Electrical conduction depends on how easily electrons can be excited into a conduction path. In insulators, the electrons are tightly bound in the outer shell and the energy gap between the valence band and the conduction band is large. When an atom has five or more valence electrons, those outer electrons are strongly bound in covalent or ionic bonds, which tends to produce a wide band gap. A wide band gap means it takes a lot of energy to free an electron to conduct, so the material resists current flow and acts as an insulator. That’s why five or more valence electrons in the outer shell is the typical rule of thumb for insulators. (There are exceptions, such as some materials with four valence electrons that still form insulators due to their network structure, but the general pattern used here centers on five or more.)

Copper is a conductor. Metals like copper have many free electrons that can move easily when an electric field is applied, creating a path for current to flow with relatively low resistance. This makes copper ideal for wiring and electrical connections. An insulator, by contrast, tightly binds its electrons and resists current flow; a semiconductor has conductivity that can be controlled and is typically between a conductor and an insulator; a superconductor carries current with zero resistance but only at very low temperatures for most materials. Under normal conditions, copper has measurable resistance, so it’s not a superconductor.

Understanding voltage terminology: EMF, or electromotive force, is another term used for voltage. It describes the energy per unit charge that a source can supply to move charges through a circuit, which is what drives current. In an ideal source with no internal resistance, the emf equals the voltage you measure at the source’s terminals when no load is connected. In real sources, internal resistance causes a small drop, so the terminal voltage under load is emf minus the I·R drop. Voltage is the general term for the potential difference between two points, which can be across a component or a source, while emf specifically refers to the driving potential generated by a source. Current is the rate of charge flow, not a measure of potential difference, and power is the rate of doing work, equal to voltage times current.

Power in electrical circuits is the rate at which electrical energy is converted or transferred to another form, such as light, heat, or motion. In SI units, this rate is measured in watts. A watt means one joule of energy moved or used each second, so P = energy per time. In practical terms for circuits, power also equals voltage times current (P = V × I), which aligns with the unit watt since volts times amperes give joules per second. The other units serve different roles: volt measures electrical potential difference, ampere measures current, and joule measures energy. So the SI unit for power is the watt.