IIT JEE , AIEEE Forum - Physics , Magnetism

astrojith

I know this is a lame question for you geniuses. But, I'm curious why this is so. Why can't DC ammeters measure AC ?

spideyunlimited

DC ammeter is designed to indicate amount of current flowing in one direction whereas in the case of AC the current reverses its direction in 1/50th of a second.. So the DC ammeter is not appropriate for measuring AC. Ac ammeters are used which are designed on a different internal working principle.

astrojith

Hey, are AC ammeters using rectifiers to trim those waves or is it completely different. Would appreciate if you can provide link to some resource or just tell me what I've to search at Google.

Googling is kinda like an art. You need the right keywords to get the right thing ;)

rhd92781

the mean value of ac current in one cycle is zero. dc ammeters measure the mean current so the fail to measure ac.

ac ammeters mostly use thermal effect of ac and show the rms value of ac current

astrojith

That maybe it, rhd. Lemme confirm. Rated you anyway. :)

spideyunlimited

LINK:

http://en.wikipedia.org/wiki/Ammeter

see moving iron ammeter inside this page... it is the second type of ammeter after moving coil ammeter and can measure both AC and DC

astrojith

It was good but I'm looking for the reason why DC ammeters can't measure AC currents. Rahul's answer was the closest. I'm digging deeper.

kishan12

They can measure yaar!!!!!!!!!!!!!!
but the problem wud be the needle wud keep deflecting as the polarity is changing and we wud nevr see a constant reading.
thats it so AC ammeters are designed.

spideyunlimited

that was what i said earlier ...bataya toh. u wont get a fixed value the needle will be oscillating between 0 and peak value of I

astrojith

Guess what ?! I found the answer from "Dinesh New Millennium Physics 2008". Here's how it goes :-

" Average value of AC over a complete cycle is zero, so we cannot measure a.c with DC ammeters. On the other hand, both depend on the heating effects of electricity. So, hot wire ammeters can be used to measure AC as well as DC."

Great !! Kudos Rahul. I guess you got it right. Answer accepted.

kishan12

Average value of AC over a complete cycle is zero, so we cannot measure a.c with DC ammeters

Hey spidey agree with this.i dont.....

spideyunlimited

Average value of AC over a complete cycle is not zero yaar ! ... Average value of AC is taken as I rms which is approx 0.693 of peak value of I ( Io).

spideyunlimited

and moving iron ammeter measures DC as well as AC from the same device!

rhd92781

average value of I over one complete cycle and rms value of I r two different things

Iav=0
Irms=Io/root(2)

Ask & Discuss Questions with Community & Experts