electric static (part i introduction)

By Mr. Chaiporn PattanajakBanphue pittayasan School

Facebook.com/chaiporn.pattanajak084-9250671 , 082-7364148

I. Electric Charge, Forces, and Fields II. Electric Potential III. Capacitance

This lesson we learn about

Around the outside of an atom are electrons, which have a negative charge.

An atom has equal amounts of negative and positive charges, which balance each other, so the atom has no overall charge.

Where does static charge come from?

The nucleus at the centre of an atom contains protons, which have a positive charge.

Electrons do not always stay attached to atoms and can sometimes be removed by rubbing.

Charging by Friction• A Styrofoam

plate when rubbed with wool gets charged by friction.

Ways to Charge an Object• Friction:

– When two objects come in contact, one object may lose electrons

– The other object will gain them!

– How easily an object gains or loses electrons depends on what it is made of!

Electric Charge

Like charges repel; unlike charges attract.

As with Mass/Energy: Charge is conserved

The net charge of an isolated system remains constant.

This is another fundamental law in physics

Properties of Charges

Properties of Charges

Like charges repel,

Unlike charges attract.+ +

+ -

Electric ChargeSI unit of charge: the coulomb, C. All charges in nature are integer multiples of the charge on the electron:

Conductors

Most atoms hold on to their electrons tightly and are insulators. In aluminum, the valence electrons are essentially free and strongly repel each other. Any external influence which moves one of them will cause a repulsion of other electrons which propagates, "domino fashion" through the conductor.

In a conducting material, the outer electrons of the atoms are loosely bound and free to move through the material.

Conductors

Property of material: conductivity

1.Conductors transmit charges readily.

2.Semiconductors are intermediate; their conductivity can depend on impurities and can be manipulated by external voltages.

3.Insulators do not transmit charge at all.

Electric charge is conserved

– the arithmetic sum of the total charge cannot change in any interaction.

Static Electricity; Electric Charge and its Conservation

METHODS OF CHARGING 1. Charging by Friction

•Transfer of electrons between the two objects that are rubbed together. •Material losing electron is positively charged and material gaining electron is negatively charged. •Amount of gained and lost electron is equal to each other.

.

Conductor:

Charge flows freely

Metals

Insulator:

Almost no charge flows

Most other materialsSome materials are semiconductors.

Insulators and Conductors

Metal objects can be charged by conduction:

2. Charging by Conduction

They can also be charged by induction, either while connected to ground or not:

Nonconductors won’t become charged by conduction or induction, but will experience charge separation:

The electroscope can be used for detecting charge:

The Electroscope

Electrostatic ChargingObjects can have excess charge of one polarity or another. An electroscope may be used to determine if an object is electrically charged.

Charging by conductionAn electroscope can be given a net charge by conduction – when it is touched with a charged object, the excess charges flow freely onto the electroscope.

The charged electroscope can then be used to determine the sign of an unknown charge.

Charging of two bodies by induction when they are in contact with each other.

3. Charging by Induction•Method used to charge an object without actually touching the object to any other charged object.

Charging by Induction• There is no contact with the charging body.

Charging by Induction

Charging by Induction

Charging by induction• In charging by

induction, a charged body A imparts some charge to another body B without any actual contact between the two.

To charge a single metal conductor by induction

To charge a single metal conductor by induction

To charge a single metal conductor by induction

To charge a single metal conductor by induction

Polarization of a Cloud

Detailed Lightning Diagrams

One mechanism incorporates friction: when moist, warm air rises, it cools and water droplets form. These droplets collide with ice crystals and water droplets in a cloud. Electrons are torn off the rising water droplets by the ice crystals. The positive droplets rise to the top of the cloud, while the negative ice crystals remain at the bottom.

A second mechanism involves the freezing process: experiments have shown that when water vapor freezes the central ice crystal becomes negatively charged, while the water surrounding it becomes positive. If rising air tears the surrounding water from the ice, the cloud becomes polarized.

There are other theories as well.

Lightning is the discharge of static electricity on a massive scale. Before a strike the bottom part of a cloud becomes negatively charged and the top part positively charged. The exact mechanism by which this polarization (charge separation) takes place is uncertain, but this is the precursor to a lightning strike from cloud to cloud or cloud to ground.

Lightning StrikesThe negative bottom part of the cloud induces a charge separation in the ground below. Air is normally a very good insulator, but if the charge separation is big enough, the air between the cloud and ground can become ionized (a plasma). This allows some of the electrons in the cloud to begin to migrate into the ionized air below. This is called a “leader.” Positive ions from the ground migrate up to meet the leader. This is called a “streamer.” As soon as the leader and streamer meet, a fully conductive path exists between the cloud and ground and a lightning strike occurs. Billions of trillions of electrons flow into the ground in less than a millisecond. The strike can be hotter than the surface of the sun. The heat expands the surrounding air; which then claps as thunder.

+ + + + + + + + +- - - - - - - - -

++ + + +

++ + +

A Van de Graaff generator consists of a large metal dome attached to a tube, within which a long rubber belt is turning on rollers. As the belt turns friction between it and the bottom roller cause the e-’s to move from the belt to the roller. A metal brush then drains these e-’s away and grounds them. So, as the belt passes the bottom roller it acquires a positive charge, which is transported to the top of the device (inside the dome). Here another metal brush facilitates the transfer of electrons from the dome to the belt, leaving the dome positively charged.

In short, the belt transports electrons from a metal dome to the ground, producing a very positively charged dome. No outside source of charge is required, and the generator could even be powered by a hand crank. A person touching the dome will have some of her e-’s drained out. So, her lightweight, positive hair will repel itself. Coming close to the charge dome will produce sparks when electrons jump from a person to the dome.

Van de Graaff Generator

Internal workings Detailed explanation