CrazyEngineers Archive
Old, but evergreen and popular discussions on CrazyEngineers, presented to you in read-only mode.
@kashmirihasan • 16 May, 2010
what is the difference between ac and dc resistance??? and why is there such a difference in both of them? i saw somewhere that ac resistance is 1.5 times dc resistance ? hows that even possible?
and how can we differentiate between ac and dc resistance?
@gaurav.bhorkar • 16 May, 2010
kashmirihasan
what is the difference between ac and dc resistance??? and why is there such a difference in both of them? i saw somewhere that ac resistance is 1.5 times dc resistance ? hows that even possible?
When you pass an Alternating Current through a wire (resistance), the current generates a magnetic field across the wire which opposes the flow of this AC along with the resistance of the wire. This oppose cause is known as Inductance. Inductance exists only in AC because the magnitude of the current is continuously changing.

Whereas, when you pass a Direct Current through a wire, the current does not generate any magnetic field as the magnitude of current is always constant in DC. Hence, there is no Inductance. So only the original resistance of wire comes into play.

Therefore, the resistance offered by a wire is greater for AC than DC.

kashmirihasan
and how can we differentiate between ac and dc resistance?
I'm not clear with this, I guess, the type of current you pass through the resistance determines the type of that resistance.
@kashmirihasan • 16 May, 2010 thanks i got your example of wire..........ok....should i take ac resistance when solving problems because in the problems they dont mention whether its ac resistance or dc resistance??
@gaurav.bhorkar • 17 May, 2010
kashmirihasan
thanks i got your example of wire..........ok....should i take ac resistance when solving problems because in the problems they dont mention whether its ac resistance or dc resistance??
If the current passed is AC then take AC resistance else if the current passed is DC, take DC resistance.
@lal • 17 May, 2010 My doubt is,
can a straight wire produce much inductance as mentioned above?
I agree that if it was a coil, then there would be good amount of inductive impedence. So the total impidence would have been higher than the original resistance of wire.
I think, as long as a straight piece of resistance wire is concerned, there won't be any difference in resistance wheter you pass dc or ac. But the opposition towards ac would be higher if you are considering a coil.
@gaurav.bhorkar • 17 May, 2010
lal
My doubt is,
can a straight wire produce much inductance as mentioned above?
I agree that if it was a coil, then there would be good amount of inductive impedence. So the total impidence would have been higher than the original resistance of wire.
I think, as long as a straight piece of resistance wire is concerned, there won't be any difference in resistance wheter you pass dc or ac. But the opposition towards ac would be higher if you are considering a coil.
Yes, a straight wire offers less impedance as compared to a coiled wire, but whenever an AC is passed through any type of wire, Impedance is bound to come into play. The impedance offered by a straight wire will obviously be less as compared to a coiled one. This is because a coil produces more magnetic field than a straight wire. That's why the lines in our homes are laid straight and it is always advised not to entangle wires.
@invincible hunk • 23 May, 2010
kashmirihasan
what is the difference between ac and dc resistance??? and why is there such a difference in both of them? i saw somewhere that ac resistance is 1.5 times dc resistance ? hows that even possible?
and how can we differentiate between ac and dc resistance?
basically, for every circuit you have a dc bias and an ac input(in most cases). so, when you are performing ac analysis you use ac resistance. and for dc analysis you use dc resistance. we generally nullify the effect of dc resistance on the output we use coupling capacitors to nullify dc. so o/p indepedent of dc.
@kashmirihasan • 24 May, 2010 thanks guys.....does ac resistance is 1.5 times of dc resistance? is that true?
i heard from some of my teachers it is more than that...
@RLSee • 24 May, 2010 • 1 like AC resistance is called impedance. In a wire there is an effect called skin effect. Skin effect is the result of the current distribution in the conductor. The current tends to flow nearer the surface and less near the center. The skin effect is proportional to the frequency. Also, the inductance of the conductor should not be forgotten. You can go here to see more information on skin effect. Skin effect - Wikipedia, the free encyclopedia.
@Naman Agarwal • 11 Mar, 2013 • 1 like AC resistance should NOT be confused with IMPEDANCE or REACTANCE.

When AC flows through a conductor, it tends to flow towards the surface of that conductor due to a phenomenon called the 'skin effect'. The skin effect, therefore, acts to reduce the effective cross-sectional area of the conductor. Since resistance is inversely-proportional to cross-sectional area, the result is an elevated value of resistance compared with that to DC current. This is called AC RESISTANCE, and it increases with frequency. At mains frequencies (50/60 Hz), AC resistance is not dramatically higher than DC resistance but the difference becomes significant at high frequencies.
@n.rajeev sharma • 27 Mar, 2013 i think resistance is same every where
@Jeffrey Arulraj • 28 Mar, 2013
n.rajeev sharma
i think resistance is same every where
Not exactly in AC there is additional resistance Due to skin effect My friend
@uche mojekwuu • 01 Feb, 2014
n.rajeev sharma
i think resistance is same every where
naman2511
AC resistance should NOT be confused with IMPEDANCE or REACTANCE.

When AC flows through a conductor, it tends to flow towards the surface of that conductor due to a phenomenon called the 'skin effect'. The skin effect, therefore, acts to reduce the effective cross-sectional area of the conductor. Since resistance is inversely-proportional to cross-sectional area, the result is an elevated value of resistance compared with that to DC current. This is called AC RESISTANCE, and it increases with frequency. At mains frequencies (50/60 Hz), AC resistance is not dramatically higher than DC resistance but the difference becomes significant at high frequencies.
IMPEDANCE IS USED FOR A DC RESISTANCE CIRCUIT WHILE RESISTANCE IS USED FOR AN AC CIRCUIT. Dont mixed them up okay? They are all resistance but in different fields of application
@Jeffrey Arulraj • 03 Feb, 2014
uche mojekwuu
IMPEDANCE IS USED FOR A DC RESISTANCE CIRCUIT WHILE RESISTANCE IS USED FOR AN AC CIRCUIT. Dont mixed them up okay? They are all resistance but in different fields of application
Sir you are a little out of the way Impedance comes for AC circuits and Resistance plays in DC circuits

Impedance Z = Resistance R + j X Reactance
@uche mojekwuu • 03 Feb, 2014 • 2 likes
Jeffrey Samuel
Sir you are a little out of the way Impedance comes for AC circuits and Resistance plays in DC circuits

Impedance Z = Resistance R + j X Reactance
Yeah jeffrey you are right. so what the questionnaire is asking is really not easy to answer in a lame language it take electrical/electronic calculations to prove is question but I think I have a simpler way to answer this. In a circuit or a system,basically electronics we have lots of components arranged in series and parallel in complexity combination of IC,CAPACITORS,RESISTORS,etc. and a whole lot of semi conductors and conductors wired to communicate with each other with gates and timers.But all this component gets their power source from Volts (AC/DC). The input power and current carrying in the system are being oppose by this component that's where resistance comes to play (system internal resistance)at each end of the component you can verify the current coming out from the end, summing all the output current of the circuit give you the total current consumed by that system.
@satyajit rules • 14 Feb, 2014
gaurav.bhorkar
Yes, a straight wire offers less impedance as compared to a coiled wire, but whenever an AC is passed through any type of wire, Impedance is bound to come into play. The impedance offered by a straight wire will obviously be less as compared to a coiled one. This is because a coil produces more magnetic field than a straight wire. That's why the lines in our homes are laid straight and it is always advised not to entangle wires.
here due to dc current magnetic field is also created. but that is static magnetic field. it cannot induce emf in other magnetuic materil. but the wire where dc current flows acts like a magnet. this was the basic principle of oersted's laws on static magnetic field which u must have aware of.

kashmirihasan
what is the difference between ac and dc resistance??? and why is there such a difference in both of them? i saw somewhere that ac resistance is 1.5 times dc resistance ? hows that even possible?
and how can we differentiate between ac and dc resistance?
the real cause of difference in ac & dc resistance is due to two causes
1. molecular study of material..in case of ac current, a magnetic field hence flux is produced around the wire or conductor. hence this magnetic field is dynamic unlike static magnetic field produced by dc current. hence it is capable of inducing emf in other magnetic materials. when current flows , the domains of materials where molecules are aligned in one direction are more. but due to ac this affects also the domains of sorrounding.hence the virtual size of the conductor changes. as size changes this causes change in ac resistance than dc resistance.
2. frequency of supply: as ac current flows conductor shows inductance property.. hence the impedance is taken into account instead of resistance :
Z=R+ jX
@Smarak Lenka • 06 Aug, 2015 In India the constant is 1.5 bcoz of the freq of 50hz and there is no specific analogy for this,it is just like a thumb rule
@Paul S. • 17 Dec, 2015 AC resistance, also known as effective resistance is often confused with impedance. Impedance is the vector combination of effective resistance and reactance due to inductance or capacitance in a circuit. However, the effective resistance itself is higher than the DC resistance due to the following: Skin effect- AC current tends to flow along the surface of a conductor while DC acts through the entire cross sectional area of a conductor in a uniform manner; Eddy current losses- currents that are induced in any metal near the conductor due to the alternating magnetic field; Hysteresis losses- the molecular friction caused by the changing direction of the magnetic field's lines of flux; Dielectric losses- a small heat loss through the insulation caused by voltage stress as the impressed voltage reaches maximum values twice per cycle. The combination of these 4 factors, plus the pure Ohmic resistance make up the AC effective resistance making it higher than the DC resistance. Normally when given a problem, it should be assumed that the resistance value given is proper for the type of voltage source unless a constant is given to distinguish between AC and DC resistance.
@Manohar Kalgunde • 30 Dec, 2015 I think, when AC is supplied, the ratio V/I is treated as impedance, This is not called as resistance of that wire. It is impedance of the wire. AC effects are considered in calling it ass 'IMPEDANCE'. Hence, i think logically it is incorrect to say as 'AC Resistance'. In fact resistance remains same in both cases AC & DC. What appears with AC supply is Resistance & Inductance (AC effect) together, therefore, it should be called impedance instead of AC resistance. This inductive effect is absent when we give DC supply to same wire.
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