Is charge conserved in this

system?

Before: 2 excess protons

After: 2 excess protons

Net charge before = net charge after

Charge conserved!

22.4 - Conductors, insulators, and grounds

Conductor: An object or

material in which charge can

flow relatively freely.

Insulator: An object or

material in which charge

does not flow freely.

Ground: Charge flows from

a charged object to a ground, leaving the object

neutral.

You can easily find conductors and insulators (also called nonconductors) in your home

or classroom. If you examine an electrical cord, you will find that it consists of a

conducting core of copper wire surrounded by an insulator such as vinyl plastic.

Charge can be moved relatively easily through a conductor such as copper using a

device like a battery. A battery will cause electrons to flow through a copper wire like the

one shown in Concept 1. In contrast, it is difficult to cause electrons to flow in insulators

like rubber or many plastics. This difference explains the design of electrical cords:

Often, they are made of copper wire wrapped with a flexible vinyl insulator so that

electrons flowing through the wire remain within the cord.

Insulators do not allow charges to flow when they are subjected to only moderate

amounts of force. When great amounts of force are applied, charge can flow through an

insulator. There are also materials called semiconductors that enable charge to flow in

some circumstances, but not others. Given their role in devices like transistors, they are

an important topic, but lie outside the scope of this section.

A ground is a neutral object that can accept or supply an essentially unlimited number

of charges. The Earth functions as an electric ground. If you touch a conducting,

charged object to the ground, the object will also become electrically neutral í in other

words, grounded. Excess electrons will flow out of a negatively charged object to the

ground, and electrons will flow into a positively charged object from the ground.

Charges move to a ground because charges of the same sign move as far away from

each other as possible due to their mutual repulsion. The ground distributes the excess

charge far enough away that it ceases to affect the object.

Protecting houses from lightning presents engineers with the need to use conductors

and grounds. A building is not usually a conductor, but lightning can transform a house

into a reluctant conductor, with disastrous, highly flammable results. A lightning rod is a

conductor that protects houses and other structures by providing an easier, alternate

The lightning rod mounted atop this cupola intercepts

lightning and protects a building during electrical storms.

Conductors and insulators

Conductor: charge moves freely

Insulator: charge does not readily flow

Ground

Makes conductors electrically neutral

(^404) Copyright 2000-2007 Kinetic Books Co. Chapter 22