Basic Electrical Engineering

Volt - unit of electrical potential or motive force - potential is required to send one ampere of current through one ohm of resistance


Ohm - unit of resistance - one ohm is the resistance offered to the passage of one ampere when impelled by one volt


Ampere - units of current - one ampere is the current which one volt can send through a resistance of one ohm


Watt - unit of electrical energy or power - one watt is the product of one ampere and one volt - one ampere of current flowing under the force of one volt gives one watt of energy


Volt-ampere (VA) - is a measurement of power in a direct current ( DC ) electrical circuit. The VA specification is also used in alternating current ( AC ) circuits, but it is less precise in this application, because it represents apparent power , which often differs from true power .


Kilovolt Ampere - one kilovolt ampere - KVA - is equal to 1000 volt amperes


Power Factor - ratio of watts to volt amperes


Most important Formulas:


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 = P / I2 = V2 / P in ohms Ω    

Power P = V × I = R × I2 = V2 / R in watts W



Electrical Potential - Ohm's Law


Ohm's law can be expressed as:


V = R I         (1a)


V = P / I         (1b)


V = (P R)1/2         (1c)


Electric Current - Ohm's Law


I = V / R         (2a)


I = P / V         (2b)


I = (P / R)1/2         (2c)


Electric Resistance - Ohm's Law


R = V / I         (3a)


R = V2/ P         (3b)


R = P / I2         (3c)


Electric Power


P = V I         (4a)


P = R I2         (4b)


P = V2/ R         (4c)




P = power (watts, W), V = voltage (volts, V)


I = current (amperes, A), R = resistance (ohms, Ω)


Electric Energy :Electric energy is power multiplied time, or


W = P t      (5)




W = energy (Ws, J), t = time (s)


Electrical Motors : Electrical Motor Efficiency


μ = 746 Php / Pinput_w         (6)




μ = efficiency


Php = output horsepower (hp)


Pinput_w = input electrical power (watts)


or alternatively


μ = 746 Php / (1.732 V I PF)         (6b)


Electrical Motor - Power


P3-phase = (V I PF 1.732) / 1,000         (7)




P3-phase = electrical power 3-phase motor (kW)


PF = power factor electrical motor


Electrical Motor - Amps


I3-phase = (746 Php) / (1.732 V μ PF)         (7)




I3-phase = electrical current 3-phase motor (amps)


PF = power factor electrical motor

Electrical measurements





frequency f

hertz (Hz)


force F

newton (N)


pressure p

pascal (Pa) = N/m²


energy E

work joule (J) = N•m


power P

watt (W) = J/s


electric charge Q

coulomb (C) = A•s


voltage V

volt (V)= W/A


current I

ampere (A) = Q/s


capacitance C

farad (F) = C/V = A•s/V = s/Ω