Understanding Ohm’s Law and Power

One of the cornerstones of electrical engineering and electronics is Ohm’s Law. It’s the simple yet powerful relationship that ties together voltage, current, and resistance—and it forms the basis for everything from circuit design to troubleshooting.

The Basics: Ohm’s Law

Ohm’s Law is expressed as:

Where:

• E is voltage in volts,
• I is current in amperes, and
• R is resistance in ohms.

This means that if you know any two of these values, you can calculate the third. The “Ohm’s Law Triangle” (or circle, depending on your preference) is a handy memory tool—cover the unknown quantity, and the relationship between the other two tells you how to solve for it.

Putting It Into Practice

The presentation walks through practical examples:

• 12 V across 96 Ω → current is 0.125 A.
• 1.5 A through 15 Ω → voltage is 22.5 V.
• 550 mV across 0.1 A → resistance is 5.5 Ω.
• 200 mA through 2.5 kΩ → voltage is 500 V.

These worked problems highlight the importance of careful unit conversion—millivolts, milliamps, and kilohms need to be translated into base units to avoid errors.

Series and Parallel Resistors

The presentation also digs into combining resistances:

• Series circuits: resistances simply add together. Current is the same everywhere, but the voltage divides across each resistor.
• Parallel circuits: resistances combine using reciprocals, and the total is always less than the smallest branch resistor. Voltage across each branch is the same, while current divides according to resistance.

Worked examples show how to calculate total resistance, individual currents, and voltage drops in both series and parallel networks.

Power in Electrical Circuits

Beyond voltage and current, we need to understand power, which is the rate at which energy is converted into work. In electrical terms:

P = E × I = E² / R = I² × R

This is where the mnemonic PIE (Power = I × E) comes in handy. The unit of power is the watt, equal to one joule per second.

Examples demonstrate that whether you start with voltage and resistance, or current and resistance, the result is the same: a 12 V source across a 50 Ω resistor dissipates 2.88 W of power, no matter which formula you use.

Practical Considerations

Resistors aren’t just about resistance—they also have power ratings. Exceeding the rated wattage leads to overheating and failure. As a general rule, always design with a 50–100% safety margin.

The presentation also explores how power ratings are applied in series and parallel circuits, reinforcing the importance of checking each component’s limits before applying a load to a circuit.

Wrapping Up

The review questions at the end drive home the fundamentals:

• Higher wattage bulbs consume energy faster.
• Open circuits have no current.
• Watts come from volts multiplied by amps.
• Resistance equals voltage divided by current.

Ohm’s Law and the concept of power are simple tools, but they’re the foundation of every circuit we build, test, or repair. Whether you’re a student learning the basics or a professional brushing up, getting comfortable with these relationships is essential.

Today was the second class in the RAC Amateur Radio License course. We were studying Chapter Two of the Study Guide, Introduction to Electronics.

This chapter lays the foundation for understanding basic electricity, beginning with the fundamental components of matter. Before delving into the technical theory, Al discussed the Amateur Radio Operator Code of Conduct. I decided that the two codes Al mentioned were important enough to warrant their own post, so you can find them here.

We looked at atoms, their structure, and how the behaviour of electrons gives rise to electrical phenomena. From there, the discussion moved into conductors and insulators—why materials like copper and gold conduct electricity so well, while glass, rubber, and plastics resist it.

Permittivity is a key idea in physics, especially in electromagnetism. It describes how a material reacts when an electric field is around. Understanding permittivity helps explain how electric fields work, as well as the functioning of capacitors, dielectrics, and electromagnetic waves.

Permittivity, denoted by the symbol ε (epsilon), is essentially a measure of how well a material can allow electric fields to pass through it. It tells us how much the electric field inside the material is weakened compared to what it would be in a vacuum or open space. This property depends on the material’s makeup, structure, and physical state.

Key ideas:

1. Electric Permittivity (ε₀): This is the permittivity of free space, often called epsilon naught. It’s a fundamental constant that describes how electric fields behave in a perfect vacuum. Its value is about 8.854 × 10^-12 farads per meter (F/m). In a vacuum, the electric field moves without distortion or loss.
2. Relative Permittivity (ε_r): Also known as the dielectric constant, this is simply the ratio of a material’s permittivity to that of free space. It’s dimensionless and tells us how well a material can store electrical energy in an electric field compared to a vacuum. Materials like glass, ceramics, and many plastics have high relative permittivity, making them very effective at energy storage.

Insulators or insulating materials are those substances which will not allow the flow of electrons through them due to very low free electrons in them, and they have a low dielectric constant (Relative permittivity = ε_r).

Examples: Porcelain insulators used in power transmission on distribution poles and towers, rubber, glass, plastic, wood, etc.

Dielectrics or dielectric materials are substances similar to insulators but allow the flow of electrons through them when subjected to an external electric field, as they can be polarized. They can also be defined as having the ability to store charge (energy) through polarization, as in a capacitor. Additionally, they have a high dielectric constant. (Relative permittivity = ε_r).

Examples: A common example of a dielectric is the electrically insulating material between the metallic plates of a capacitor, (such as mica, laminated paper). Other examples include air, ceramic, etc.

• All dielectrics are insulators, but not all insulators are dielectrics.
• Everything becomes a conductor at certain temperatures or electric fields due to breakdown, as every insulator has its limits to withstand a potential difference across the material.

Key electrical concepts were introduced, including charge, current, voltage, and resistance. Al Penny VO1NO, our instructor, explained the coulomb as the standard unit of charge, the ampere as the rate of electron flow, and voltage as the “pressure” that pushes electrons through a conductor. Resistance and the factors that affect it—material type, length, diameter, and temperature—are also covered, along with the role of resistors and potentiometers in circuits.

The class then explored magnetism as one of the four fundamental forces of nature, showing how magnetic fields, poles, and materials influence electrical behaviour.

This naturally led to a discussion of direct current (DC), its sources, and the role of cells and batteries. Al explained the difference between primary (non-rechargeable) and secondary (rechargeable) cells, the chemistry behind common examples like zinc–carbon and lead–acid batteries, and how cells can be connected in series or parallel to change voltage or current capacity.

By the end, the chapter tied together the essential elements of electricity—atomic theory, conductors and insulators, current, voltage, resistance, magnetism, and electrochemical cells—providing a solid grounding for anyone beginning their journey into radio and electronics.

This was a review for me, as I have worked with electronics and electricity throughout my career. However, for anyone who doesn’t have a grounding in these subjects or feels like they need a refresher, this was a great place to start.

If you are thinking about studying for an Amateur Radio Certificate, there are some excellent flashcard decks available for free on Ankiweb. One that I am using is the ISED basic amateur questions (2025) deck.