Before starting this module, you should be able to: When you complete this module, you should be able to: 
  • Describe Ohm's Law for resistance, and explain how the value of resistance affects the amount of current flowing through a DC circuit.
  • Cite the equation for determining the value of inductive reactance, given the values of applied frequency and inductance.
  • Note: Use the BACK function of your browser to return to this page. 
  • Cite Ohm's Law for inductive reactance.
  • Explain how Ohm's Law for XL is similar to Ohm's Law for R.
  • Use Ohm's Law to solve for voltage, current, or inductive reactance for an inductor.
  • Solve Ohm's Law for inductance, given values of f and L rather than XL.

Topic 6-4.1 Basic Application of Ohm's Law for XL

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Ohm's Law applies directly to an inductor: 

VL = ILXL

where: 

    VL = voltage across the inductor 
    IL = current through the inductor 
    XL = inductive reactance

 
Just as resistance opposes the flow of current through a resistor, inductive reactance opposes the flow of current through an inductor.

Ohm's Law for a resistor:

V = IR 

Ohms' Law for an inductor 

VL = ILXL

 

 

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

Ans: 8.4 V

Solution: 

This is a straightforward application of Ohm's Law for XL

VL = ILXL

 

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

Ans: 1.05 mA

Solution: 

Given the values for XL and VL, use this form of Ohm's Law to solve for IL

IL= VL / XL  

 

lblexa.gif (1250 bytes) The current through an inductor is 250 mA when 16 V is dropped across it. What is the value of XL

Ans: 64 W

Solution: 

Given the values for IL and VL, use this form of Ohm's Law to solve for XL

XL= VL / IL

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

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XL = 2pfL Basic definition of inductive reactance.
VL = ILXL Ohm's Law for inductive reactance.

It follows that: 

VL =2pfL IL

There aren't many electronics labs that are equipped to measure the XL of an inductor directly. Instead, XL is usually calculated from the value of the inductor 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 an inductor, you must first calculate the value of XL, then solve for the current or voltage. 

    Step 1: Calculate XL = 2pfL 
    Step 2: Substitute the calculated value of XL into Ohm's Law, VL = ILXL.

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

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