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 a capacitor.
  • Solve Ohm's Law for inductance, given values of f and L rather than XL.
  • Describe the fact that the current of a capacitor always leads the voltage across the capacitor by 90 degrees.
  • Sketch a vector diagram showing how the current leads the voltage.

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Capacitor Current and Voltage
  • Attempting to change the amount of voltage applied to a capacitor instantaly causes a corresponding change in current. 
  • Due to the basic properties of capacitance, however, the voltage across a capacitor cannot change instantly.
  • Therefore, changes in capacitor current always lead the corresponding changes in capacitor voltage.

Notes: 

When the applied voltage is a sinusoidal waveform, the current waveform is also a sinusoidal waveform. 

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

 

lbldef.gif (1302 bytes)

The current of a capacitor leads the voltage applied to the capacitor by 90° 

 

It is also correct to say that the voltage applied to a capacitor lags the current by  90° 
 
The phase difference between capacitor current and voltage can also be shown with a vector diagram:

 

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