Resistance

Resistance

Resistance is the opposition that a substance offers to the flow of electric current. It is represented by the uppercase letter R.

Formula

 R  is the resistance in ohms (Ω). 

 ρ  is the resistivity in ohms-meter (Ω×m) 

 l   is the length of the conductor in meter (m) 

A  is the cross sectional area of the conductor.

The standard unit of resistance is the ohm. Ohms are named after Georg Simon Ohm (1784-1854), a German physicist who studied the relationship between voltage, current and resistance. Symbolized by the uppercase Greek letter omega: 

Resistance of resistors in series

The total equivalent resistance of resistors in series is the sum of the resistance values:

R_Total = R₁+ R₂+ R₃+...

Resistance of resistors in parallel

The total equivalent resistance of resistors in parallel is given by:

Measuring electrical resistance

• Electrical resistance is measured with ohmmeter instrument.
• In order to measure the resistance of a resistor or a circuit, the circuit should have the power supply turned off.
• The ohmmeter should be connected to the two ends of the circuit so the resistance can be read.