What is resistance ? in electrical.

Electrical resistance is a property of a material by virtue of which it oppose the flow of the electrons through the material.

or

The property of a material which obstructs the flow of electric current through a conductor is called resistance.

Electric current is the flow of free electrons, electrical resistance is the opposition given by the substance for free electrons to flow. This opposition occurs because atoms and molecules of the substance these obstruct the flow of electrons.

Some substances which are metals like silver, copper, aluminum have little resistance to the flow of electric current and is know as conductors.

On the other hand those substances which provide high opposition to the flow of electric current ( flow of free electrons ) is know as insulator example of insulator glass, rubber, mica and dry wood.

It may be noted that electrical resistance is the electric friction given by the substance and produces heat with the flow of electric current. Moving electrons collide with atoms or molecules of matter, each collision results in the release of a small amount of heat.

See also this  What is vector diagram of loaded alternator ?

The unit of resistance 

The resistance’s unit is  ‘Ω’ (ohm). And it is often denoted by ‘R’ .

According to ohm’s law

V ∝ I

V = RI

R = V/I

So , the resistance’s formula  is  R = V/I .

For What is current in electrical engineering ?click here

Ohmic and non ohmic resistance

Those conductors that obey ohm’s law is know as ohmic resistance such as a manganese coil.

Those conductor who do not obey ohm’s law are know as non-ohmic resistance. Such as the resistance of triode and diode valve.

What is effect of temperature on resistance 

It is found that in the normal range of temperature, the resistance of metal conductors increases with increase in temperature and increases linearly. Hence resistance/temperature graph is a straight line as you can see in the fig.

Consider a metal conductor having resistance R₀ at 0° C and R₁ at t₁° C. Then in the normal range of temperatures the increase in resistance that is R₁ – R₀ .

1. Is directly proportional to the initial resistance that is  R₁ – R₀ ∝ R₀ .
2. Is directly proportional to the rise in resistance that is R₁ – R₀ ∝ t₁
3. Depends upon the nature of the material.

 Combining the first 2, then we get 

R₁ – R₀ ∝ R₀ t₁

R₁ – R₀ = α₀ R₀ t₁

Here α₀ is constant and is know as temperature of co-efficient of resistance at 0° C.

From eq (i) we get, R₁ – R₀ = α₀ R₀ t₁                                                                                                                                                                                                                                R₁ = R₀ ( 1 + α₀ t₁ )

We get the definition of α₀ from eq 1, So α₀ =  R₁ – R₀ / R₀ × t₁ 

What is the temperature coefficient of resistance

Therefore, the temperature co-efficient of resistance of a conductor is the increase in resistance per ohm original resistance per °C rise in temperature.

 (i) Temperature co-efficient α₁ at t₁° C is  α₁ = α₀ /1+α₀ t₁

similarly, temperature co-efficient α₂ at t₂° C is α₂ = α₀ / 1+α₀ t₂

The relation between α₁ and α₂ is = α₂ = 1/ (1/α₁ ) + (t₂ – t₁ )

 (ii) If the resistance of a conductor is R₂ at t₂° C and R₁ at t₁° C ( t₂ > t₁ ), then α₁ = temperature coefficient at t₁° C.

So R₂ = [ 1 +  α₁ ( t₂ – t₁ ) ]

In the effect of temperature on resistance : What happens when the temperature rises –

 As the temperature increases, the electrical resistance of the conductor increases and that of the semiconductor decreases and there is no effect of any kind in the dielectric. And remember one thing when the temperature of the conductor is increased from 0° to t° then the increased resistance will be R_t = R₀ ( 1 ± αt ).

Where, R₀ = 0° Resistance of the conductor

α = Resistance heat coefficient

t = Increase in temperature

Define resistivity or specific resistance

Where  ‘R’ is resistance of a material  ‘l’ is length of material and  ‘A’ is the area of cross-section.

‘ρ’ is the Greek  letter ‘rho’ , ρ (rho) is called resistivity or specific resistance of the material. So the value of rho depends on the nature and temperature of the material. If  L = 1 m , A = 1 m² , then  R = ρ .

Hence the specific resistance or resistivity of a material is the resistance offered by 1m length of wire material having area of ​​cross-section of 1m². The SI unit of resistivity is  Ω  (ohm).

So , different materials have different resistivity for example the resistivity of copper is 1.7 × 10^-8 Ω. That means if you take a copper wire 1 meter long and having a cross-sectional area of 1m². Then the resistance of this piece of copper wire will be 1.7 × 10^-8 Ω.

To make a network, we connect different resistances mainly in two ways and they are 1. series connection 2. Parallel connection. Both are briefly explain in below.

Resistance in series connection

In series connection current remain same or series connected electrical element. The voltage across each resistances ( R₁ , R₂ , R₃ ) respectively. So in the series circuit voltage gets divided across each resistance.

So, the source voltage  V = V₁ + V₂ + V₃ .   

V = V₁ + V₂ + V₃   ,  [ we know  V =IR ]

Then , I R_eq  =  IR₁ + IR₂ + IR₃

I R_eq  = I ( R₁ + R₂ + R₃ )

Then we get , [ R = R₁ + R₂ + R₃ ]  

 R_eq  = R₁ + R₂ + ……………+ R_n . for ‘n’ number of series connected resistances.  

Resistance in parallel connection

In parallel connection the voltage remains the same and the current is divided. So it is depending on the value of individual resistance.

V_AB = V = VR₁

V_AB = V = VR₂

Similarly, V_AB = V = VR₃

So now we can write, I = I₁ + I₂ + I₃

V/R_eq = V₁/R₁ + V₂/R₂ + V₃/R₃  [ Using Ohm’s law : V = IR ]

V/R_eq = V/R₁ + V/R₂ + V/R₃

Here we take V common

V/R_eq = V( 1/R₁ + 1/R₂ +1/R₃ )

  or

NOTE – ‘R’ is the equivalent resistance for both parallel circuit and series circuit.

Resistance of human body

About 50,000 Ω is the resistance of a dry body and about 10,000 Ω is the resistance of a wet body. About 1/1000 ampere of electric current is require to produce a shock in the human body.

So in the above post I have discussed what is resistance in electrical? and I hope you will get a lot of information from this article, This chapter in electrical engineering is a basic thing and your basic concept should be clear and also it is a very important chapter for you, So i share this concept through this post I hope your maximum doubt Will be clear, Thank you.