Before starting this module, you should be able to: When you complete this module, you should be able to: 
  • Cite Ohm's Law for resistance.
  • Explain the meaning of each term in the reactance equation,   XC = 1 / (2pfC).
  • Cite Ohm's Law for capacitive reactance.
  • Explain how Ohm's Law for XC is similar to Ohm's Law for R.
  • Use Ohm's Law to solve for voltage, current, or capacitive reactance for a capacitor.
  • Solve Ohm's Law for capacitance, given values of f and L rather than XC.

 
Topic 11-4.1 Basic Application of Ohm's Law for XC  

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Ohm's Law applies directly to an capacitor:

VC = ICXC

where: 

    VC = voltage across the capacitor 
    IC = current through the capacitor 
    XC = capacitive reactance

     

  

Just as resistance opposes the flow of current through a resistor, capacitive reactance opposes the flow of current through a capacitor circuit.

  • Ohm's Law for a resistor:   V = IR   

  • Ohms' Law for an capacitor:   VC = ICXC

 

lblexa.gif (1250 bytes) For a certain capacitor, IC = 20 mA and XC = 420 W. What is the voltage across this capacitor? 

Ans: 8.4 V

Solution: 

This is a straightforward application of Ohm's Law for XC.  VC = ICXC

 

lblexa.gif (1250 bytes) What is the current through a capacitive reactance of 12 kW when the voltage across it is 12.6 V? 

Ans: 1.05 mA

Solution: 

Given the values for XC and VC, use this form of Ohm's Law to solve for IC:  IC= VC / XC  

 

lblexa.gif (1250 bytes) The current through a capacitor is 250 mA when 16 V is dropped across it. What is the value of XC? 

Ans: 64 W

Solution: 

Given the values for IC and VC, use this form of Ohm's Law to solve for XC: 

XC= VC / IC

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Topic 6-4.2 Ohm's Law when XC is Not Known 

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Given: 
 
XC = 1 / (2pfC) Basic definition of capacitive reactance.
VC = ICXC Ohm's Law for capacitive reactance.

 It follows that: 

    VC =IC / (2pfC)

 There aren't many electronics labs that are equipped to measure the XC of a capacitor directly. Instead, XC is usually calculated from the value of the capacitor and the frequency of the sine waveform applied to it.  

So when you want to use Ohm's Law to determine the current of voltage across a capacitor, you must first calculate the value of XC, then solve for the current or voltage. 

    Step 1: Calculate XC = 1 / (2pfC) 
    Step 2: Substitute the calculated value of XC into Ohm's Law, VC = ICXC

Or you can combine the two equations to produce the single equation as shown on the left.

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