There is a lot of confusion among most people without an electrical background as to what voltage, current, and resistance mean. It’s typical to hear people say things such as, “this battery has a current of 12 volts”. I will try to explain in simple terms what these three fundamental terms mean.
Voltage
Voltage (V) is also called potential difference or electrical pressure and is measured in volts (V). The concept of voltage can be explained using the concept of a water tower. A water tower’s purpose is to supply homes with water at a sufficient pressure. The water tower is generally taller than the homes it supplies. When the water inside it is released, it will flow through the pipes and out of the faucets. If the water tower was at the same height as the houses it supplies, there would be no flow since there is no difference in pressure.
Water held high inside the water tower contains potential energy. The higher the tower, the higher the potential energy and the higher the pressure of the water coming out of the faucets. This pressure difference between the water tower and the house is analogous to voltage. Voltage is defined as the potential difference between two points. In a battery, you would measure the voltage between the positive (+) terminal and the negative (–) terminal. Voltage does not exist at a point by itself; it is always measured with respect to another point.
Current
Electric current (I) is the flow of electrons through a conductor and is measured in amperes or amps (A). The conductor is usually a wire but can be any conductive material such as electrolyte, graphite, plasma, semiconductors, etc. Going back to the water tower example, current is analogous to the water flowing through the pipe. There are two major factors determining the flow of water in a pipe:
- The water pressure.
- Piping restrictions such as pipe diameter, pipe internal smoothness, number of bends in the pipe, etc.
If water pressure is increased, the flow of water is increased. This is what happens when you increase voltage; current goes up by the same factor. If you increase the voltage by a factor of 2, then the current will also increase by a factor of 2 if resistance remains the same. Thus voltage and current are directly proportional. The piping restrictions are analogous to the conductor resistance which brings us to the next concept.
Resistance
Resistance (R) is the opposition to current flow through a conductor and is measured in ohms (Ω). Using the water flow analogy, the pipe represents the wire and piping restrictions represent electrical resistance. You can reduce piping restrictions by increasing the size of the pipe, using a different pipe material, reducing the number of bends, or using a shorter pipe. Likewise in an electrical conductor you can reduce resistance by increasing the thickness of the conductor, using copper instead of steel which reduces resistance by a factor of 8.5, reducing the number of splices, or using a shorter wire. Current and resistance are indirectly proportional. That means if you increase resistance by a factor of 2, current decreases by a factor of 2 if voltage remains the same.
Ohm’s Law
George Simon Ohm (1787-1854). German physicist and mathematician.
George Simon Ohm was a German physicist and mathematician who studied the relationship between voltage, current, and resistance. While working with a newly developed battery and wires of different materials, length, and thickness, he discovered that current flow depended on voltage and resistance. The relationship between voltage, current, and resistance is named after him and is called Ohm’s law. Ohm’s law states that for a fixed resistance, current is directly proportional to voltage. If you halve the voltage, current is also halved. If you double the voltage, the current is also doubled, and so on. This is the mathematical expression for Ohm’s law.
Using Ohm’s law formula above, one can calculate the value of one of the variables as long as the other two variables are known.
Example 1: A 250Ω resistor is connected to a 12V battery. What is the current flow through the circuit?
V=12V, R=250Ω
Example 2: Resistor R is connected to a 12V battery. If the current flow is 1mA, what is the value of resistor R?
V=12V, I = 1mA or 0.001A therefore using simple algebra,
Example 3: A light bulb has a resistance of 154Ω and draws 1.8 amps when lit. What is the voltage required to light the bulb?
R = 154Ω, I = 1.8A therefore using simple algebra,
Ohm’s law is the foundation of electricity upon which other electrical concepts are built on. It is important to understand Ohm’s law whether you are the homeowner who occasionally likes to do their own electrical work or someone who hopes to start a career in the electrical industry. Hopefully this article helps to make these electrical fundamentals more clear.
