14.1. The Big Ideas http://www.ck12.org
14.1 The Big Ideas
In the last chapter, we looked at static configurations of charges. In general, problems with moving charges are very
difficult to solve; the field that deals with these is calledelectrodynamics. In this chapter, we consider how charge
can flow through conducting wires connecting opposite ends of a battery. Such a setup, called acircuitusually
involves a current, a voltage source, and resistors.
Conductors have an effectively infinite supply of charge, so when they are placed in an electric field, aseparation
of chargeoccurs. A battery with a potential drop across the ends creates such an electric field; when the ends are
connected with a wire, charge will flow across it. The term given to the flow of charge iselectric current, and it is
measured in Amperes (A) — Coulombs per second. Current is analogous to a river of water, but instead of water
flowing, charge does.
Voltageis the electrical energy density (energy divided by charge) and differences in this density (voltage) cause
electric current.Batteriesoften provide a voltage difference across the ends of a circuit, but othervoltage sources
exist. If current is a river, differences in voltage can be thought of as pipes coming out of a water dam at different
heights. The lower the pipe along the dam wall, the larger the water pressure, thus the higher the voltage.
Resistanceis the amount a device in the wire resists the flow of current by converting electrical energy into other
forms of energy. A resistor could be a light bulb, transferring electrical energy into heat and light or an electric
motor that converts electric energy into mechanical energy. The difference in energy density across a resistor or
other electrical device is calledvoltage drop.Resistance is analogous to rocks and other objects that impede the flow
of water, transforming the water’s kinetic energy into heat, sound, and other forms of energy through contact forces.
This is what a typical circuit looks like: