The Electrical Nature of Matter

Atoms always contain electric charges, but we don’t notice them until we make them move from their normal positions.

Atoms have equal numbers of protons and electrons.

Protons cannot move; electrons move. Protons and electrons have the same

amount of charge, but their charges are opposite.

When atoms become charged, only the electrons move from atom to atom.

In each atom the number of electrons surrounding the nucleus equals the number of protons and so a single atom is electrically neutral.

In some elements (e.g. copper - Cu) the nucleus has a weaker attraction to its electrons and the electrons are able to move freely from atom to atom.

In elements such as sulfur (S) the electrons are strongly bonded to the atom and do not move freely.

If an atom gains an extra electron, the overall (net) charge on the atom is negative and the atom is called a negative ion.

If the atom loses an electron, the overall charge is positive and the atom is called a positive ion.

Like charges repel. Unlike charges attract.

Think about this: Clothing often becomes electrically charged when the articles rub against one another in the dryer. One substance loses electrons and the other gains electrons, so there is a charge separation: one substance will have a positive charge (it lost electrons) and the other will have a negative charge (it gained the electrons the other substance lost).

The study of charge separation (“static electricity”) is called electrostatics.

There are 3 ways to make an object have an electrical charge:

by friction by contact and by induction

Charging by FrictionElectrons are transferred from one

material to another by direct rubbing.

One material will pick up electrons more easily than the other and become negatively charged while the material that loses electrons more easily will become positively charged.

Examples: Tumbling clothes in a dryer Walking across a carpet Gasoline rushing out of a hose at

a gas station Dry air rushing over the surface

of a car or a plane Wearing pantyhose and a skirt Combing your hair

The Electrostatics Series Acetate weak hold on electrons Glass Fur, human hair, wool Calcium, lead, magnesium Silk increasing tendency Aluminum to gain electrons Cotton Paraffin wax Ebonite Plastic/ polyethylene Carbon, copper, nickel Rubber Sulfur Platinum, gold strong hold on electrons

A plastic comb and a woolen sweater are both uncharged – electrically neutral. Neither will attract bits of paper.

However, when the comb is rubbed by the woolen sweater, the comb gains electrons and becomes negatively charged and the sweater loses electrons and becomes positively charged.

Both will now attract neutral objects.

Charging by Contact

One of the objects is already electrically charged. The other object may or may not be charged, too.

Touching the second object causes a transfer of the same charge that the first object had. Both objects will have the same charge.

e.g.A doorknob is usually uncharged, and so

are you until you walk across the carpeted floor, picking up extra electrons.

When you touch the doorknob, you transfer excess negative charge to the doorknob. These charges are shared very rapidly, and a surprising – even painful, shock is experienced.

The transfer of electrons can be heard and even seen as a spark of light!

Insulators and Conductors

An electrical insulator is a

material in which electrons cannot move freely from atom to atom.

If some atoms of an insulator gain some electrons, these electrons stay on the same atoms until removed by a substance that exerts an even stronger force on the electrons.

e.g. When we polish wooden furniture or glass (both insulators), the electric charges stay on the surfaces and don’t move easily through the surfaces. These built-up static charges actually attract uncharged dust particles!

Electrons cannot be conducted through insulators, and so these materials can protect us from electric shocks.

e.g. Plastic or rubber wrapping on household appliance cords does not allow the charge to move freely when we touch the cord.

An electrical conductor is a material in which electrons can move freely from one atom to another.

If a conductor becomes negatively charged with extra electrons, they move freely along the conductor.

e.g. Copper wires are used to conduct electricity in our homes. Touching a bare wire would cause a severe shock, even death, because the electrons are able to move so freely. Also, you must never stick a metal knife into a toaster to remove a jammed piece of bread as the metal knife would easily transfer electrons into your hand when it comes in contact with the electricity in the toaster. Ouch!!

GOOD CONDUCTORS Silver Ag Tungsten WCopper Cu Nickel NiGold Au Mercury HgAluminum Al Platinum PtMagnesium Mg Iron FeSelenium (in the light) Se

FAIR CONDUCTORS Carbon C Acid solutionsNichrome Salt waterHuman body EarthMoist human skin Water vapour in air

GOOD INSULATORS Oil Plastic EboniteFur Wood GlassSilk Paper WaxWool Rubber Selenium (in the dark)

If a charged object has all the excess charges removed, it is said to be discharged or neutralized.

  The simplest way to discharge an

object is to connect it to the Earth itself by means of a conductor, such as a wire or metal rod buried in the ground. This is called grounding. A grounded object shares its charge with the entire Earth.

Examples: No matter how hard you polish a metal tap in your

kitchen or bathroom, it never builds up static charge. The extra electrons repel one another and are conducted away from the taps along metal water pipes to the main water supply pipe where they move into the ground. The electron charge is conducted away as soon as it is produced.

Gasoline pumps are grounded. Lightning rods protect our homes by discharging

the energy from a lightning strike into the ground. People who work assembling sensitive electronic

equipment usually wear metal straps on their wrists which are attached to a grounding system.

Cars and aircraft build up static charges but grounding wires don’t usually work for them, although some vehicles do have strips or chains that ground them to the road. However, the surface shape of a charged conductor affects the rate at which it becomes discharged.

An example of this would be static wicks on planes. Electrons at the sharply curved point of a negatively charged rod repel one another so strongly that the electrons at the tip are actually pushed off in a continuous stream. This is called discharge at a point.

Charged objects can, over a period of time, be discharged by exposure to the air as well as exposure to light and radioactivity. Although dry air is an insulator, water molecules in moist air easily transfer electric charges.

Electrons are always moving. 

Charging by Induction Electrons in an object or material can

move around within it. A charged object can induce (cause) a

movement of electrons in another object without touching it.

The electrons realign themselves, moving away from a like charge or towards an unlike charge.

The electrons do not move into the other object but change position.

Induction explains why there is an attraction between a neutral object and a charged object.

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A comb can be charged by friction. Rubbing it with wool will cause it to gain electrons from the wool, so the comb will have a negative charge.

If you bring the now negatively charged comb near a neutral pith ball, the electrons in the pith ball will realign themselves and move away from the side of the pith ball nearest the comb. (The pith ball is still neutral overall because no electrons have been gained or lost.)

However, the side of the pith ball nearest the charged comb will be attracted to the comb as if it had an opposite charge, which it only has on that one side because the electrons have moved away from the like charge on the comb. This is charge by induction.

Negatively charged comb attracts neutral pith ball but they do not touch.

neutral pith ball Induction

If the pith ball and comb touch, electrons actually move from the comb to the pith ball. The side of the pith ball that has touched the comb now has the same charge as the object that touched it. The pith ball repels from the comb. This is charge by contact.

Neutral pith ball becomes negative when it contacts comb

Contact

Negative pith ball and negative comb repel each other.

Like charges

repel.

In your notebooks, write down three things you learned in class today.

On your copy of these notes, highlight the most important words.

Then, write three questions you would like us to answer in this unit.