A charging cable contains a small lesson in materials science. Inside, metal provides a path for electric current. Outside, an insulating covering helps keep that current where the design intends it to go.
Both materials contain electrons. Plastic is not electrically empty, and copper is not filled with a special electrical fluid. The difference lies in how charges are held within the material and how easily they can respond to an electric field.
Charges need room to move
In a metal, some electrons are not tied to a single atom. They can move through the structure. Apply an electric field and their motion develops a directed component, producing an electric current.
The atoms do not all march down the wire with them. A useful distinction is between the material's structure and the mobile charges it contains. Conductivity depends strongly on the availability and mobility of those charge carriers.
In many insulating materials, electrons are much less able to move between sites under ordinary conditions. The material can still respond electrically—for example, through small shifts in charge distribution—without carrying a substantial current across it.
Current does not always mean moving electrons
Metals carry current mainly through electrons. Salt water provides a different example. Dissolved salts supply charged atoms or groups of atoms called ions, which can move through the liquid.
That is why ordinary water containing dissolved substances can conduct much better than highly purified water. The familiar advice to keep electricity away from water does not require water molecules themselves to behave like electrons in a copper wire.
The state of a substance can matter too. Ions locked into a solid crystal are not free to travel as they can when the appropriate substance is dissolved or melted. The ingredient list alone does not determine conductivity.
There is a useful middle ground
Semiconductors, including silicon, have electrical behaviour that engineers can control through composition, electric fields, temperature and light. Their importance comes from that controllability, not simply from conducting halfway as well as a metal.
In a solar cell, absorbed light can give electrons energy and help create mobile charge carriers. The device's internal structure then helps separate and collect charges, making useful electrical power possible.
In electronic circuits, carefully designed semiconductor structures can control current. The ability to alter electrical behaviour is the foundation of components far more versatile than a permanently open or permanently blocked pathway.
Insulation has limits
An insulator is not an absolute promise that current can never pass. A sufficiently strong electric field can cause electrical breakdown. Moisture, contamination, damage and temperature can also change how a real object behaves.
That is why an everyday plastic object should not be treated as certified electrical protection. Engineering insulation involves the material, its thickness, its condition and the conditions it must withstand.
The cable's simple design therefore rests on a subtle distinction. Every material contains electric charges, but not every material gives those charges the same freedom to move. Technology works by choosing—and sometimes deliberately changing—that freedom.
Sources and further reading
OpenStax: Conductors and insulators