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.