The difference between instantaneous power and average power. The distinction is important because we use AC power. This means that since the current is alternating, then the power is also cycling and only reaches some average level after some time. This is illustrated below.

You can see in the above that in AC power, the voltage is continuously varying so that one can define a Peak Voltage and an Average Voltage. The average voltage is usually called the root mean square (rms voltage). Note that 1.414 is the square root of 2.

V_rms = V_peak/1.414

Current, I, varies in a similar way as the Voltage such that power curve for AC would look like this:

Now let's consider this situation of powering a speaker from an amplifier.

• Peak Current = V/R = 34/8 =4.25
  
  
• Peak Power = V x I = 34 x 4.25 = 144.5
  
  
• V_rms = 34/1.414 = 24V
  
• I_rms = 4.25/1.414 = 3A
  
• P_rms = VxI = 24x3 = 72 Watts

  
  
  Below are some example circuits. You should study them in some detail.