Electrical Resistance

Resistance definition

 

Resistance is an electrical quantity that measures how the device or material reduces the flow electric current through it.

Resistance symbol

Resistance symbol

Resistance  units

 

 The resistance is measured in units of Ohms (Ω).

Resistance  units

 

Resistance calculation

 

R= ƿ × L\ A

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

 It is easy to understand this formula with water pipes analogy:

If we make an analogy to water flow in pipes, the resistance is bigger when the pipe is thinner, so the water flow is decreased.

when we use the Resistance equation in the pipe:

• when the pipe is longer, the length is bigger and the resistance will increase.
• when the pipe is wider, the cross sectional area is bigger and the resistance will decrease.

Resistance calculation with ohm's law

 

R = V \ I

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).

Temperature effects of resistance

 

R₂ = R₁ × ( 1 + α(T₂ - T₁) )

R₂ is the resistance at temperature T₂ in ohms (Ω).

R₁ is the resistance at temperature T₁ in ohms (Ω).

α is the temperature coefficient.

 we see that 

The resistance of a resistor increases when temperature of the resistor increases.

Connecting Resistors

Individual resistors can be connected together in either a series connection, a parallel connection or combinations of both series and parallel together, to produce more complex resistor networks whose equivalent resistance is a combination of the individual resistors. Then complicated resistor networks or impedances can be replaced by a single equivalent resistor, R_EQ or impedance, Z_EQ