The unit of resistance is called Ohm in honor of a German scientice by the name of Georg Simon Ohm, who discovered that when a conductor has a resistance of 1 ohm than an emf of 1 volt will cause a current of 1 amp to flow through a conductor.
Ohms Law. I = E/R or R = E/I or E = IR.
DC. or  Direct Current  is one that the current always flows in one direction.
The letter for Current is I and is expressed in amperes.
The letter for voltage is E and is expressed in volts.
The letter for resistance is R and is expressed in ohms.
The letters for power or wattage are P and W and are expressed power and watts.
The formulas for ohms law are.
If you know R the resistance and E the voltage the formula for finding I current is I = E/R.
If you know E the voltage and I the current the formular for R resistance is R =E/I.
If you know I the current and R the resistance the formula for E voltage is E = IR.
If you know I the current and E the voltage the formula for P power or W watts is P =EI.
If you know W watts and I current the formula for E voltage is E = P/I.
If you know W Watts and E voltage the formula for I current is I = P/E.
 Example<>If you have a 1200 watt or 1.2 KVA electrical device and the voltage is 120 volts, the amperage rating for this device is approximately 10 amps. I amperage = P or watts divided by E voltage. The K in KVA stands for 1,000.
 AC. or  Alternating Current  is one that the current periodically changes directions.
The formulas for AC current involve impedance, effective value and  Power Factor. But for home devices such as lights and appliances, there would not be much of a change when using the Ohms Law formulas above.