Everything around us is made of atoms. At the center of each atom is a nucleus, containing positively charged protons and neutral neutrons. Orbiting the nucleus are tiny, negatively charged electrons. The concepts of positive and negative are just labels for two opposite types of electric charge.

In many materials, electrons are tightly bound to their atoms. In others, particularly metals, the outermost electrons are held more loosely. These are called valence electrons, and their ability to move is what makes electricity possible. When these electrons are stripped away from an atom, the atom is left with a net positive charge and is called an ion.

Materials behave differently when it comes to letting electrons move. We can group them into three main categories.

Next, we have insulators.

Between these two extremes lies a special category of materials.

Charges don't have to touch to interact. They exert forces on each other through invisible fields. An electric charge creates an electric field in the space around it. When another charge enters this field, it feels a force. Think of it like a tiny version of gravity, but for charge instead of mass. The field tells other charges how to move.

When electric charges move, they create another kind of field: a magnetic field. This is the same force that makes magnets stick to a refrigerator. A wire with electricity flowing through it generates a circular magnetic field around it. Electric and magnetic fields are deeply connected and are two aspects of the same fundamental force.

The flow of electric charge is called electric current. In a copper wire, this current is a river of countless electrons moving together. We measure current in amperes (A), or amps for short. A flow of one coulomb of charge (about $6.24 \times 10^{18}$ electrons) past a point in one second is one ampere.

But what causes the charge to move? For that, you need a potential difference, more commonly known as voltage. Voltage is the 'push' or 'pressure' that drives the current through a circuit. It represents the difference in electric potential energy between two points. A battery, for example, creates a voltage between its positive and negative terminals. Voltage is measured in volts (V).

As current flows, it often encounters opposition. This opposition to the flow of current is called resistance. Think of it like friction. A wider pipe offers less resistance to water flow than a narrow one. Similarly, thick copper wires have less resistance than thin ones. Resistance is measured in ohms (Ω).

These three concepts are tied together by a simple but powerful rule.

In the 1820s, German physicist Georg Ohm discovered a fundamental relationship between voltage, current, and resistance. Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points. The relationship is elegantly simple.

Here, $V$ is voltage, $I$ is current, and $R$ is resistance. This means if you increase the voltage (the push), the current (the flow) will increase, as long as the resistance stays the same. If you increase the resistance (the opposition), the current will decrease for a given voltage.

This simple formula is one of the most important tools in electrical engineering. Now, let's review these core concepts.

Time to check your understanding.

These basic principles of charge, fields, and circuits are the foundation for everything from the power grid to the smartphone in your pocket.