What is Resistance?
Resistance is defined as the property of substance due to which it opposes the electric current flow through it.
For the better understanding take an example.
When a potential difference or voltage is applied across a conducting substance, the free electrons start flowing through it. As we know that every substance consists of atoms therefore when the free electrons move through substance they have to collides with the atoms of substance and this reduced the average speed of electrons. As a result of this current flow is also decreases.
As atom are present in all substances therefore every substance has property to resist current flow through it and this property is known as electric resistance.
Factor effecting resistance
1. Nature of material
2. Area of cross section
Different substances have different number of electron valence electron, atomic density and strength of bonding between electrons and nucleus
Stronger bonding between electrons and nucleus, higher the resistance offer.
Substance which have large number of free electrons and less atomic density offer less resistance.
Therefore, the resistance of different material is different.
What happened to resistance if the area of conductor is increases?
When the cross sections area is increases, the electron have more cross-sections area to move through it, which decreases the chances of collision. As a results it gets easier to electrons moves through conductor. Hence the resistance of conductor decreases.
It states that the resistance of is substance is inversely proportional to its cross section area. Greater the area of cross sections, lesser the chances of collision
R ∝ 1/A ……….. (1)
What happen to resistance if the length of wire is increases?
When is the length of wire is increases, the electron have to travel larger distance which increases the chances of collision. As a result it get harder to electron move through it. Hence the resistance of conductor increases.
It state that the resistance of a conductor is directly proportional to length.
R ∝ L …………. (2)
From equations 1 and 2
R ∝ ρ × L/A
Where ‘ρ’ is the constant of proportionality and known as resistivity or specific resistance of the material.
It is the property of material that determine the resistance of a piece of a given dimension.
In simple resistivity is defined as the resistance offered by a material which have length of 1m and area of cross section 1m².
Suppose you have a piece of wire which has length of 1m and area of cross sections 1m². Now checked the resistance of that wire with the help of ohm-meter.
The value of resistance shown in ohm-meter is called resistivity of that material.
Effect of temperature on resistance of conductor
When the temperature of conductor is increases, its atom start vibrating. This increases the chances of collision between electrons and atom. As a results it gets harder to electron to move through it. Therefore resistance of conductor increases. It means that with the rise in temperature, resistance of conductor increases.
Temperature coefficient of resistance
As we discussed when the temperature of conducting material increases the speed of its electrons increases as well as its atom start vibrating this increases the chances of collision between electrons and atom. Therefore resistance of conducting material increases linearly with rise in temperature.
Draw the graph between resistance and temperature.
Let us consider
R0 = Resistance of material at 0°C
Suppose temperature changes from 0°C To T. Therefore resistance also changes from R0 to R.
R = Resistance of materials at temperature T.
Therefore, when the temperature changes from 0°C to T, the change in resistance is represented by ΔR
ΔR = R – R0 = change is resistance
The change in resistance depends on the following factors:
1. It is directly proportional to initial resistance.
ΔR ∝ (R0). ………. (1)
2. It is directly proportional to temperature.
ΔR ∝ (T°C – 0°C)
ΔR ∝ T°C ………..(2)
3. It is it depends on nature of material.
Combining equations 1 and 2 we have
ΔR ∝ R0 × T
R – R0 ∝ R0 T
R – R0 = α R0 T
α = (R – R0) / R0 T
Where ‘α’ is a constant of proportionality it is called temperature coefficient of resistance.
Its value is different for different material.
The temperature coefficient of resistance is defined as the change in resistance of material with per degree Celsius change in temperature.
In simple, temperature coefficient of resistance refer that if we change the temperature of a material by 1°C then how much will change in the resistance of material.
Types of temperature coefficient of resistance
1. Positive temperature coefficient of resistance :
when the resistance of material increases with increase in temperature, the material is said to have a positive temperature coefficient of resistance.
2. Negative temperature coefficient of resistance :
When the resistance of material decreases with increase in temperature, the material is said to have a negative temperature coefficient of resistance
3. Zero temperature coefficient of resistance :
When the resistance of material is remained unchanged with change in temperature, the material is said to have zero temperature coefficient of resistance.
Effect of temperatures on resistance of material
When the temperature of conductor increases, the speed of its electron increases as well as its atoms start vibrating. This increases the chances of Collision between electron and atom. As a results, it gets difficult for the electron to move through conductor. Hence the resistance of conductor increases and it increases linearly or regularly.
Conductor has positive temperature coefficient of resistance.
When the temperature increases, the outermost electron of semiconductor acquires energy and jump from valence band to conduction band. As a result of it, the number of careers in conductor increases which increases the conductivity and decrease the resistance.
Semiconductor has negative temperature coefficient of resistance.
The effect of temperature on insulator is same as semiconductor but it has a very large forbidden energy gap. It requires a huge amount of energy or temperature change to move the electrons from valence band to conduction band.
Once the electrons move from Valence band to conduction band the number of carriers in conduction band increases. Therefore the conductivity increases and resistance decreases.
Insulators also have negative temperature coefficient of resistance.
When the temperature is increases, the resistance of a alloy increases. But unlike a pure conductor the increase is very small and irregular.
Alloy have almost zero temperature coefficients of resistance.