Properties of Objects

TOC

1

Physics 212Electric Charge

Properties of Objects

Fundamental Particles

Atoms and Ions

Quantization of Charge

Conservation of Charge

Conductors, Insulators and Semiconductors

How to Put Charge on an Object

Polarization

TOC

2


Physical Properties

That which is proper to anything;a peculiar quality of a thing;that which is inherent in a subject, or naturally essential to it;an attribute; as, sweetness is a property of sugar. Physical properties, or those which result from the relations of bodies to the physical agents,light, heat, electricity, gravitation, cohesion, adhesion, etc.,and which are exhibited without a change in the composition or kind of matter acted on. http://www.wordiq.com/dictionary/Properties

Examples of physical properties of objects include…

ColorMassHardness

TOC

3


Intrinsic Physical Properties

An intrinsic physical property is a property of a set of objects that is constant throughout the entire set over space and time.

Examples of intrinsic physical properties of an NFL football include…

shape

color

texture

material

size

weight

TOC

4


Intrinsic Physical Properties of Fundamental Particles

Everything in the universe is made up of a finite number of fundamental particles.

Each of these particles has the following intrinsic physical properties.

mass

spin

size

charge

TOC

5


There are only two types of charge called positive and negative

Unlike charges attractand like charges repel.

TOC

6


Why positive and negative?

Imagine what it would be like for the types of charges to be named in a different way.

Which of these would best lead to an understanding of the difference between these types of charge and why?

A and BGood and EvilLeft and RightJack and Jill ?21 Evil Good

TOC

7


All figures shown here are from Lawrence Berkley National Laboratory“The Particle Adventure”

The universe is made of 28 fundamental particles6 quarks, 6 leptons, their antiparticles and some force particles.

TOC

8


Most charge that we experience is found in the quarks and the electron.

The fundamental unit of charge is

(C) Coulombs 1910602.1 e

Each quark carries a fraction of this charge.

Electrons have a charge of -e


TOC

9


Fundamental Charge

Quarks are only typically in combinations of three called protons and neutrons

A proton contains 1 down and 2 up quarks (+e charge)A neutron contains 1 up and 2 down quarks (0 charge)An electron is a fundamental particle (-e charge)


TOC

10


Positive charge is created when an atom loses an electron.

This is called a positive ion.

Negative charge is created when an atom gains an electron.

This is called a negative ion.

Charge found on most objects comes typically from one place…

The object has either too many or too few electrons.

TOC

11


All ordinary matter consists of …

Protons Mass = 1.67x10-27 kg Charge = +eNeutrons Mass = 1.67x10-27 kg Charge = 0Electrons Mass = 9.11x10-31 kg Charge = -e

All ordinary matter must therefore havea total charge which is an integermultiple of e.

Charge is designated by eitherQ or q.

eNeNQ electronsprotons

TOC

12


Example 1

Dr. Mike puts electrons on a neutral balloon by rubbing it in his hair. The balloon gains -6.3 μC of charge. How many excess electrons did the balloon receive from Dr. Mike?

Answer

The number is given by Nelectrons in the equation from the previous slide.

electrons

QN

e

6

19

6.3 10 C

1.602 10 C

133.93 10

TOC

13


Example 2

Dr. Mike puts electrons on a neutral balloon by rubbing it in his hair. The balloon gains -6.30 μC of charge. By how much did Dr. Mike’s weight change?

Answer

In Example 1, we calculated that the balloon received 3.93 x 1013 electrons. The weight of these additional electrons is then found as follows.

electrons electronM N m

13 313.93 10 electrons 9.11 10 kg 173.58 10 kg

17 163.58 10 kg 9.81 m/s 3.51 10 Nw Mg

TOC

14


Example 3

Dr. Mike puts electrons on a neutral balloon by rubbing it in his hair. His hair loses 3.5 x 1016 electrons. How much charge does Dr. Mike have when he is finished?

Answer

The charge is given by Q in the equation from the previous slide.

protons electronsQ N e N e

16 19

19

3.5 10 protons 1.602 10 C

0 electrons 1 602 10 C-

- .

35.6 10 C 5.6 mC

TOC

15


Conservation

The amount of charge in the universe never changes.

Example

Dr. Mike puts electrons on a balloon by rubbing it in his hair. The balloon gains -6.3 μC of charge. How much charge does Dr. Mike’s hair have after he has done this?

Answer

The electrons moved from Dr. Mike to the balloon. Thus, the charge that move to the balloon was taken away from Dr. Mike and he has +6.3 μC of charge more than he had when he began.The total charge in the universe is still the same as it was when he began.

TOC

16


You can easily determine the number of electron and protons in a mole of some element.

Example

How much negative electron charge is in 3 moles of oxygen?

Answer

There are Avogadro’s number of oxygen atoms in one mole. Using the periodic table (look it up on the web or in your chemistry textbook), we see that there are 8 electrons in on oxygen atom. And there is –e charge in each electron. So we have…

electron

C10602.1

atomoxygen

electrons8

mole

atoms1002.6moles 3 1923Q

C1031.2 6Q Note: There are just as many protons. So 3 moles of oxygen is actually neutral.

TOC

17


In an insulator, all of the electrons are bound to a nucleus…even the excess charges which stay right where they were forced to be by friction.

TOC

18


In a conductor, some of the electrons are free to move between atoms.

Excess electrons will move freely as well, but they will stay on the surface of the conductor.

TOC

19


In a semiconductor, the electrons cannot move unless given a high enough energy.

They are insulators when the energy is low.

and conductors when the energy is high

TOC

20


A photoconductor is a semiconductor that uses light as its input energy.

They are insulators when the light energy is low.

and conductors when the light energy is high

TOC

21


Dipoles

Dipoles are pairs of equal positive and negative charge that can rotate but not translate.

The intrinsic physical properties of the dipole are its charge and its length. (It also has mass and spin but we will almost never speak of the mass or spin of a dipole.)

The dipole is sometimes described by its dipole moment. The magnitude of the dipole moment is given by the equation

and it points from positive to negative in direction.

L

qLp

-q

+qp

TOC

22


Polarization

Dipoles always attract charged objects.

1. A negatively charged object causes the dipole to rotate so that the positive charge is closer to the conductor.

2. A positively charged object causes the dipole to rotate so that the negative charge is closer to the conductor.

TOC

23


Inductors and conductors sometimes exchange charge by rubbing them together.

Use the Tribolectric Table to know which way the charges move.

TOC

24


In conductors, electrons are free to move…

There are two ways to add charge to a conductor.

1. Touching a charged rod to a conducting sphere transfers electrons to the sphere.

This is called charging by contact.

A negative total charge remains

What would happen if the rod were positive to begin with?

TOC

25


2. If a charged rod is simply brought near a conducting sphere, the electrons will begin to move. And, some will go into the ground.

This is called charging by induction.

In conductors, electrons are free to move…

There are two ways to add charge to a conductor.

A positive total charge remains

What would happen if the rod were positive to begin with?

TOC

26


In insulators, electrons are not free to move. They stick to the atom…

Insulators can shift their charges and will therefore attract any charged conductor.

1. A negative conductor repels the electrons and is then attracted by the closer protons.

2. A positive conductor attracts the electrons and is then attracted by these closer electrons.

TOC

27


Dipoles are pairs of positive and negative charge that can rotate but not translate.

Like insulators they always attract the conductor.

1. A negative conductor causes the dipole to rotate so that the positive charge is closer to the conductor.

2. A positive conductor causes the dipole to rotate so that the negative charge is closer to the conductor.

TOC

28


Properties of Objects

Fundamental Particles

Atoms and Ions

Quantization of Charge

Conservation of Charge

Conductors, Insulators and Semiconductors

How to Put Charge on an Object

Polarization

Documents

Properties of Objects