What is Resistance?

Resistance definition:

Resistance is property of any materiel to reduce the flow of current. 

Its unit is ohm (Ω). 

Resistance calculation:

The resistance of a conductor is resistivity of the conductor's material times the conductor's length divided by the conductor's cross sectional area.

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 in square meters (m²).

•  The length is bigger and the resistance will increase.
•  The cross sectional area is bigger and the resistance will decrease

Resistance calculation with ohm's law:

R is the resistance of the resistor in ohms (Ω).

V is the voltage drop on the resistor in volts (V).

I is the current of the resistor in amperes (A).

Resistance in series circuit:

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

R_T= R₁+ R₂+ R₃+...

Resistance in parallel circuit:

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.