Before starting this module, you should be able to: When you complete this module, you should be able to: 
  • 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.
  • Describe the fact that the current through an inductor always lags the voltage across the inductor by 90 degrees.
  • Sketch a vector diagram showing how the current lags the voltage.


  • Changing the amount of voltage applied to an inductor causes a corresponding change in current through the inductor. 
  • Due to the property of self-inductance, however, changes in inductor current always lag behind the changes in applied voltage.
  • When the applied voltage is a sinusoidal waveform, the voltage is changing constantly and the current is constantly lagging behind.


Unless stated otherwise, discussions of inductive reactance in AC circuits assume a sinusoidal voltage source.


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The current through an inductor lags the voltage applied to the inductor by 90 


It is also correct to say that the voltage applied to an inductor leads the current through the inductor by  90 

The 90 phase difference between current and voltage of an inductor applies only to ideal inductors—inductors that have no internal resistance. The internal resistance of real-world inductors causes the phase difference to be something less than 90.   

The phase difference between inductor current and voltage can also be shown with a vector diagram: