A conductor contains mobile charge carriers that can move through the material when an electric field is applied, whereas an insulator has charges that are strongly bound and cannot move freely through the material. This distinction is part of IB Physics B.5 Current and circuits and applies at both SL and HL.
Particle-Level Mechanism
In a metal conductor, some outer electrons become delocalized electrons. They are not attached to one particular atom and move randomly through a lattice of positive ions. Without a potential difference, their random motion produces no net charge flow.
When a potential difference is applied, an electric field is established. The delocalized electrons gain a small drift velocity opposite to the conventional-current direction, producing current:
Electrons frequently collide with vibrating lattice ions, transferring energy to the material. These interactions produce electrical resistance and heating.
In an insulator, electrons remain tightly bound to atoms or molecules. An applied electric field may slightly displace the bound charges, causing polarization, but normally does not create a sustained current. A sufficiently strong field can cause electrical breakdown, after which the material may conduct.
| Property | Conductor | Insulator |
|---|---|---|
| Charge carriers | Many mobile, delocalized electrons | Electrons are strongly bound |
| Response to an electric field | Carriers acquire drift velocity | Mainly polarization; negligible charge flow |
| Typical resistance | Low | Very high |
| Examples | Copper, aluminium | Glass, rubber, plastic |
A common misconception is that conductors “contain electricity” while insulators contain none. Both contain charged particles; the crucial difference is whether charge carriers are sufficiently mobile.
IB Exam Technique
For an Explain question, link microscopic structure to macroscopic behaviour: mobile charge carriers respond to an electric field, producing current; bound charges do not produce sustained charge flow. Do not merely state that conductors have low resistance and insulators have high resistance, that describes the effect but not the particle-level cause.