Before starting this module, you should be able to: When you complete this module, you should be able to: 
  • Cite the units of measure for capacitance.
  • Explain how a capacitor can oppose current flow in an AC circuit.
  • Cite the equation for determining the value of capacitive reactance, given the values of applied frequency and capacitance.
  • Solve the equation, given two of the three variables.

 

lbldef.gif (1302 bytes)

The equation for calculating the amount of capacitive reactance in an ac circuit is given by: 
 

    equ110202.gif (1031 bytes)

where: 

    XC = capacitive reactance in ohms  
    f = frequency in hertz 
    C = capacitance in farads 

This equation demonstrates the relationship between capacitive reactance (XC), the frequency (f) of the waveform applied to the circuit, and the value of the capacitance (C). 

The amount of capacitive reactance (XC) changes inversely with the applied frequency (f):

Increasing frequency causes XC to decrease.   

Decreasing frequency causes XC to increase.

 

The amount of capacitive reactance (XC) changes inversely with the value of capacitance (C):   

Increasing capacitance causes XC to decrease.   

Decreasing capacitance causes XC to increase.

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lblexa.gif (1250 bytes) What is the value of capacitive reactance for a 10 nF capacitor operating in a 200 kHz circuit? 

Ans: 79.6 W 

Hints  Use the basic form of the equation:

equ110201.gif (1031 bytes)

 

lblexa.gif (1250 bytes)  What value of capacitance is required for producing a capacitive reactance of 120 W when 1.2 MHz is applied to it? 

Ans: 1.1 nF 

Hints 

Rearrange the basic equation: 

equ110202.gif (1002 bytes)

 

 

lblexa.gif (1250 bytes) At what frequency will the XC of a 1 mF capacitor be 1.2 kW

Ans: 133 Hz 

Hints 

Rearrange the basic equation:

equ110201.gif (1013 bytes)

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