Conductors and insulators are materials that respond very differently when electric charges are present. In a conductor, some electrons can move freely through the material, so charge and electrical energy can flow easily. In an insulator, electrons are tightly bound to atoms, so charge usually stays localized.
This difference explains why metals are used for wires while rubber and plastic are used for protective coatings.
A semiconductor sits between these two extremes because its ability to conduct can be controlled. Temperature, light, impurities, or applied voltage can increase the number of mobile charge carriers in a semiconductor. At the microscopic level, the key idea is whether electrons are free to move through the material or trapped near atoms.
This makes conductors, insulators, and semiconductors essential for circuits, sensors, computers, power systems, and safety design.
Key Facts
- Current is the rate of charge flow: I = ΔQ/Δt.
- Ohm's law relates voltage, current, and resistance: V = IR.
- Resistance depends on material and shape: R = ρL/A.
- Conductivity is the inverse of resistivity: σ = 1/ρ.
- Metals are good conductors because they contain many mobile electrons.
- Insulators have very few mobile charge carriers, so their resistivity is high.
Vocabulary
- Conductor
- A conductor is a material that allows electric charge to move through it easily because it has mobile charge carriers.
- Insulator
- An insulator is a material that strongly resists the flow of electric charge because its electrons are tightly bound.
- Semiconductor
- A semiconductor is a material with electrical conductivity between that of a conductor and an insulator, often controllable by doping, temperature, or light.
- Free electron
- A free electron is an electron in a material that can move from atom to atom and contribute to electric current.
- Resistivity
- Resistivity is a property of a material that measures how strongly it opposes electric current.
Common Mistakes to Avoid
- Thinking insulators contain no electrons is wrong because all ordinary materials contain electrons, but in insulators they are mostly bound and cannot move freely.
- Confusing voltage with current is wrong because voltage is electric potential difference, while current is the flow rate of charge.
- Assuming all metals conduct equally well is wrong because different metals have different resistivities, so copper, aluminum, iron, and silver do not carry current equally for the same size wire.
- Ignoring wire length and thickness is wrong because resistance increases with length and decreases with cross-sectional area according to R = ρL/A.
Practice Questions
- 1 A wire carries 6.0 C of charge past a point in 3.0 s. What is the current in the wire?
- 2 A copper wire has resistance 4.0 Ω and is connected to a 12 V battery. What current flows through the wire?
- 3 Explain why a metal spoon becomes part of a charge-sharing process when touched to a charged object, but a plastic spoon usually keeps charge localized near the contact point.