Hi folks.
A very tricky interview question we are going to discuss now.
Q. Resistance of a conductor depends on the frequency of the supply. True or false?
As soon as we hear this, we say false. Why? because we don't find any angular frequncy term introduced in R which is there in Xc or Xl.
But the answer my friends is TRUE.
Let us see why.
Have you heard that the AC resistance of the conductor is more than its DC resistance?
The reason for this is Skin effect. As the frequency of the supply increases, the current surprisingly tends to flow in the skin instead of body of the conductor.This is due the flux linkages in the conductor due to AC current carried by itself. You can understand it something like, An straight conductor when carries AC has inductance even though it is not in the coil form as we always imagine. For 50 Hz the value of resistance is 1.6 times that of DC resistance. We take this 1.6 as an approximation constant to account for skin effect. So as the frequency increases, the tendency of current to flow through skin increases and the effective cross section of the conductor decreases. Effective cross section means the cross section of conductor through which current is actually flowing. (It will be ring like structure if you imagine it and current concentrated in the outer parts).
In the basic resistance formula, we find that Area of cross section is inversely proportional to resistance. So The resistance of conductor increases.
If this is so, Why do we take Resistance constant in Electrical circuit analysis or day to day life?
Answer to 1st Question is ECA is an ideal subject where we assume that F parameter has no effect on R and in day to day life we ignore the skin effect which enables us to say firmly- R IS INDEPENDENT OF FREQUENCY..!
Skin effect is the base of design of ACSR conductors that we see on distribution poles. It is helpful there. As we know current is actually flowing in outer parts, We can use aluminum outside (high conductivity and low mechanical strength) and we use steel inside (low conductivity and high mechanical strength). As a result we get conductor that has good conductivity as well as mechanical strength at low cost.
So next time when you will keep resistance constant under changing frequency, just keep in mind that you are ignoring skin effect. Thank you for you time and please feel free to comment or mail me about your areas of interest. It will be my genuine pleasure to post about them.
A very tricky interview question we are going to discuss now.
Q. Resistance of a conductor depends on the frequency of the supply. True or false?
As soon as we hear this, we say false. Why? because we don't find any angular frequncy term introduced in R which is there in Xc or Xl.
But the answer my friends is TRUE.
Let us see why.
Have you heard that the AC resistance of the conductor is more than its DC resistance?
The reason for this is Skin effect. As the frequency of the supply increases, the current surprisingly tends to flow in the skin instead of body of the conductor.This is due the flux linkages in the conductor due to AC current carried by itself. You can understand it something like, An straight conductor when carries AC has inductance even though it is not in the coil form as we always imagine. For 50 Hz the value of resistance is 1.6 times that of DC resistance. We take this 1.6 as an approximation constant to account for skin effect. So as the frequency increases, the tendency of current to flow through skin increases and the effective cross section of the conductor decreases. Effective cross section means the cross section of conductor through which current is actually flowing. (It will be ring like structure if you imagine it and current concentrated in the outer parts).
In the basic resistance formula, we find that Area of cross section is inversely proportional to resistance. So The resistance of conductor increases.
If this is so, Why do we take Resistance constant in Electrical circuit analysis or day to day life?
Answer to 1st Question is ECA is an ideal subject where we assume that F parameter has no effect on R and in day to day life we ignore the skin effect which enables us to say firmly- R IS INDEPENDENT OF FREQUENCY..!
Skin effect is the base of design of ACSR conductors that we see on distribution poles. It is helpful there. As we know current is actually flowing in outer parts, We can use aluminum outside (high conductivity and low mechanical strength) and we use steel inside (low conductivity and high mechanical strength). As a result we get conductor that has good conductivity as well as mechanical strength at low cost.
So next time when you will keep resistance constant under changing frequency, just keep in mind that you are ignoring skin effect. Thank you for you time and please feel free to comment or mail me about your areas of interest. It will be my genuine pleasure to post about them.
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