Chapter 1

Electrostatic phenomena

CLIL

Unit 1

PHYSICS Electromagnetism, Relativity and Quantum physics

by Borracci and Carbone

FREELY ADAPTED FROM

Electrostatic Phenomena – Foundations of Electricity, Lesson 38

by Julius Sumner Miller

http://www.youtube.com/watch?v=o33VNRQdmAo

AND

• V sec B.C.

• Thales of Miletus

• Pre-socratic phylosopher

• Mathematician

• Great physical discovery:

A piece of history

If amber is rubbed with a piece of fur

it acquires the property of attracting onto itself

light bits of straw and dust.

→ some iron dust →1 hard rubber

rod

1° experiment

Nothing happens!

electrostatically neutral

=

identical number of positive and negative electricity

There is no interaction between the rod and the iron dust!

• Electric charge is a fundamental physical property of

matter.

• Charges come in two types arbitrarily named

+ positive charge, as the charge of the proton,

- negative charge as the charge of the electron.

• When on an object the net amount of positive charge is

equal to the net amount of negative charge, we say that

this object is neutral.

Charges

Like charges repel,

while unlike, or opposite charges attract.

Rubbing the rod is a mechanical action that permits to separate the charges.

The fur loses electrons

becoming positively charged

while the rod acquires electrons.

The rod is charged .

Static electricity caused by friction

Only the electrons can move from an object to another one.

1° experiment bis …

→ a piece of fur

• Rub the rod

• Approach it to the iron dust

• Iron dust is attracted by the rod!

By convention we define

• the charge on the rubber rod, rubbed with a fur, as negative

• the charge on the glass rod with silk we will define the charge on the rod as positive.

What happens with the glass rod and the silk?

The glass loses electrons , becoming positively charged,

while the silk acquires electrons becoming negatively charged.

Rubber and glass:

two different behaviors

glass & glass rubber & rubber glass & rubber

silk silk

glass

silk

glass glass

fur fur fur

rubber rubber rubber Total

charge =0

Total

charge =0

The law of conservation of charge

The net amount of charge that we have totally

(considering the rod and the silk) is zero, as zero was the total amount of charge

before I rubbed the glass rod with the silk.

The net charge of an isolated system remains constant.

Electric charge can neither be created nor destroyed

The total amount of charge in the Universe is constant.

The change in the amount of electric charge in any volume of space is

exactly equal to the amount of charge flowing into the volume minus the

amount of charge flowing out of the volume.

• Insulators:

the charge doesn’t flow.

It remains in the same place

where it was generated.

• Conductors:

the charge flows freely.

2° experiment

Conductors and insulators

pieces of paper

attracted

pieces of paper

not attracted

The electroscope

metal

knob

metal

rod

light

gold

leaves

• Rub the rubber rod on the fur

• Touch the metal knob with the rod

• Some negative charge is deposited

on the knob

3° experiment

The conduction

The electroscope is charged negatively by conduction

The electroscope has been grounded

• Touch the metal knob with your finger

• The electroscope is still neutral

• Some negative charge has gone to the earth

Charging by conduction is typical of metals.

Touch the electroscope

with a charged sphere.

Observe that the leaves

diverge.

4° experiment Determine the nature of an unknown charge

Ground the electroscope.

Touch the electroscope

with another charged

sphere and observe that

the leaves diverge further.

Now we can choose a unit of charge.

Charge on the electron:

In the SI, the unit of electric charge is the Coulomb.

Electric charge is quantised in units of electron

charge. The total charge carried by any object

is a whole multiple of the electron charge.

e = −1.602× 10−19

C

A polar molecule is neutral

overall, but its charge is not evenly

distributed.

water molecule

Only within the past century has it become clear that an

understanding of electricity originates from within

the atom itself. This simplified model of an atom shows

a small, dense, positively charged nucleus containing

a mixture of protons and neutrons surrounded by

negatively charged electrons.

All protons and electrons have exactly the same magnitude of electric charge.

The Coulomb

5° experiment

The induction

Approach the knob

with a charged rod. The leaves diverge. The leaves collapse.

The electroscope makes connection with the charged rod

because of induction

+ +

+

+

- - -

-

L1 COMPRENSIONE 1

Metodi di elettrizzazione a confronto

L1 COMPRENSIONE 2

Guardando più a fondo: la polarizzazione

L1 COMPRENSIONE 3

Domande aperte

L1 COMPRENSIONE 4

Quesiti a scelta multipla

Soluzioni: 1b,2b,6c,7b

Soluzioni: 3b,4b,5a,8b

L1 COMPRENSIONE 4

Esercizi