Question

Incorrect statement are:
(a) A.C. meters can measure D.C. also
(b) If A.C. meter measures D.C. their scale must be linear and uniform
(c) A.C. and D.C. meters are based on the heating effect of current.
(d) A.C meter reads rms value of current.

(A) a, b
(B) b, c
(C) c, d
(D) d,a

Answer 1

A D.C. meter measures the D.C. current or voltage across a circuit and an A.C. meter measures the A.C. current or voltage across a circuit. D.C current flows only in one direction while A.C. reverses its direction in intervals.

Complete step by step answer: First consider the statement (a) “A.C. meters can measure D.C. also”.

We generally use two types of A.C ammeters for current measurements. One is the moving iron type and the other one is the electrodynamometer type. Both the meters give the reading of the rms value of the A.C. current. The rms value of the A.C. is the equivalent value of the corresponding D.C measurement. Thus, in D.C., the rms value is the average value of the current itself. Hence, if we connect a DC current source to the A.C. meter, it will give the reading of the D.C. current. Therefore, we can say that A.C meters can give measurements of D.C. also.

Thus, the statement (a) is correct.

Now, let us consider the statement (b) “If A.C. meter measures D.C. their scale must be linear and uniform”.

In an A.C ammeter of the moving iron type, the deflection depends on the square of the current passing through it. Therefore, the scale for the meter is usually non-linear. As we know, the A.C. meter measures the rms value which is equivalent to the D.C. value. Only the average value of the D.C. is measured. So, we can say that an A.C. meter need not be linear and uniform for it to measure D.C.

Thus, the statement (b) is incorrect.

Now, we consider the statement (c) “A.C. and D.C. meters are based on the heating effect of current”.

A.C. and D.C meters work based on the torque created by the current. The needle in the meters deflects depending on the amount of torque experienced by the coil inside them. As said, the amount of torque depends on the amount of current. Hence, the heating effect of the current is not the principle used in the working of A.C. and D.C. meters.

Thus, the statement (c) is incorrect.

Finally, let us check the statement (d) “A.C meter reads rms value of current”.

As explained above, A.C. meter measures the root mean square value of the current or the rms value of the current.

Thus, the statement (d) is correct.

Since only statements (a) and (d) are correct, we find that option (D) is correct.

Note: It should be noted that while an A.C. meter measures the rms value of current, A D.C. meter measures the peak value of the current. Also, moving iron ammeters can be used to measure both D.C. and A.C. currents while moving coil ammeters can measure only D.C.