Ohm's Law and the Beginnings of Voltage, Current, and Resistance Calculations

June 6, 2019 Cadence PCB Solutions

Testing a circuit board for volts, amps, or ohms in ohm’s law

 

No matter if you are a hobbyist, a technician, or if you are pursuing a career in design, you should start with a good understanding of what you are working with. For electronics this means working with ohm’s law. The basic principle of ohm’s law, which is the relationship between voltage, current, and resistance, is the foundation that electronic design is built on. And just as a building requires a solid foundation under it to support it, your ability to work with and utilize electricity relies on the solid foundation of ohm’s law and your understanding of it.

An ohm is the standard unit of electrical resistance, but you may be wondering where that term came from and how it all factors into ohm’s law. It all started with a German physicist and mathematician named Georg Ohm, and the term “ohm” was named after him. So where did ohm’s law originate from and what is this law? Let’s take a closer look.

Georg Ohm, the Originator of Ohm’s Law

Born in 1789, Georg Ohm started his professional life as a teacher of mathematics. He was not happy with his profession at first, however, and the different schools that he initially taught at were not the best positions either. Eventually, though he ended up teaching mathematics and physics at the Jesuit Gymnasium of Cologne. This was much more to his liking as the school had a good reputation for science education.

Georg Ohm had started experimenting with electricity which led him to publishing some papers on the subject. In 1827 he published his book; “The Galvanic Circuit Investigated Mathematically” where he gave his complete theory of electricity including the formula for what would eventually be called Ohm’s law. Unfortunately the school where he was employed did not appreciate his work causing Ohm to ultimately resign. Later he would go on to positions at other schools where his work was more widely recognized, and his discovery of the relationship between voltage, current, and resistance would become known worldwide as ohm’s law.

 

Graphical representation of ohm’s law

Formulas used to depict Ohm’s law

 

What is Ohm’s Law?

What Georg Ohm discovered and defined was how electrical voltage, current, and resistance affect each other in a circuit. A circuit is a closed loop of electricity, or charge. The components we use in a circuit will control the charge so that it will behave the way we need it to for the function of a particular circuit. To begin with the first thing we will do is to define these terms:

 

  • Voltage: In a circuit one point will have more charge than another. This difference in charge between the two points is voltage, and is measured in volts.

  • Current: The amount or volume of charge flowing through the circuit over a period of time. Current is measured in amperes, which is usually referred to as amps.

  • Resistance: Certain materials will resist the flow of electricity, and when these materials are used in a circuit they create resistance. The greater the resistance in a circuit, the less charge will flow. Resistance is measured in ohms.

 

It can be helpful to use the analogy of a water tank with a host connected to the bottom to better explain the relationship between voltage, current, and resistance.

 

  • The charge will be represented by the water in the tank that will flow out of the hose.

  • Voltage will be represented by the pressure of the water flow.

  • Current will be represented by the volume of water flow.

  • Resistance will be represented by the diameter of the hose.

 

If our water tank is elevated above the ground, the pressure of the water flowing out of the hose at the bottom would represent voltage while the volume of water flowing would represent current. If the diameter of the hose were to get smaller, the flow of water would be constricted resulting in less water flowing through. If the diameter of the hose were to get larger, then more water would be able to flow through. To summarize, the circuit with the higher resistance will restrict how much charge will flow through it.

To express ohm’s law in a formula where voltage equals current times resistance, you specify V = voltage in volts, I = current in amps, and R = resistance in ohms. This allows you to easily solve for each of the three elements:

 

  • V = I x R

  • I = V / R

  • R = V/ I

 

Therefore in a 12 volt circuit that has a component with 6 ohms of resistance, we can calculate that 12 volts divided by 6 ohms of resistance will result in a current of 2 amps for our circuit. Although this is an extremely simple explanation, you can see how ohm’s law will be part of almost everything in electronics.

 

Schematic made in OrCAD Capture ready for PSpice Simulation using Ohm’s Law

PSpice is a circuit simulator that will allow you to find parameters in your design based on circuit goals

 

Why is Ohm’s Law Important?

V=IR, or ohm's law, is the most fundamental law in electricity, and it will impact every circuit that you will work with. The efforts of Georg Ohm marked the first work in the subject of electrical theory and analysis, which is the process of finding the voltages across and the currents through all the components in a circuit.

Although Ohm was at first confronted with a cold reception of his discovery, he persisted and eventually the importance of ohm’s law was recognized universally. It remains the most widely used of all the different rules and laws relating to electrical circuits and their behavior.

As you apply ohm’s law and other electronic principles and rules to your work, you will need analysis tools that can simulate your circuits and report back to you exactly what their behavior is. Utilizing proper SPICE tools can confirm theoretical concepts through simulation, allow you to create more complex circuit combinations before layout, as well as save an enormous amount of time from doing hand calculations.

PSpice, from the Cadence design tool suite, will give you the ability to simulate your circuits in software saving you the time to build and manually test these circuits yourself. PSpice is the simulation tool of choice for users whether you are just starting out in the business or you are a seasoned engineer. You can completely recreate the circuit you have in the schematic tool of OrCAD Capture, as well, to be readily simulated with PSpice.

If you want to know more about how the simulation tools from Cadence will be the best solution for you, talk to us and our team of experts.

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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