Electrostatics

ObjectivesTo define Electrostatics, Electric Force, and the

Law of Conservation of charge

ElectrostaticsDefinition: Electricity at rest

Involves electric charges, the forces between them, the aura that surrounds them, and their behavior in materials.

Electrical ForcesElectric force is the attraction or repulsion of two

particles.

Like particles repel and unlike particles attract

Electrical ForceClusters of positive and negative particles have

been pulled together by the enormous attraction of the electrical force.

By forming the compact and evenly mixed clusters of positives and negatives, the huge forces have balanced themselves out almost perfectly.

These clusters are the atoms of matter.

Electrical ForceWhen two or more atoms join to form a

molecule, the molecule also contains balanced positives and negatives. The same happens with trillions of molecules.

Between two pieces of ordinary matter, there is scarcely any electrical attraction or repulsion at all, because each piece contains equal numbers of positives and negatives

When this happens, there is no net electrical force.

FactWhich charges are called positive and which are

called negative is the result of a choice made by Benjamin Franklin. It was completely arbitrary and could have easily been the other way around.

Electrical ForceSo, like charges repel and unlike charges attract.

Electric ChargesThe terms positive (+) and negative (- ) refer to

electric charge.

Protons (+)

Electrons (-)

Neutrons (0)

The attractive force between these particles causes them to lump together into incredibly small units – atoms.

Electrical ChargesImportant facts about atoms:

Every atom is composed of a positively charged nucleus surrounded by negatively charged electrons.

The electrons of all atoms are identical. Each has the same quantity of negative charge and the same mass.

Electric ChargesFacts about atoms:

Protons and neutrons compose the nucleus. Protons are about 1800 times more massive than electrons, but they carry an amount of positive charge equal to the negative charge of electrons. Neutrons have more mass than protons, but no net charge.

Atoms usually have as many electrons as it does protons, so the atom has zero net charge.

Electric ChargeSo, why don’t protons pull the oppositely

charged electrons into the nucleus?

Because an electron behaves like a wave and requires a certain amount of space related to its wavelength.

So what holds the nucleus together?

In addition to the electrical forces in the nucleus, there are even stronger nonelectrical nuclear forces that are able to hold the protons together in spite of the electrical repulsion.

QuestionHow does the charge of an electron differ from

the charge of a proton?

There is a fundamental rule of charges. This rule is?

Conservation of ChargeIn a neutral atom, there are as many electrons

as protons, so there is no net charge.

The positive balances the negative exactly.

If an electron is removed from an atom, then it is no longer neutral.

The atom then has one more positive charge (proton) than negative charge (electron) and is said to be positively charged

A charged atom is called an ion.

Conservation of ChargeA positive ion has a net positive charge, so it has

a lack of electrons.

A negative ion has a net negative charge, so it has an excess of electrons.

Conservation of ChargeObjects ordinarily have equal numbers of

electrons and protons and, therefore, are electrically neutral.

An imbalance comes about when electrons are added to, or removed from, an object.

Although electrons closest to the atomic nucleus, the innermost electrons, are bound very tightly to the oppositely charged atomic nucleus, the outermost electrons, are bound very loosely and can be easily dislodged.

Conservation of ChargeSo how much work is required to tear an

electron away from an atom varies from one substance to another.

Different substances give up their electrons more readily.

Electrons can be rubbed off of different materials and added to others.

Conservation of ChargeFor example, you comb your hair.

The electrons are held more firmly to the plastic of the comb than in your hair. So when you comb, the electrons transfer to the comb from your hair.

This leaves the comb negatively charged (it has more electrons)

And your hair positively charged (it lost electrons, so it has more protons than electrons)

Conservation of ChargeIf it has more electrons than protons it is

negatively charged

If it has more protons than electrons it is positively charged.

When we charge something, no electrons are created or destroyed, they are simply transferred from one material to another. Charge is conserved.

This is the law of Conservation of Charges.

Conservation of ChargeAny object that is electrically charged has an

excess or deficiency of some whole number of electrons.

Electrons cannot be divided into fractions of electrons.

This means that the charge of the object is a whole number multiple of the charge of an electron.

Conservation of ChargeCharge is “grainy” or made up of elementary

units called quanta.

We say that charge is quantized, with the smallest quantum of charge being that of the electron or proton.

No smaller units of charge have ever been observed.

QuestionIf you scuff electrons onto your feet while

walking across a rug, are you negatively or positively charged?