Ohm’s law defines a linear relationship between the voltage and the current in an electrical circuit. The DC current flow through a resistor is set by the resistor’s voltage drop and the resistor’s resistance. With water flow analogy we can imagine the electric current as water current through pipe, the resistor as a thin pipe that limits the water flow, the voltage as height difference of the water that enables the water flow.
Ohm’s law
Ohm’s law definition :- The resistor’s current I in amps (A) is equal to the resistor’s voltage VR=V in volts (V) divided by the resistance R in ohms (Ω)
V is the voltage drop of the resistor, measured in Volts (V). Sometimes E is used instead of V . E denotes electromotive force. I is the electrical current fl0wing through the resistor, measured in Amperes (A) R is the resistance of the resistor, measured in Ohms (Ω)
Voltage calculation
When we know the current and resistance, we can calculate the voltage. The voltage V in volts (V) is equal to the to the current I in amps (A) times the resistance R in ohms (Ω):
Resistance calculation
When we know the voltage and the current, we can calculate the resistance. The resistance R in ohms (Ω) is equal to the voltage V in volts (V) divided by the current I in amps (A)
Since the current is set by the values of the voltage and resistance, the Ohm’s law formula can show that:
If we increase the voltage, the current will increase.
If we increase the resistance, the current will reduce.
Example =1 Find the current of an electrical circuit that has resistance of 50 Ohms and power supply of 5 Volts.
Solution:V = 5V
R = 220Ω
I = V / R = 5V / 220Ω = 0.02A = 22 mA.
Unit Name Unit Symbol Quantity.
Unit Name |
Unit Symbol |
Quantity |
Ampere (amp) | A | Electric current (I) |
Volt | V | Voltage (V, E)Electromotive force (E)Electric potential (φ) |
Ohm | Ω | Resistance (R)Impedance (Z) |
Watt | W | Electric power (P) |
Decibel-milliwatt | dBm | Electric power (P) |
Decibel-Watt | dBW | Electric power (P) |
Volt-Ampere-Reactive | VAR | Reactive power (Q) |
Volt-Ampere | VA | Apparent power (S) |
Farad | F | Capacitance (C) |
Henry | H | Inductance (L) |
siemens / mho | S | Conductance (G)Admittance (Y) |
Coulomb | C | Electric charge (Q) |
Ampere-hour | A·h | Electric charge (Q) |
Joule | J | Energy (E) |
Kilowatt-hour | kWh | Energy (E) |
Electron-volt | eV | Energy (E) |
Ohm-meter | Ω∙m | Resistivity (ρ) |
siemens per meter | S/m | Conductivity (σ) |
Volts per meter | V/m | Electric field (E) |
Newtons per coulomb | N/C | Electric field (E) |
Volt-meter | V·m | Electric flux (Φ_{e}) |
Tesla | T | Magnetic field (B) |
Gauss | G | Magnetic field (B) |
Weber | Wb | Magnetic flux (Φ_{m}) |
Hertz | Hz | Frequency (f) |
Seconds | s | Time (t) |
Meter / metre | m | Length (l) |
Square-meter | m^{2} | Area (A) |
Decibel | dB | |
Parts per million |
ppm |
Units prefix table
Prefix |
PrefixSymbol |
Prefix factor |
Example |
pico |
p |
10^{-12} |
1pF = 10^{-12}F |
nano |
n |
10^{-9} |
1nF = 10^{-9}F |
micro |
μ |
10^{-6} |
1μA = 10^{-6}A |
milli |
m |
10^{-3} |
1mA = 10^{-3}A |
kilo |
K |
10^{ 3} |
1KΩ = 1000Ω |
mega |
M |
10^{ 6} |
1MHz = 10^{6}Hz |
giga |
G |
10^{ 9} |
1GHz = 10^{9}Hz |