Welcome to our third lesson. In our previous session, we introduced the fundamental relationship governing voltage, current, and resistance: Ohm's Law. You learned the core equation, , which describes how these three quantities are inextricably linked.
Today, we will move from theoretical understanding to practical application. Our goal is to use Ohm's law to calculate any one of these three values when the other two are known. This is the first essential calculation skill in electronics, forming the bedrock of circuit analysis. Mastering this will allow you to start making design decisions, such as selecting the correct components for the robotics and IoT projects you're aiming to build.
Putting Ohm's Law to Work
The formula is a simple algebraic equation. By rearranging it, we can solve for any of the three variables. This gives us three essential tools for circuit analysis:
- To find Voltage (V):
- To find Current (I):
- To find Resistance (R):
The video below offers a great overview of these three formulas and provides a clear, worked example for each one.
Ohms Law Explained - The basics circuit theory
This video from The Engineering Mindset provides a concise and practical introduction to applying all three forms of Ohm's Law.
Watch the video from the beginning up to the start of the practice problems. It will walk you through the Ohm's Law triangle mnemonic and demonstrate how to calculate voltage, current, and resistance with simple examples.
Now that you have a clear overview, let's solidify these calculations with more detailed examples.
A Deeper Dive into Calculations
To really understand how to apply Ohm's law, it's best to walk through the process for each variable. We will use the concept of a "simple circuit," which consists of a single voltage source and a single resistor.
This image shows a simple circuit. A voltage source (V) of 4.5V is applied across a resistor (R) of 78Ω. Ohm's Law predicts the resulting current (I) will be .
The following video provides a very methodical explanation of how to analyze these simple circuits.
03 - What is Ohm's Law in Circuit Analysis?
The "Math and Science" channel gives a detailed, step-by-step guide to applying Ohm's law in single-loop circuits.
Please watch from the beginning to the end of the second example. Pay close attention to how the voltage of the source is applied directly across the resistor and how changes in resistance affect the current.
As you saw, if you know any two values, you can find the third. Let's look at one of the most important practical applications of this: choosing a resistor to protect a component.
Example: Protecting an LED
Light Emitting Diodes (LEDs) are common in almost every electronics project. However, they are sensitive components that can be easily destroyed if too much current flows through them. We use a "current-limiting resistor" to protect them. Ohm's law is how we calculate the value of this resistor.
Voltage, Current, Resistance, and Ohm's Law
This section from a SparkFun tutorial provides an excellent, real-world example of using Ohm's Law to calculate the correct resistor for an LED circuit.
Please read the section titled An Ohm's Law Experiment. The key part is the calculation that determines the necessary resistance (R = V / I) to keep the current at a safe level for the LED.
This example perfectly illustrates why Ohm's law is not just an abstract formula. It's a design tool you will use constantly to ensure your circuits work correctly and safely.
Working with Units
In real-world electronics, you will rarely work with nice, round numbers of Volts, Amps, and Ohms. It's much more common to see values like "millivolts" (mV), "milliamps" (mA), or "kilo-ohms" (kΩ).
- milli (m): one-thousandth (1/1,000 or ). So, 20 mA = 0.020 A.
- kilo (k): one-thousand (1,000 or ). So, 2.2 kΩ = 2,200 Ω.
It is critical to convert all values to their base units (Volts, Amps, Ohms) before using Ohm's law. A common mistake is to mix units, for example, dividing Volts by kilo-ohms, which will lead to an incorrect answer.
Let's return to the "Math and Science" video to see how to handle these conversions properly.
03 - What is Ohm's Law in Circuit Analysis?
This segment specifically addresses how to handle calculations involving units like millivolts and milliamps.
Watch the portion of the video from the introduction of non-base units. The instructor demonstrates how to convert values into scientific notation or decimals representing the base units before performing the calculation.
Practice Problems
The best way to solidify your understanding is to solve a few problems. The following worksheet provides some excellent practice scenarios. Try to solve them on your own before revealing the answer.
Ohm's Law Practice Worksheet With Answers
This worksheet from All About Circuits contains a variety of problems to test your understanding.
First, solve Question 6. This is a straightforward calculation of current. Next, try Question 9. This is a great conceptual problem. You're given one voltage and two different resistors. Apply Ohm's law (I = V/R) to each resistor independently to find the current through each path. This problem helps dispel the common myth that electricity only takes the path of least resistance; in reality, it takes all available paths, with more current flowing through the less resistive ones. Finally, consider the thought experiment in Question 23. According to Ohm's Law, what happens to current as resistance approaches zero? This introduces the important real-world concept of a "short circuit" and hints that even components we consider ideal, like wires and batteries, have some internal resistance.
Conclusion
In this lesson, you have moved from knowing what Ohm's law is to knowing how to use it as a practical tool for circuit analysis. This is a significant step forward.
Key Takeaways:
- Ohm's Law can be algebraically rearranged to solve for voltage (), current (), or resistance ().
- These calculations are fundamental to analyzing circuit behavior and designing circuits, such as choosing a current-limiting resistor for an LED.
- It is crucial to convert all values to their base units—Volts (V), Amperes (A), and Ohms (Ω)—before applying the formulas.
In our next lesson, we will build directly on this foundation by exploring electrical power. You will learn how to calculate the power dissipated by a component, which is essential for selecting parts that can handle the heat generated during operation—a critical consideration in the design of any robust robotic or IoT system.