Electricity follows certain rules that do not change. The most useful rule is called Ohm's Law, usually stated as a simple equation

E = I ´ R

Where

E = Voltage
I = Current
R = Resistance

Before proceeding, we need to understand some electrical terminology. E stands for Voltage, the electrical pressure causing electrons to flow in a circuit. The higher the voltage the greater the pressure that causes electrons to flow. An electron is the smallest known particle present in every atom. A circuit is the path for an electrical current. The most familiar circuits are the wires in our houses and cars which connect bulbs, motors, and appliances to a power source.  These power sources can be a battery or the power company's electrical lines and generators. The I in the Ohm's Law equation stands for Current, the flow of the electrons in a circuit. Current is measured in amperes or amps, for short. Finally, the R stands for Resistance, the opposition to the flow of electrons or current in a circuit; it is measured in ohms.

So with this information, we can apply the mathematical relationships in electricity known as Ohm's Law. If you know two of the items in the equation you can solve for the third.

How much current is used in a car if a light bulb with 50 ohm's resistance has a voltage of 12 volts in the circuit ?

E = I ´ R Solving for I or current, we can change the equation to:

I = E / R

I = 12 volts / 50 ohms

I = 0.24 amps or 240 milliamps

If a current of 2 amps flows through a heater having a 50 ohm resistance, what is the voltage across the heater ?

Estimate the answer:

Less than 60 volts
Between 60 and 120 volts
Over 120 volts