97 quantities · symbol · unit

Electrical
quantities

Each quantity in the course with its symbol, its unit and the lessons where it is used: voltage, current, resistance, power, energy, capacitance, inductance, impedance.

Search a quantity Prefixes

U	Voltage	volt	V
I	Current	ampere	A
R	Resistance	ohm	Ω
P	Power	watt	W
W	Energy	joule	J

01

Search a quantity

By name, symbol or unit.

Symbol	Quantity	Unit	Lessons
Pa	Absorbed power	W	Electric power
P	Active power	W	Power factor and AC power
r_A	Ammeter resistance	Ω	Voltmeter and ammeter
ω	Angular frequency	rad/s	Alternating current, RLC circuits and impedance
S	Apparent power	VA	Power factor and AC power
S	Area	m²	Magnetism and magnetic force (F = BIL)
E'	Back-EMF	V	EMF and internal resistance
C	Capacitance	F	Capacitors and RC circuits, RLC circuits and impedance
C	Capacitance to add	F	Power factor and AC power
q	Charge	C	Electrostatics and Coulomb's law
Q	Charge	C	Capacitors and RC circuits
l	Conductor length	m	Magnetism and magnetic force (F = BIL)
k	Constant (9 × 10⁹)	N·m²/C²	Electrostatics and Coulomb's law
S	Cross-section	mm², m²	Voltage and current, Ohm's law
I	Current	A	Electrical units and conversions, Voltage and current, Ohm's law, Electric power, Series and parallel circuits, EMF and internal resistance, Kirchhoff's current and voltage laws, Magnetism and magnetic force (F = BIL), Electromagnetic induction
I	Current (common)	A	Series and parallel circuits
I	Current delivered by the generator	A	Series and parallel circuits
J	Current density	A/mm²	Voltage and current
I_n	Current in branch n	A	Series and parallel circuits
I₁	Current in R₁	A	Series and parallel circuits
I₂	Current in R₂	A	Series and parallel circuits
I_n	Current through it	A	Series and parallel circuits
d	Diameter	mm	Voltage and current
d	Distance	m	Electrostatics and Coulomb's law
η	Efficiency	-	Electric power
E	Electric field	V/m	Electrostatics and Coulomb's law
E	EMF	V	EMF and internal resistance, Kirchhoff's current and voltage laws
W	Energy	J	Electrical units and conversions, Electric power, Capacitors and RC circuits, Electromagnetic induction
Céq	Equivalent capacitance	F	Capacitors and RC circuits
Réq	Equivalent resistance	Ω	Series and parallel circuits
R₂₃	Equivalent resistance of R₂ + R₃	Ω	Series and parallel circuits
R₂₃	Equivalent resistance of R₂ ∥ R₃	Ω	Series and parallel circuits
Φ	Flux	Wb	Magnetism and magnetic force (F = BIL), Electromagnetic induction
F	Force	N	Electrostatics and Coulomb's law, Magnetism and magnetic force (F = BIL)
f	Frequency	Hz	Alternating current
i	Full-scale current	A	Voltmeter and ammeter
U	Generator voltage	V	Series and parallel circuits
Z	Impedance	Ω	RLC circuits and impedance
E	Induced EMF	V	Electromagnetic induction
L	Inductance	H	Electromagnetic induction, RLC circuits and impedance
φ	Initial phase	rad	Alternating current
r	Internal resistance	Ω	EMF and internal resistance
r'	Internal resistance	Ω	EMF and internal resistance
l	Length	m	Ohm's law, Magnetism and magnetic force (F = BIL), Electromagnetic induction
B	Magnetic flux density	T	Magnetism and magnetic force (F = BIL), Electromagnetic induction
R	Measured resistance	Ω	Voltmeter and ammeter
n	Number of resistors	-	Series and parallel circuits
N	Number of turns	-	Magnetism and magnetic force (F = BIL), Electromagnetic induction
Umax	Peak value	V	Alternating current
T	Period	s	Alternating current
φ	Phase shift	rad	RLC circuits and impedance
φ₁, φ₂	Phase shifts before / after	rad	Power factor and AC power
e	Plate spacing	m	Electrostatics and Coulomb's law
P	Power	W	Electrical units and conversions, Electric power
P_n	Power dissipated by resistor n	W	Series and parallel circuits
cos φ	Power factor	-	Power factor and AC power
Q	Quantity of electricity	C	Electrical units and conversions, Voltage and current
X	Reactance	Ω	RLC circuits and impedance
Q	Reactive power	var	Power factor and AC power
R	Resistance	Ω	Electrical units and conversions, Ohm's law, Electric power, Capacitors and RC circuits, RLC circuits and impedance
R₀	Resistance at 20 °C	Ω	Ohm's law
R_n	Resistance of branch n	Ω	Series and parallel circuits
R	Resistance of each branch	Ω	Series and parallel circuits
R_l	Resistance of one wire	Ω	Series and parallel circuits
ρ	Resistivity	Ω·m	Ohm's law
R_n	Resistor n	Ω	Series and parallel circuits
I	RMS current	A	RLC circuits and impedance
Ueff	RMS value	V	Alternating current
U	RMS voltage	V	RLC circuits and impedance
R_add	Series multiplier	Ω	Voltmeter and ammeter
R_s	Shunt resistance	Ω	Voltmeter and ammeter
v	Speed	m/s	Electromagnetic induction
ΣR	Sum of loop resistances	Ω	EMF and internal resistance
Δθ	Temperature change	°C	Ohm's law
α	Temperature coefficient	/°C	Ohm's law
U	Terminal voltage	V	EMF and internal resistance
t	Time	s	Voltage and current, Electric power, Electromagnetic induction
τ	Time constant	s	Capacitors and RC circuits, Electromagnetic induction
I	Total current	A	Series and parallel circuits
I	Total current (split)	A	Series and parallel circuits
I	Total current to measure	A	Voltmeter and ammeter
Réq	Total equivalent resistance	Ω	Series and parallel circuits
P	Total power	W	Series and parallel circuits
U	Total voltage	V	Series and parallel circuits
U	Total voltage (split)	V	Series and parallel circuits
Pu	Useful power	W	Electric power
µ₀	Vacuum permeability (4π × 10⁻⁷)	T·m/A	Magnetism and magnetic force (F = BIL)
U	Voltage	V	Electrical units and conversions, Electrostatics and Coulomb's law, Ohm's law, Electric power, Capacitors and RC circuits
U	Voltage (common)	V	Series and parallel circuits
U_n	Voltage across resistor n	V	Series and parallel circuits
U_n	Voltage across resistor n	V	Series and parallel circuits
U	Voltage across the network	V	Series and parallel circuits
ΔU	Voltage drop	V	Series and parallel circuits
R·I	Voltage drop	V	Kirchhoff's current and voltage laws
U₁	Voltage on R₁	V	Series and parallel circuits
U₂₃	Voltage shared by R₂ and R₃	V	Series and parallel circuits
r_V	Voltmeter resistance	Ω	Voltmeter and ammeter

02

Unit prefixes

A prefix multiplies the unit by a power of 10.

Prefix	Symbol	Factor	Example
mega	M	10⁶ = 1,000,000	1 MΩ = 1,000,000 Ω
kilo	k	10³ = 1,000	4.7 kΩ = 4,700 Ω
milli	m	10⁻³ = 0.001	20 mA = 0.02 A
micro	µ	10⁻⁶	100 µF = 0.0001 F
nano	n	10⁻⁹	10 nF = 0.00000001 F

Before calculating, convert everything to base units: formulas only work with them.

03

Don't mix up

The pairs that trip up the most students.

Volt and wattThe volt measures voltage; the watt measures power (P = U × I).

Power and energyPower is a rate (W); energy builds up over time (J, kWh).

Resistance and resistivityR in Ω belongs to the wire; ρ in Ω·m belongs to the material.

Voltage and potentialVoltage is a difference between two potentials: U_AB = V_A − V_B.

Frequently asked questions about quantities

What is the unit of voltage?

The volt (V). It is measured with a voltmeter connected in parallel. An AA battery gives 1.5 V, a wall socket 230 V.

What is the difference between a volt and a watt?

The volt measures voltage (what pushes the current), the watt measures power (energy per second). They are linked by P = U × I.

Why is current written with the letter I?

It comes from the French "intensité du courant" (current intensity). I is used in every language, even though English simply says "current".

What does kWh mean?

Kilowatt-hour: the energy used by a 1,000 W appliance for one hour. 1 kWh = 3,600,000 J; it is the unit on the electricity bill.

What is the symbol for resistivity?

The Greek letter ρ (rho), in ohm-meters (Ω·m). Copper has ρ ≈ 1.7 × 10⁻⁸ Ω·m.

Electrical
quantities

Search a quantity

Unit prefixes

Don't mix up

Frequently asked questions about quantities

Circuit diagram CEI 60617

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