Ohm's Law

• A fundamental rule of electricity, Ohm's Law states that resistance in ohms equals a circuit's voltage in volts divided by its current in amperes. It also says that you can calculate power by multiplying current by voltage, or by multiplying the resistance by the square of the current.
  
  

DC

• You can easily work with resistance in DC circuits because DC is a steady, constant flow of current. The relationships between resistance, voltage, current and power come directly from Ohm's Law.
  
  

AC

• In contrast to DC's constant flow, AC alternates between negative and positive peak values 60 times a second. The 110 volt AC is 110 volts only at its momentary maximum. Though Ohm's Law applies to AC at any given instant, you determine power and heat by finding an average current value.
  
  

RMS Power

• To calculate the true power in an AC circuit, you first multiply the square of .707 by the current, then multiply by resistance. This is called the root mean square (RMS) value. This averages the peaks of AC current.
  
  

Resistors

• You don't need different resistors for AC and DC circuits. The materials resistors use respond instantly to changes in voltage and current, so they can handle either steady DC or alternating AC.