Question

In a circuit when ac ammeter is connected it reads I current as if a student uses dc ammeter in place of ac ammeter the reading in the dc ammeter will be
A:\[\dfrac{i}{{\sqrt 2 }}\]
B:\[i\]
C:\[0.637i\]
D:\[zero\]

Answer 1

Hint: DC ammeter is a device used to measure the direct current.
As the direction of DC does not change, the ammeter doesn’t average the value over a half cycle.
AC changes its direction periodically.

Complete step by step answer:
To solve this question, we need to have a proper idea about alternating current, direct current and ac and dc ammeter.
Alternating current like the name says is an electric current which alternates or in other words which periodically reverses directions and changes its magnitude with change in time.
On the other hand, direct current(dc) is unidirectional, it is not mandatory that its magnitude does not change but the direct current remains the same. Which is why if we average it’s value in time it can either be positive or zero but never negative.
An AC ammeter is the device which is used to measure the value of alternating current. But as we know the magnitude as well as direction of an Alternating current varies in time periodically and thus measuring the instantaneous value would not be of much use as it would continuously fluctuate.
Hence, an AC ammeter is designed in a manner as to average the value of the alternating current in every half cycle. If it averaged the value in a full circle the average would contain current in both directions and net average would be zero, which is why the averaging is done on a half cycle. So, the student in the question obtained I because I is the average current of AC in the half cycle.
But unlike the ac meter the dc ammeter is not averaging the value of the half cycle, because in direct current the direction of current doesn’t change and thus we do not face the problem of receiving a zero reading on averaging on a full cycle.
Therefore, when a DC ammeter is connected to AC the ammeter would show a zero reading as the average current is zero in AC.

So, the correct answer is “Option D”.

Note:
The ac ammeter if connected to DC would not show zero reading.
Because the average value in time would not be zero.
Alternating current can be converted to direct current with the help of a rectifier.
The value in a full circle the average would contain current in both directions.