Alternating current cannot be measured by dc ammeter because, (a) ac cannot pass through dc ammeter (b) Average value of complete cycle is zero (c) ac is virtual (d) ac changes its direction

Alternating current (AC) is an electrical current that changes its direction periodically, whereas direct current (DC) only flows in one direction. AC is commonly used for power transmission and distribution, while DC is employed in batteries, electronic devices, and other applications where a constant voltage is required.

A DC ammeter is an instrument designed to measure the flow of direct current in a circuit. It operates on the principle of measuring the voltage drop across a known resistance (shunt resistor) proportional to the current flow. Since a DC ammeter is specifically designed to measure direct current, it cannot be used to accurately measure an alternating current.

(a) ac cannot pass through dc ammeter: This statement is not entirely accurate. AC current can pass through a DC ammeter; however, the measurement will not be correct due to the alternating nature of AC current. (b) Average value of complete cycle is zero: This is the correct answer. The average value of an AC waveform over a complete cycle is zero, as the positive and negative portions of the waveform cancel out each other. A DC ammeter mainly measures the average current, which will be zero for AC. (c) ac is virtual: This statement is incorrect. AC is not virtual; it is a real form of electrical current used in many applications, such as power transmission and distribution. (d) ac changes its direction: While this statement is true, it doesn't explain why a DC ammeter is unable to measure AC current. The main reason why AC cannot be measured by a DC ammeter is due to the averaging effect on a complete cycle, not simply because it changes direction.

The correct answer to the question, "Alternating current cannot be measured by dc ammeter because" is option (b): Average value of complete cycle is zero. The DC ammeter measures the average current, and since the average value of an AC waveform over a complete cycle is zero, the DC ammeter will not provide an accurate measurement of alternating current.