Electric resistance is a measure of how much a material opposes the flow of electric current. The SI unit of electrical resistance is the ohm (Ω), which represents the resistance of a conductor through which a current of one ampere flows when a potential difference of one volt is applied.
Common Electric Resistance Units
Ohm (Ω): The SI unit of electric resistance. Named after German physicist Georg Simon Ohm.
Milliohm (mΩ): Equal to 10-3 ohms. Used for measuring very small resistances like those in connectors and PCB traces.
Microohm (μΩ): Equal to 10-6 ohms. Used in precision measurements of very low resistances like those in superconductors.
Kilohm (kΩ): Equal to 103 ohms. Commonly used in electronics for resistors.
Megohm (MΩ): Equal to 106 ohms. Used for high-resistance components and insulation measurements.
Gigaohm (GΩ): Equal to 109 ohms. Used for extremely high-resistance measurements in specialized applications.
Abohm: A unit in the CGS electromagnetic system, equal to 10-9 ohms.
Statohm: A unit in the CGS electrostatic system, equal to approximately 9 × 1011 ohms.
Ohm's Law
The fundamental relationship between voltage (V), current (I), and resistance (R):
R = V / I
Where R is in ohms, V is in volts, and I is in amperes.
Resistivity and Resistance
The resistance of a specific conductor can be calculated using its resistivity (ρ), length (L), and cross-sectional area (A):
R = ρ × (L / A)
Where R is in ohms, ρ is in ohm-meters, L is in meters, and A is in square meters.
Typical Resistance Values
Material/Component
Typical Resistance Range
Application/Notes
Superconductors (below critical temperature)
0 Ω
MRI machines, particle accelerators
Copper wire (typical household)
0.1 - 10 Ω
Electrical wiring
Standard resistors (electronics)
1 Ω - 10 MΩ
Circuit components
Human body (hand to hand)
1 kΩ - 100 kΩ
Varies with moisture, contact area
Electrical insulation
1 MΩ - 10 TΩ
Electrical safety
Air (dry)
1016 Ω·m (resistivity)
Electrical isolation
Resistors in Circuits
Series Connection
When resistors are connected in series, their resistances add:
Rtotal = R1 + R2 + R3 + ...
Parallel Connection
When resistors are connected in parallel, their conductances add:
1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ...
Electric Resistance Applications
Electronics: Resistors are fundamental components in electronic circuits used to control current flow, divide voltage, and provide biasing.
Heating Elements: Electric heaters use resistive elements to convert electrical energy to heat (Joule heating).
Sensors: Many sensors like thermistors, strain gauges, and photoresistors work based on changes in electrical resistance.
Power Transmission: Minimizing resistance in power lines is crucial for efficient electricity distribution.
Electrical Safety: Insulation resistance measurements ensure electrical systems are safely isolated.
Medical Devices: Bioelectrical impedance analysis uses resistance measurements to estimate body composition.
Power Dissipation in Resistors
Power (P) dissipated in a resistor is related to current (I) and resistance (R) by:
P = I2 × R = V2 / R = V × I
Where P is in watts, I is in amperes, R is in ohms, and V is in volts.
