Electric Resistance

The electrical resistance of an electrical conductor is a measure of the difficulty to pass an electric current through that conductor. A conductor has free electrons randomly moving inside. When electric potential is applied to it, the electrons collide with other atoms and molecules of the conductor. The atoms and molecules create the obstruction in the flow of electrons. This obstruction is called resistance.

Each and every material possesses the property of resistance because every conductor has atoms or molecules which can obstruct flowing electrons. If the material has less electrical conductivity, it means resistance of that material is high. So conductor like copper, gold, etc. has less resistance while insulators like wood, glass, etc. has higher resistance.

Unit of resistance: Unit of resistance is ohms and it is denoted by Ω.

Laws of resistance

Value of resistance is directly proportional to length of the conductor and inversely proportional to area of the conductor.

R α l/a

Therefore, R = ρl/a

Where ρ is constant of proportionality and also known as resistivity of the material.

Another law is Ohm’s law which states that voltage is directly proportional to current. A constant resistance is used and this resistance is obstruction.

V = IR

If the circuit has no resistance then voltage will be equal to current. V = I. But this is not possible; every conductor has its own definite amount of resistance.

Resistivity of the material

It is a measure of how strongly a material opposes the flow of electric current. Resistance of the conducting wire depends on the material of which wire is made. Every conductor has its own resistivity. Resistivity is denoted by ρ.

According to our formula of resistance,

R = ρl/a

Ρ = Ra/l

Therefore, resistivity is directly proportional to area of conductor and inversely proportional to length of the conductor.

Unit of resistivity = ohms×m×m/m = ohms×m

Resistor vs. Resistivity

Resistance is the property of material which opposes the flow of current while resistivity is the resistance of the conductor having specific length and area. Resistance is dependent on length, cross sectional area and temperature and resistivity is only dependent on temperature.

Conductivity of the material

Conductivity of the material will be more if resistivity of that material is less and if resistivity is higher, conductivity of the material will be less. So conductivity of the material is inverse of resistivity. Conductivity is represented by σ.

σ = 1/ρ

Unit of conductivity is 1/ohms×m or Siemens.

Resistor

Resistor is an electrical component that implements electrical resistance to circuit. These components reduce the current flow in the circuit.

 Resistors are denoted as:

Effects of temperature on resistor

Resistance of the resistor changes with the change in temperature in surroundings. The temperature coefficient is generally taken to show the change. Temperature coefficient of the resistor is the amount of change in resistor per degree Celsius change in temperature. It is denoted as α. If the resistor have +ve temperature coefficient, then as the temperature increases the resistance in resistor also increases. If the resistor has –ve temperature coefficient, then as the temperature increases the resistance in the resistor decreases.

Effects of pressure on resistor

There is not much change in resistivity of the conductor in small changes of pressure but only if the change in pressure is as high as 1 mega bar i.e. 1Mbar. Thus, there is no notable change in resistor until this much pressure is applied. However,  If the pressure is increased the resistivity of the resistor decreases.

Almost every electronic circuit consists of resistor. No electric circuit can work without resistors because controlling of voltage is most important thing of a circuit. Resistors are unsung heroes of electronics without which we cannot make some complex electronic systems.