Question

Of the following about capacitive reactance which is correct,
A. The reactance of the capacitor is directly proportional to its ability to store charge.
B. Capacitive reactance is inversely proportional to the frequency of the current.
C. Capacitive reactance is measured in Farad.
D. The reactance of a capacitor in an AC circuit is similar to the resistance of a capacitor in a DC
circuit.

Answer 1

Hint: First we should know the definition of capacitive reactance and its characteristics. Its measuring unit is in ohms like resistance and inductive reactance.

It can be explained using the above figure. A capacitor is comprised of normally two or more
electrically conducting surfaces called plates, insulated from each other by a material called a
dielectric. Materials like air, paper, mica and oil are called dielectrics. When current flows the
electrons accumulate in one plate more and once the plate becomes saturated with electrons. The plate is said to be fully charged. The amount of charge a capacitor can store is called capacitance
and is measured in Farads or micro farads.
The opposition to the flow of alternating current due to capacitance is called capacitive
reactance. It is measured in ohms just like resistance and inductive reactance. In an AC circuit,
the voltage is sinusoidal and is continuously changing from a positive cycle to a negative cycle at
a rate which is determined by the supply frequency. It is denoted by ${X_c}$and is inversely
proportional to the supply frequency and the capacitance $C$. The expression for capacitive
reactance is given as,

${X_c} = \dfrac{1}{{2\pi fC}}$
Where, f is the supply frequency, C is the capacitance.
It can be observed that capacitive reactance is inversely proportional to the frequency.
Hence, the correct answer is (B).
Note: In the solution, the students can understand the theory of capacitance reactance and its
characteristics from the relation of capacitive reactance with frequency and capacitance.Even Though Capacitance and resistance have the same units, they are entirely different quantities.