In a Capacitive Circuit, Why the Current Increases When Frequency Increases?

Why the Current (I) Increases, When Frequency Increases in a Capacitive Circuit & Vice Versa?

Another question from electrical and electronics engineering interviews question and answers series.

Explain the statement that “In a capacitive circuit, why the circuit current increases when frequency increases“.

Related Questions:

• Why Current Increases When Capacitance Increases or Capacitive Reactance Decreases?
• Why Current Decreases When Inductance or Inductive Reactance Increases?

Explanation:

We know that in DC circuits:

I = V / R,

But in case of AC circuits:

I = V / Z

Where “total resistance of AC circuits = Impedance = Z = √ (R² + (X_L – X_C²)”

In case of a capacitive circuit:

• Z = √ (R² + X_L²)
• I = V / X_C or I = V / Z

It shows that in a capacitive circuit, Current is directly proportional to the capacitance “C” and inversely proportional to the capacitive reactance as capacitance and capacitive reactances “X_C” are inversely proportional to each others.

Related Questions:

• Which Transformer is More Efficient When Operates on 50Hz or 60Hz?
• Can We Operate a 60Hz Transformer on 50Hz Supply Source and Vice Versa?
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Let’s check with an example to see how current increases by increase in frequency in case of a capacitive circuit.

When Frequency = 5 µF

Suppose a capacitive circuit where:

• Voltage = V = 3000 V
• Capacitance = C = 5 µF
• Frequency = f = 50 Hz

To find the capacitive reactance;

X_C = 1 / 2πfC

X_C = 1 / (2 x 3.1415 x 50 x 5×10^-6)

X_C = 636.94 Ω

Now, current in the capacitive circuit:

I = V / X_C

I = 3000 V / 636.94 Ω

 I = 4.71 A 

Related Questions:

• Why Power Factor Decreases When Inductance or Inductive Reactance Increases?
• Why Power Factor Decreases When Capacitive Reactance Increases or Capacitance Decreases?

When Frequency = 60 Hz

Now we increased the frequency form 50Hz to 60Hz.

V = 3kV,  C = 5 µ Farads,  f = 60 Hz.

X_C = 1 / 2πfC = 1 / (2 x 3.1415 x 60 x 5×10^-6) = 530.63Ω

I = V / X_C = 3 kV / 530.63Ω

 I = 5.65 A 

Conclusion:

We can see that, When frequency was 50Hz, then the circuit current were 4.71 A,

But when circuit frequency increased from 50Hz to 60Hz, then the current increases as well from 4.71 A to 5.65 A.

Hence proved,

In a capacitive circuit, when frequency increases, the circuit current also increases and vice versa.

f ∝ I

• Related Post: In an Inductive Circuit, Why the Current Increases When Frequency Decreases?

In oral or verbal,

• Capacitive reactance is a kind of resistance. When resistance increases, the circuit current decreases and vice versa.
• Capacitance is inversely proportional to the capacitive reactance and frequency.

C ∝ 1 / X_C   and   C ∝  1 / f

• Current is directly proportional to the capacitance and inversely proportional to capacitive reactance and impedance.

I ∝ C      and      I ∝ 1 / X_C   and   I ∝ 1 / Z

• Impedance is directly proportional to the capacitive reactance

Z ∝ X_C

• In a capacitive circuit, Frequency is directly proportional to the current.

I ∝  f

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