistance R. Ohm's law equation (formula): V = I × R and the power law equation (formula): P = I × V. P = power, I or J = Latin: influare, international ampere, or intensity and R = resistance. V = voltage, electric potential difference Δ V or E = electro motive force (EMF = voltage).

Enter any two of the following values and click the calculation button. The missing values will be calculated. Enter only two values.

Voltage or volts E or V =  volts V Amperage or current I = amperes, amps A   Resistivity or resistance R =  ohms Ω Wattage or power P = watts W   For R take impedance Z

Basic Principles of Electric Circuits Physics Formulary and Electricity Equations    Formula wheel ▼  Important formulas Electrical engineering laws   Electronic engineering laws Electrical Formulas Pie Chart

 
The Formula Wheel of Electrical Engineering  

V comes from "voltage" and E from "electromotive force". E means also energy, so V is chosen. Energy = voltage × charge. E = V × Q. Some like better to stick to E instead to V, so do it.

 

Voltage V = I × R = P / I = √(P × R) in volts V          Current I = V / R = P / V = √(P / R) in amperes A Resistance R = V / I = V2 / P = P / I2 in ohms Ω      Power P = V × I = R × I2 = V2 / R in watts W

 
See also: The Formula Wheel of Acoustics (Audio)

The Big Power Formulas         Electrical and mechanical power calculation

Formula 1 − Electrical power equation: Power P = I × V = R × I2 = V2 ⁄ R  where power P is in watts, voltage V is in volts and current I is in amperes (DC).  If there is AC, look also at the power factor PF = cos φ and φ = power factor angle  (phase angle) between voltage and amperage.  Formula 2 − Mechanical power equation: Power P = E ⁄ t = W ⁄ t  where power P is in watts, Energy E is in joules, and time t is in seconds. 1 W = 1 J/s.  Power = force multiplied by displacement divided by time P = F × s / t or:  Power = force multiplied by speed (velocity) P = F × v.  Electric Energy is E = P × t − measured in watt-hours, or also in kWh. 1J = 1N×m = 1W×s    Undistorted powerful sound is not found in these formulas. Please, mind your ears!  The eardrum and microphone diaphragms are really only moved by the waves of the  sound pressure. That does not do either the intensity, nor the power or the energy.  If you are in the audio recording business, it is wise not to care much about the energy,  power and intensity as the cause, care more about the effect of sound pressure p  and sound pressure level on the ears and the microphones and the corresponding  audio voltage V ~ p; see: Sound pressure and Sound power − Effect and Cause  Very loud sounding speakers will have a lot of power, but better look closer at the very  important efficiency of loudspeakers. This includes the typical question:  How many decibels (dB) are actually twice or three times as loud?  There is really no RMS power. The words "RMS power" are not correct. There is a calculation  of power which is the multiplication of a RMS voltage and a RMS current.  Watts RMS is meaningless. In fact, we use that term as an extreme shorthand for power in  watts calculated from measuring the RMS voltage. Please, read here:  Why there is no such thing as 'RMS watts' or 'watts RMS' and never has been.  "RMS" power is a rather silly term which has gathered currency among audio people.  Power is the amount of energy that is converted to a unit of time. Expect to pay more when  demanding higher power.

Power is like all energy sizes primarily a calculated value.

The word "power amplifier" is a misnomer. Power is not really something that can be "amplified". Voltage and current can be amplified. The term "power amplifier" although technically incorrect has become understood to mean an amplifier that is intended to drive a load such as a loudspeaker. We call the product of current and voltage gain just "power amplification". The total energy within a closed system due to the conservation of energy is neither increased nor decreased.

Courtesy of Brian Willis and Causeway Art Studio Courtesy of Brian Willis and Causeway Art Studio

Tip: The electric power triangle (power formula)

The magic triangle can be used to calculate all formulas of the "electric power law". You hide with
a finger the value to be calculated. The other two values show then how to do the calculation.

Please enter two values, the third value will be calculated.

Electric power P:  watts Voltage V:  volts Amperage I:  amps

Calculations: Ohm's law - Ohm's magic triangle
Measurement of input impedance and output impedance

ALTERNATING  CURRENT (AC) ~

V_l = line voltage (volts), V_p = phase voltage (volts), I_l = line current (amps), I_p = phase current (amps)
Z = impedance (ohms), P = power (watts), φ = power factor angle, VAR = volt-amperes (reactive)

Current (single phase): I = P / Vp×cos φ	Current (3 phases): I = P / √3 Vl×cos φ or I = P / 3 Vp×cos φ
Power (single phase): P = Vp×Ip×cos φ	Power (3 phases): P = √3 Vl×Il×cos φ or P = √3 Vp×Ip×cos φ

Power factor PF = cos φ = R/(R2 + X2)^1/2, φ = power factor angle. For the purely resistive circuit, PF = 1 (perfect).
The apparent power S is calculated according to Pythagoras, the active power P and reactive power Q. S = √(P² + Q²)  
The true power factor and not the conventional 50/60 Hz displacement power factor

Electrical Measurement Definitions
QUANTITY	NAME	DEFINITION
frequency f	hertz (Hz)	1/s
force F	newton (N)	kg·m/s²
pressure p	pascal (Pa) = N/m²	kg/m·s²
energy E	work joule (J) = N·m	kg·m²/s²
power P	watt (W)= J/s	kg·m²/s³
electric charge Q	coulomb(C) = A·s
voltage V	volt (V)= W/A	kg·m²/A.s³
capacitance C	farad (F)= C/V = A·s/V = s/Ω	A²·s4/kg·m²
inductance L	henry (H)= Wb/A = V·s/A	kg·m²/A²·s²
resistance R	ohm (Ω) = V/A	kg·m²A²·s³
conductance G	siemens (S)= A/V	A²·s³/kg·m²
magnetic flux Φ	weber (Wb) = V·s	kg·m²/A·s²
flux density F	tesla (T) = Wb/m² = V·s/m²	kg/A·s²

The flow of electric charge Q is referred to as an electrical current I. The amount of charge per unit time is the change in electrical current. A current flows at a constant value I. during the time t, it transports the charge Q = I × t. For a temporally constant power, the relationship between the charge and current: I = Q / t or Q = I × t. Through this relationship, the basic units of amps and second the Coulomb in International System of Units is set. The Coulomb unit can be represented as 1 C = 1 A × s.

In acoustics we have an "Acoustic equivalent for ohm's law"

Relationships of acoustic sizes associated with plane progressive sound waves

Conversions of many units, like power and energy

prefixes | length | area | volume | weight | pressure | temperature | time | energy | power | density | velocity | acceleration | force

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