Before starting this module, you should be able to: When you complete this module, you should be able to: 
  • Define the terms voltage and current.
  • Cite the units of measure for voltage and current.
  • Describe the difference between DC and AC voltage and current.
  • Describe the shape and main features of a sinusoidal waveform.
  • Calculate the instantaneous value of a current or voltage sine waveform, given the maximum value and angular displacement.

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DC Current vs. AC Current  
 
  • Direct current (DC) flows in one direction the circuit. 
  • Alternating current (AC) flows first in one direction then in the opposite direction.

The same definitions apply to alternating voltage (AC voltage): 

  • DC voltage has a fixed polarity.
  • AC voltage switches polarity back and forth.
There are numerous sources of DC and AC current and voltage. However: 

Sources of DC are commonly shown as a cell or battery:  

Sources of AC are commonly shown as an AC generator: 

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The Sinusoidal AC Waveform 

The most common AC waveform is a sine (or sinusoidal) waveform. 
 

The vertical axis represents the amplitude of the AC current or voltage, in amperes or volts. 

The horizontal axis represents the angular displacement of the waveform. The units can be degrees or radians. 
 

The sine waveform is accurately represented by the sine function of plane trigonometry: 

y = rsinq

where:  

y = the instantaneous amplitude
r = the maximum amplitude
q = the horizontal displacement

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Instantaneous Current and Voltage 
 
i = Ipsinq

where  

i = instantaneous current in amperes 
Ip = the maximum, or peak, current in amperes 
q = the angular displacement in degrees or radians

v = Vpsinq

where 

v = instantaneous voltage in volts 
Vp = the maximum, or peak, voltage in volts 
q = the angular displacement in degrees or radians

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