Chapter 1

Introduction

In the previous part, we followed a single photon from the heart of the Sun to a solar cell, where its energy freed an electron and created the voltage needed to drive an electric current.

But what happens next?

How does the movement of tiny electrons become the electricity that powers our homes, charges our phones and boils our kettles?

Most of us use electricity every day without ever stopping to think about what it actually is. We flip a switch, plug in a laptop or switch on the television, yet the invisible process taking place inside the wires remains a mystery.

The truth is surprisingly simple.

Electricity is not a substance flowing through a cable, nor is it a fuel waiting to be used. It is the movement of electrically charged particles carrying energy from one place to another.

Understanding this simple idea is the foundation of everything that follows, from solar panels and batteries to inverters and the electricity grid itself.

Electric Charge

Everything around us is made from atoms, and every atom contains electrically charged particles.

As we discovered in Part 3, electrons carry a negative electric charge, while protons carry an equal but positive charge. Charge is one of the fundamental properties of matter. Just as mass determines how strongly an object responds to gravity, electric charge determines how particles interact with electric fields and with one another.

Opposite charges attract, while like charges repel. This simple rule lies behind everything from the crack of a lightning bolt to the operation of a solar cell.

Whenever electrical charge moves in an organised way, an electric current is produced.

Static and Current Electricity

Not all electricity behaves in the same way. 

Sometimes electrical charge builds up in one place without flowing anywhere. This is known as static electricity. You've probably experienced it after walking across a carpet and touching a metal door handle, or by rubbing a balloon against your hair. In both cases, electrical charge gradually accumulates until it suddenly finds a path to discharge.

Solar panels operate very differently.

Rather than allowing charge to build up, they create a continuous flow of electrons through an electrical circuit. This flowing electricity is called electric current, and it is this steady movement that allows useful work to be performed.

Static electricity stores charge. Current electricity transports energy.

Electrons Carry the Energy

One common misconception is that solar panels create new electrons. They don't.

The electrons were already present inside the silicon solar cells, the copper wiring and every electrical component connected to the system.

The Sun simply provides the energy needed to set those electrons in motion.

A useful analogy is a line of dominoes. When the first domino falls, the movement travels along the entire line, even though each individual domino only moves a short distance before coming to rest. The motion is passed from one domino to the next.

Electricity behaves in a similar way. The electrons themselves travel only relatively short distances, but the energy they carry is passed rapidly through the electrical system.

The energy comes from the Sun. The electrons simply deliver it.

Electricity Needs a Complete Path

Imagine trying to drink through a straw with one end blocked. No matter how hard you suck, nothing moves because there is no complete path for the liquid to follow.

Electricity behaves in much the same way.

For electrons to keep moving, they must have a continuous path from one side of the power source to the other. This complete pathway is called an electrical circuit. If the path is broken by opening a switch, disconnecting a wire or a component failing, the electrons can no longer flow and the transfer of electrical energy stops immediately.

Whether it's a torch, a mobile phone or a utility-scale solar farm, every electrical system depends on a complete circuit.

Electricity Is Energy in Motion

Perhaps the most important idea to remember is that electricity is really a way of transporting energy. Inside a solar power system, that journey begins deep within the Sun, where nuclear fusion produces photons. Those photons travel 150 million kilometres through space before releasing their energy inside a solar cell.

The moving electrons then carry that energy through cables to wherever it is needed.

• A light bulb converts electrical energy into light.

• A kettle converts it into heat.

• An electric motor converts it into movement.

• A battery converts it into stored chemical energy.

In every case, the electrons are simply the delivery system, transporting the Sun's energy from one place to another before it is converted into the form we actually need.

Looking Ahead

We now know what electricity is: the organised movement of electrically charged particles carrying energy through a complete circuit. The next question is just as important.

Not all electrical systems behave the same way. Some can push electrons with greater force, while others move larger numbers of electrons every second.

To describe these differences, electrical engineers use four fundamental quantities: voltage, current, power and energy. Together, they form the language of electricity and provide the foundation for understanding every solar energy system.