Ohm's Law for Electrical Circuits
The most fundamental equation in electrical circuits is called Ohm's Law. While doing experiments on how well metals conducted electricity, German physicist Georg Ohm discovered the law in 1827. Ohm's Law is the equation V = I R and is used in both AC and DC circuits. Knowing two items in this equation allows you to calculate the third.
Questions you may have include:
- What do the parameters in the equation mean?
- What are AC and DC circuits?
- How can you use Ohm's Law?
Parameters
Ohm's Law states that in a simple electrical circuit, the voltage equals the electrical current times the resistance.
V = IR
where:
V is the voltage in volts
I is the current in amperes or amps
R is the resistance in ohms
IR is I times R
Voltage
Voltage is the electrical potential energy and is measured in volts.
A good analogy is to think of a water hose. There is water pressure or potential energy on the other side of the faucet or outlet valve. Once you open the faucet, the pressure causes the water to rush through the hose.
The unit symbol for volts is V, as in 110V.
Current
Current indicates the amount of electrons passing through the wire and is measured in amperes or amps for short. For some reason, they use I to indicate current instead of a different letter. The unit symbol for amps is A, as in 2.0A.
Electrical current is similar to the rate of water flowing through a hose.
Resistance
Electrical resistance can be thought of as the "friction" on the movement of electrons in a wire. Resistance is measured in ohms, and the unit symbol for it is the Greek letter omega, Ω. Thus 3 ohms is often written as 3 Ω.
Most devices in an electrical circuit can be considered resistors, including light bulbs and electric motors. Even the wire itself provides some resistance. Just as you get some heat from friction, electrical resistance also results in heat. That is why the light bulb filament gets hot and glows.
Following the water hose analogy, resistance is similar to the friction inside the hose. But also, the resistance increases with a narrower hose, just like a thin copper wire has more electrical resistance than a thick wire.
Circuits
Ohm's Law applies to electrical circuits. The equation V = IR says that in a simple electrical circuit with a source of resistance—such as a resistor or a light bulb—the voltage equals the current times the resistance. This is true in a DC circuit and in an AC circuit.
Ohm's Law applies whether the current flows in one direction or alternates.
AC circuit
A simple AC circuit is illustrated below. A circle with the sine wave symbolizes an AC generator with some given voltage.
The resistor could be a single light bulb or an electrical motor. Or it could be a combination of resistive devices, such as several bulbs. Complex AC sub-circuits that include capacitors and inductors, besides resistors, are not being discussed in this lesson.
DC circuit
The power source for a DC circuit could be a battery or DC generator. The (+) and (−) indicate the direction of the current.
Using equation
The importance of Ohm's Law is that if you know the value two of the variables in the equation, you can then determine the third.
You can measure any of the parameters with a voltmeter. Most voltmeters or multi-meters measure voltage, current and resistance for both AC and DC.
Find voltage
If you know current and resistance, you can find voltage from V = I R. For example, if the current I = 0.2A and the resistance R = 1000 ohms, then
V = 0.2A * 1000 Ω = 200V
Find current
If you know voltage and resistance, you can use algebra to change the equation to I = V / R to find the current. For example, if V = 110V and R = 22000 ohms, then
I = 110V / 22000 Ω = 0.005A
Find resistance
If you know voltage and current, you can use algebra to change the equation to R = V / I to find the resistance. If V = 220V and I = 5A, then
R = 220V / 5A = 44 Ω
Summary
Ohm's Law is the equation V = I R that shows the relationship between voltage, current and resistance in a simple electrical circuit. It applies both the AC and DC circuits.