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Chapter 15 Static Electricity

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• Chapter 15

Static Electricity

• At the end of this chapter, you should be able to

� Show understanding that electrostatic charging by rubbing involves a transfer of electrons.

� State that there are positive and negatives charges and that charge is measured in coulombs.

� State that unlike charges attract and like charges repel.

• At the end of this chapter, you should be able to

� Describe an electric field as a region in which an electric charge experiences force.

� Draw the field of an isolated point charge and show understanding that the direction of the field lines gives the direction of the force acting on a positive charge.

� Describe experiments to show electrostatic charging by conduction.

• Quiz Time

Use a piece of tissue paper to rub a plastic ruler. Place the rubbed side of the plastic ruler near your friend’s hair. What do you observe and why did it happen?

A few strands of hair are attracted to the rubbed surface of the ruler. This is caused by electrostatics.

• 15.1 Laws of Electrostatics

� Electrostatics is the study of

� Static electricity happens when

within a material are

static electric charges or electric charges at rest.

positive charges and negative chargesnot equal.

• Structure of an Atom

� The atoms of all substances contain both positively-charged particles and negatively-charged particles.

––

++

+

+

electron

proton

neutrons

nucleus

nucleus

orbits of

electrons

• Structure of an Atom

� All matter are made of invisible particles

called

––

++

+

+

electron

proton

neutrons

nucleus

nucleus

orbits of

electrons

atoms. (Can be seen using an electron microscope.)

• Structure of an Atom

� Each atom has negatively-charged orbiting round a

massive which consists of positively-charged particles called and neutral particles called

- electron

- proton

- neutron

electrons nucleus

protons neutrons

• Structure of an Atom

� An atom is electrically balanced (uncharged) when the amount of protons is to the amount of electrons.

+ ++−− −

A substance that is uncharged.

equal

• Structure of an Atom

� Atom is positively-charged when the amount of protons is the amount of electrons.

+ ++−−

A substance that is positively-charged.

+

more than

• Structure of an Atom

� Atom is negatively-charged when the amount of protons is the amount of electrons.

+ +−−

A substance that is negatively-charged.

−−

less than

• Measurement of electric charges.

� Electric charges are measured in( )coulomb C

Q = I × t

where Q is electric charges (C)

I is electric current (A)

t is time (s)

• Measurement of electric charges.

� The electric charge for one electron or proton is 1.6 x 10-19 C

• 15.2 Electric Field

� An electric field

� The direction of the field is defined as

is a region where an electric charge experiences an electric force.

the direction of the force on a small positive charge.

• 15.2 Electric Field

+

• 15.2 Electric Field

-

• 15.2 Electric Field

� The strength of the field is indicated by

The closer the field lines, the is the electric field in that region.

� The strength of the field is

how close the field lines are toeach other.

stronger nearer the charge and decreases further away from the charge.

stronger

• 15.2 Electric Field

-+

• 15.2 Electric Field

++

• 15.2 Electric Field

-

+ + + + + + +

• 15.2 Electric Field

++

++

++

+

ll

ll

ll

l

ll

ll

++

++

• 15.3 Laws of Electrostatics

� A material can be charged by

� Static electricity occurs when two different materials are

against each other.insulating

rubbed

friction.

• 15.3 Laws of Electrostatics

� During charging, are transferred from one material to another material

electrons

• +

+

+

-

--

+ + +

+-

-

--

15.3 Laws of Electrostatics

� A material become positively-charged when some electrons are

-

--

wool

polytheneWool becomes

positively-charged

lost by the material.

• +

+

+

-

--

+ + +

+-

-

--

15.3 Laws of Electrostatics

� A material become negatively-charged when some electrons are

-

--

wool

polythenePolythene becomes

negatively-charged

gained by the material.

• 15.3 Laws of Electrostatics

� Charges arein the friction process, it

is only

(Similar to Energy)

neither created nor destroyed

transferred from one place to another.

• Quiz Time

Rub a balloon on a woolen sweater. Then place it near some bits of paper. Observe what has happen?

The paper will be attracted to the balloon.

• Quiz Time

Run another balloon on a woolen sweater and placed it near to the balloon that was previously rubbed with the woolen sweater. Observe has happen?

The balloons will repel one another.

• From what we have learned so far, we can conclude that:

� Only two charges (positive and negative) exist.

� Friction produces 2 different kinds of charges on different materials.

� Positively-charged material gives away electrons while negatively-charged material takes in electron.

� Unlike charges attract while like charges repel.