A. resistance in ac > resistance in dcB. resistance in ac = resistance in dcC. resistance in ac < resistance in dcD. none of theseAnswer: A. resistance in ac > resistance in dcExplanation: Skin Effect- Skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the conductor's surface and decreases with greater depths in the conductor.
- The phenomenon arising due to unequal current distribution over the conductor's entire cross-section is referred to as the skin effect.
- Such a phenomenon does not have much role to play in the case of a very short transmission line, but with an increase in the effective length of the conductors, the skin effect increases considerably.
- The distribution of current over the entire cross-section of the conductor is quite uniform in the case of a DC system.
- But in the alternating current system, current tends to flow with higher density through the surface of the conductors (i.e., the skin of the conductor), leaving the core deprived of current. Increased effective resistance but reduced effective internal reactance of the conductor.
- Hence due to the skin effect effective area of the conductor of the line decreases, so that the resistance of the line increases.
- The electric current flows mainly at the skin of the conductor, between the outer surface and a level called the skin depth. the skin depth is inversely proportional to the root of frequency. In a good conductor, the skin depth is proportional to the square root of the resistivity. This means that better conductors have a reduced skin depth. The overall resistance of the better conductor remains lower even with the reduced skin depth.
- The skin effect increases with the increase of cross-section, permeability, and supply frequency. It reduces with the increase in resistivity of the conductor material.
- Hence, The skin effect is negligible when the supply frequency is low and the conductor diameter is small.
- Skin effect is the tendency for high-frequency currents to flow on the surface of a conductor.
- The skin effect reduces the effective area available for conduction due to self-linked flux. Now, we know that resistance is inversely proportional to the area of conduction so the effective value of resistance increases with the skin effect.
- If the resistance is constant over a large range of current and voltage values, the resistor is an ohmic device.
- Hence if the current is uniformly distributed in the conductor cross-section, the effective resistance of a conductor will be the same as ohmic resistance.
Watch Video For Full Explanation About Skin Effect
Factors affecting skin effects in transmission lines are:Frequency : - The skin effect increases with the increase in frequency.
- At low frequencies, such as 50Hz, there is a small increase in the current density near the surface of the conductor.
- At high frequencies, such as radio-frequency, practically the whole of the currents flows on the surface of the conductor, and the Skin effect is more noticeable at high frequencies.
- With DC supply (frequency is zero), the current passed in a conductor is uniformly distributed over the cross-section of the conductors.
Diameter : - It increases with the increase in the diameter of the conductor.
The shape of the conductor :- The skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.
Type of material :- The skin effect increase with the increase in the permeability of the material (Permeability is the ability of the material to support the formation of the magnetic field).
Important Points:- The Skin effect is negligible if the frequency is less than 50Hz and the diameter of the conductor is less than 1cm.
- In stranded conductors like ACSR (Aluminium Conductor Steel Reinforced) the current flows mostly in the outer layer made of aluminium, while the steel near the centre carries no current and gives high tensile strength to the conductor.
- The concentration of current near the surface enabled the use of an ACSR conductor.
Important Point: Skin effect ∝ 1/skin depth
Skin effect is inversely proportional to skin depth.
skin depth = 1/√(Ï€fµσ)
Where,
f = frequency
µ = permeability
σ = conductivityIf skin depth is more, then the skin effect is less and vice versa.
In the case of communication, line frequency is higher. So that skin depth is very small and hence skin effect is larger.The depth to which the electromagnetic waves pass through the conductor is very small. It is measured in μm.In the case of power lines frequency is small and hence skin depth is larger, so the skin effect can be neglected.
Therefore, Skin effect ∝ √(Ï€fµσ)When resistivity decrease, its conductivity will increase. This will increase the skin effect.
A. resistance in ac > resistance in dc
B. resistance in ac = resistance in dc
C. resistance in ac < resistance in dc
D. none of these
Answer: A. resistance in ac > resistance in dc
Explanation:
Skin Effect
- Skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the conductor's surface and decreases with greater depths in the conductor.
- The phenomenon arising due to unequal current distribution over the conductor's entire cross-section is referred to as the skin effect.
- Such a phenomenon does not have much role to play in the case of a very short transmission line, but with an increase in the effective length of the conductors, the skin effect increases considerably.
- The distribution of current over the entire cross-section of the conductor is quite uniform in the case of a DC system.
- But in the alternating current system, current tends to flow with higher density through the surface of the conductors (i.e., the skin of the conductor), leaving the core deprived of current. Increased effective resistance but reduced effective internal reactance of the conductor.
- Hence due to the skin effect effective area of the conductor of the line decreases, so that the resistance of the line increases.
- The electric current flows mainly at the skin of the conductor, between the outer surface and a level called the skin depth. the skin depth is inversely proportional to the root of frequency. In a good conductor, the skin depth is proportional to the square root of the resistivity. This means that better conductors have a reduced skin depth. The overall resistance of the better conductor remains lower even with the reduced skin depth.
- The skin effect increases with the increase of cross-section, permeability, and supply frequency. It reduces with the increase in resistivity of the conductor material.
- Hence, The skin effect is negligible when the supply frequency is low and the conductor diameter is small.
- Skin effect is the tendency for high-frequency currents to flow on the surface of a conductor.
- The skin effect reduces the effective area available for conduction due to self-linked flux. Now, we know that resistance is inversely proportional to the area of conduction so the effective value of resistance increases with the skin effect.
- If the resistance is constant over a large range of current and voltage values, the resistor is an ohmic device.
- Hence if the current is uniformly distributed in the conductor cross-section, the effective resistance of a conductor will be the same as ohmic resistance.
Watch Video For Full Explanation About Skin Effect
Factors affecting skin effects in transmission lines are:
Frequency :
- The skin effect increases with the increase in frequency.
- At low frequencies, such as 50Hz, there is a small increase in the current density near the surface of the conductor.
- At high frequencies, such as radio-frequency, practically the whole of the currents flows on the surface of the conductor, and the Skin effect is more noticeable at high frequencies.
- With DC supply (frequency is zero), the current passed in a conductor is uniformly distributed over the cross-section of the conductors.
Diameter :
- It increases with the increase in the diameter of the conductor.
The shape of the conductor :
- The skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.
Type of material :
- The skin effect increase with the increase in the permeability of the material (Permeability is the ability of the material to support the formation of the magnetic field).
Important Points:
- The Skin effect is negligible if the frequency is less than 50Hz and the diameter of the conductor is less than 1cm.
- In stranded conductors like ACSR (Aluminium Conductor Steel Reinforced) the current flows mostly in the outer layer made of aluminium, while the steel near the centre carries no current and gives high tensile strength to the conductor.
- The concentration of current near the surface enabled the use of an ACSR conductor.
Important Point:
Skin effect ∝ 1/skin depthSkin effect is inversely proportional to skin depth.
skin depth = 1/√(Ï€fµσ)
Where,
f = frequency
µ = permeability
σ = conductivity
If skin depth is more, then the skin effect is less and vice versa.
In the case of communication, line frequency is higher. So that skin depth is very small and hence skin effect is larger.
In the case of communication, line frequency is higher. So that skin depth is very small and hence skin effect is larger.
The depth to which the electromagnetic waves pass through the conductor is very small. It is measured in μm.
In the case of power lines frequency is small and hence skin depth is larger, so the skin effect can be neglected.
Therefore, Skin effect ∝ √(Ï€fµσ)
When resistivity decrease, its conductivity will increase. This will increase the skin effect.
