Question

Alternating current cannot be measured by D.C ammeter because:

Answer 1

In this question, we will try to find the answer mathematically also, by using the common relation between the alternating current, amplitude and time. This solution will help us to get our required result. Also, we will study the basics of a simple electric circuit, for our better understanding.

Formula used:
$I = A\sin \left( {\omega t} \right)$

Complete solution:
As we know an alternating current keeps on changing its polarity periodically. When we see the positive half cycle, the alternating current has positive polarity whereas when we observe for in the negative half cycle, the alternating current has negative polarity.
So, the average value of an alternating current for one complete cycle is zero.
Let us understand with the help of the expression of current, given by:
$I = A\sin \left( {\omega t} \right)$
Now, taking the average of current we get:
${I_{avg}} = \int_0^T {\dfrac{{A\sin \left( {\omega t} \right)dt}}{T}}$
By integrating the above equation with respect to time t, we get:
${I_{avg}} = \dfrac{A}{T}\left[ { - \cos (\omega t)} \right]_0^T$
\eqalign{ & \Rightarrow {I_{avg}} = \dfrac{A}{T}\left[ {\cos (0) - \cos \dfrac{{2\pi }}{T}T} \right] \cr & \therefore {I_{avg}} = 0 \cr}

Therefore, as we get the average value of alternating current for one complete cycle as zero, we cannot measure it from as D.C ammeter. So, we get the required result.

Additional information:
As we know that in a series circuit, the output current of the given first resistor flows into the input of the given second resistor. So, we can say that the current is the same in each resistor. As shown in the diagram below.

Also, in a given parallel circuit, all of the resistors are connected together on one side of the circuit and all the leads on the other side are connected together. As shown in below diagram:

In a given circuit if the resistance is constant over a certain range of voltage, then current I is given as: I = V/R, this relation can be used to find the behavior of the given material. This involves DC current and voltage, it is the same for the resistors. Further, a material obeys Ohm's law or the material does not obey Ohm's law; so, the resistance of the material is described in terms of its bulk. The resistivity, and the resistance both, is temperature dependent. Also, for certain ranges of temperature, the temperature dependence of the material can be predicted from the resistance.

Note:
We should remember that in high voltage transmission, we cannot step-up the D.C voltage. Also, D.C switches and circuit breakers are limited and they are expensive compared to A.C switches and circuits. Also, in series connection the resistances are directly added whereas in parallel connection resistance is added inversely.