conductors
Summary
Conductors are materials, such as metals, whose charges move freely, so they carry current with low resistance. In electrostatic equilibrium, the electric field inside a conductor is zero, excess charge sits on its surface, and the whole conductor is at one potential.
Detail
Meaning: A conductor has charge carriers that move easily: free electrons in a metal, or ions in a salt solution. Its resistivity is low. When a conductor is placed in an electric field, the free charges shift until their own field cancels the applied field inside. At electrostatic equilibrium, then, E = 0 inside the material, any excess charge lies on the outer surface, the field just outside is perpendicular to the surface, and every point of the conductor has the same potential. Insulators hold their charges in place and do not do this.
Equation: R = ρL/A, where R is resistance (Ω), ρ is resistivity (Ω·m), L is length (m), and A is cross-sectional area (m²). Conductors have small ρ.
On the MCAT: Expect a charged object brought near a neutral conductor, which causes induced charge separation, or a grounded conductor that gains or loses electrons. Other setups are shielding by a hollow conductor (Faraday cage), capacitor plates, and wires whose resistance you must compare after a change in length or diameter. For metals, resistivity rises with temperature.
Common confusion: Zero field inside applies to a conductor at electrostatic equilibrium. A wire carrying current does have an internal field driving the charges. Also, a conductor can carry net charge, and its constant potential is not necessarily zero.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.