Question

٢٠) قراءة الأميتر الحراري في دائرة تيار متردد لمثل .......

• A. القيمة المتوسطة لدورة كاملة من دورات التيار المتردد.
• B. القيمة العظمى للتيار المتردد.
• C. القيمة اللحظية للتيار المتردد.
• D. القيمة الفعالة للتيار المتردد.

Answer 1

Answer: (D)

The reading of a hot-wire ammeter in an AC circuit represents the القيمة الفعالة للتيار المتردد (The RMS value of the alternating current).

Answer 2

A hot-wire ammeter operates based on the heating effect of electric current. The heat produced in a resistor is proportional to the square of the current ($H \propto I^2$). In an alternating current (AC) circuit, the current continuously changes direction and magnitude. Due to the rapid oscillations, the needle of the ammeter cannot follow the instantaneous variations. Instead, it responds to the average heating effect over a cycle. The average of the square of the current over a full cycle is related to the Root Mean Square (RMS) value of the current. Therefore, a hot-wire ammeter measures the RMS (effective) value of the alternating current. The average value of AC over a full cycle is zero, so it cannot be measured. The peak value and instantaneous value are not what a hot-wire ammeter directly indicates.