Chapter 3

Introduction

Imagine trying to run through an empty corridor. You could move quickly with very little effort.

Now imagine trying to run through a crowded room. Every step becomes more difficult as you weave around the people in front of you.

Electricity experiences something very similar.

Although voltage pushes electrons through a circuit, they don't move effortlessly. As they travel through a material, they constantly interact with the atoms around them. These interactions make it harder for the electrical current to flow.

This opposition to the movement of electrical charge is known as resistance.

Understanding resistance helps explain why some materials make excellent electrical conductors while others are used as insulators. It also reveals why wires become warm, why power can be lost during transmission and why choosing the correct cable size is so important in every solar installation.

What Is Resistance?

Resistance is the opposition to the flow of electrical current. You can think of it as friction for electricity.

Just as friction makes it harder to push a heavy object across the floor, resistance makes it harder for electrons to move through a material. When resistance increases, fewer electrons are able to flow for the same voltage.

Resistance is measured in ohms (Ω), named after the German physicist Georg Ohm.

Every electrical material has some resistance. The only question is how much.

Conductors and Insulators

As we learnt in Part III, not all materials allow electricity to flow equally well. Some materials offer very little resistance and allow electrons to move easily. These are called conductors. Copper is one of the most common conductors because it combines excellent electrical performance with strength, flexibility and relatively low cost. This is why most electrical wiring is made from copper.

Other materials strongly resist the movement of electrons. These are known as insulators. Plastic, rubber, glass and dry wood are all good insulators. Rather than carrying electricity, they help keep it safely contained inside electrical cables and equipment.

Every electrical system relies on both. Conductors carry the electricity. Insulators keep it under control.

Why Wires Get Warm

As electrons move through a conductor, they continually interact with the atoms that make up the material. Each interaction transfers a tiny amount of energy to the atoms, causing them to vibrate more vigorously.

Those vibrations are what we experience as heat.

This effect is happening in every electrical wire, although under normal operating conditions it is usually very small.

If too much current flows through a cable, however, far more heat is produced. The cable temperature rises, potentially damaging the insulation and creating a serious fire risk.

This is one reason electrical systems include protective devices such as circuit breakers, which disconnect the circuit before dangerous overheating can occur.

Ohm's Law

Voltage, current and resistance are closely related. Imagine blowing air through a drinking straw. If you blow harder, more air flows through the straw. If the straw becomes narrower, less air can flow even though you're blowing just as hard.

Electricity behaves in much the same way.

Increasing the voltage provides a greater electrical push, encouraging more current to flow. Increasing the resistance makes it harder for electrons to move, reducing the current. This relationship is known as Ohm's Law, one of the most important principles in electrical engineering.

It is commonly written as:

I=V/R

Where:

• I = electric current, measured in amperes (A)

• V = voltage, measured in volts (V)

• R = resistance, measured in ohms (Ω)

Although this equation appears throughout electrical engineering, the idea behind it is surprisingly simple. A greater push creates a greater flow. More resistance reduces that flow.

Understanding this relationship allows engineers to predict how electrical circuits will behave and forms the foundation of almost every electrical calculation.

Why Resistance Matters in Solar

Every metre of cable in a solar installation has some resistance. Although copper is an excellent conductor, it isn't perfect.

As electricity travels from the solar panels to the inverter, and then onwards to the home or the electricity grid, a small amount of energy is inevitably lost as heat.

For this reason, engineers carefully select cable sizes that keep resistance as low as reasonably practical. Thicker cables have a larger cross-sectional area, allowing electrons to flow more easily and reducing energy losses. These losses may seem small, but over the lifetime of a solar installation they can have a noticeable impact on performance and efficiency.

Understanding resistance helps engineers design systems that deliver as much of the Sun's energy as possible to where it is actually needed.

Looking Ahead

Resistance explains why electricity doesn't flow equally well through every material. Conductors allow electrons to move easily. Insulators restrict their movement.

As electrons push through resistance, some of their energy is converted into heat, making resistance one of the key considerations in every electrical system.

But electricity has another important characteristic.

Sometimes it flows steadily in one direction. At other times, it repeatedly changes direction many times every second.

In the next chapter, we'll explore the difference between direct current (DC) and alternating current (AC), and discover why solar panels and the electricity grid use different types of electricity.