Chapter 15Charge

ConductorsInsulatorsInduction

We’re now going to start down the path ofexamining forces whose origin is not visibleto us. We call the quality of matter responsiblefor these forces “charge,” a substance neverdirectly observed in the history of the humanrace!

Electrical Forces and Fields

Amber (elektron in Greek) attracts straw/feathers when rubbed (observed by Thales of Miletus ~ 600 B.C.).

Iron ore from the country ofMagnesia seemed to have anatural affinity for metals.

When released, all objectsseem to fall toward the ground.

Attraction in Nature

Electrostatic and Magnetic Forces

William Gilbert (1540 - 1603, English physician)- clarifies the difference between the attraction

of amber and that of magnetic iron ore;- shows that many materials besides amber

exhibit electrical attraction- showed that the behavior of a compass needle

results from the magnetic field of the Earthitself!

Stephen Gray (early 1700’s) shows that the electrostaticattractive and repulsive forces can be transferredthrough contact alone…Metals didn’t need to berubbed.

Charles Du Fay (1698 - 1739) first postulated the existenceof two distinct kinds of electricity: vitreous (theglass rod) and resinous (the silk). But thought theyexisted together in most matter and when separatedthrough friction, resulted in an electrical force.

Benjamin Franklin (1706 - 1790) hypothesized a one-fluidmodel of electricity: charge is transferred from onebody to another (e.g. through rubbing); but the totalcharge on the two bodies combined remains thesame. This theory is known as the…

Conservation of Charge.

Two Kinds of Charge

Franklin decided to call the materials which he believed had excess charge “positively” charged materials. Those with a deficiency of charge he called “negatively” charged.

Parenthetical Remark:

Unfortunately, as we would later learn, it is the electrons (negative charge carriers) that are mobile while the protons (positive charge carriers) generally remain fixed in the nucleus of atoms. So materials with excess electrons appear negatively charged. Nevertheless, Franklin’s convention has stuck with us…quite literally!

Hence, the glass rod (vitreous) was positive when rubbed with silk while the amber (resinous) was negative when rubbed with wool.

Charge is QuantizedRobert Millikan (1868 - 1953) in a very clever

experiment showed that electrical chargecame in quantized units. In other words,charge of 0, +/- 1e, +/- 2e, +/- 3e,...+/- ne (where n is an integer) couldbe observed, but never a charge of1.5e (see section 15.7).

An electron carries a charge of -1e.A proton carries a charge of +1e.

•Electrons and protons carry charge•They are responsible for the electrical

forces we encounter•Atoms are made up of electrons and

protons•Matter is composed of atoms

So why doesn’t every object we encounter exert an electric force on every other object around it?

P+ElectronCloude-

Hydrogen atom

Neutral Space

Electrically Neutral...Most objects in nature are electricallyneutral (i.e. they contain an equal number of protons and electrons).

Ne = Np

Therefore most objects exert no electrical force on the objects around them.Atoms in which Ne< Np or Ne> Np arecalled ions.

Insulators and ConductorsObjects on which electrons move freely areknown as conductors. Most metals are goodconductors. Electrical wires are made ofgood conductors.

Objects on which charges do not move freelyare known as insulators. Glass, amber,rubber, silk and cloth are all examples ofgood insulators.

If metals are good conductors, why is it hard to charge them by rubbing them with wool?

If rubber is such a bad conductor, why is it so easy to put a charge on rubber by rubbing it with wool?

GROUND

RUBBER

The charges remain near the end of the rubber rod--right where we rubbed them on!

GROUND

COPPER

Rub charges on here

They move down theconductor toward our hand

Eventually ending up in the ground.

GROUND

COPPER

Rub charges on here

A good conductor distributesthe charge uniformly over itssurface.

Rubber glove insulates copper rodfrom us and therefore the ground.

So, we can charge up a conductor!

And human beings must be fairly goodconductors of electricity, too.

Notice that the Earth’s surface (ground)acts as a vast source and sink for electricalcharge. Touching a conductor to groundwill neutralize the charge on the conductor:If the conductor is positively charged, electronsflow from the ground to the conductor. If theconductor is negatively charged, electrons flowoff the conductor into the Earth.

So when we talk about an object beinggrounded, we literally mean that it isconnected via a conductor to the Earth’ssurface.

All electrical outlets now have a groundprong. And most electrical devices usea 3-prong plug that requires the groundconnection.

Well….just in case the device malfunctions,it’s nice to be able to siphon off the excesselectrical charges to ground rather thanallowing them to accumulate in the device.

If they build up in the device, they willeventually find their way to ground. If a person comes in contact with the device, the resulting flow of charges through the body can be deadly.

The ground prong provides a nice safety feature.

We can take advantage of the Earth’sability to accept and provide charges toplace a net charge on a conductor….

Here’s how you do it!

GROUND

COPPER Bring negatively charged rubber ball close to the a copper rod. The copper rod is initially neutral.

Negative charges on the copper runaway from the rubber ball and into theground.

Rubber

GROUND

COPPER The copper rod is now positively charged. The electrons originally on it were forced away into the ground by the negative charges on the rubber ball.

Rubber

+++

++

++

GROUND

COPPER

Put a rubber glove on your hand to insolate the copper rod from ground.

Rubber

+++

++

++

Finally, remove the rubber ball...

GROUND

COPPER+

+ ++

+++

The excess positive charge is trapped on the copper rod with no path to ground. It redistributes itself uniformly over the

copper rod. We have taken an initially neutral copper rod and induced a positive charge on it!