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Physics 212Charge

Properties of Objects

Fundamental ParticlesFundamental Particles

Atoms and Ions

Quantization of Charge

Conservation of Charge

C d t I l t d S i d t

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Conductors, Insulators and Semiconductors

How to Put Charge on an Object

Polarization

Physics 212Charge

Physical Properties

That which is proper to anything;a peculiar quality of a thing;th t hi h i i h t i bj t t ll ti l t itthat 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

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http://www.wordiq.com/dictionary/Properties

Examples of physical properties of objects include…

ColorMassHardness

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Physics 212Charge

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…

shapematerial

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color

texturesize

weight

Physics 212Charge

Intrinsic Physical Properties of Fundamental Particles

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

Each of these particles has the following intrinsic physical properties.

masssize

4 spin

size

charge

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Physics 212Charge

There are only two types of charge called positive and negative

Unlike charges attractUnlike charges attractand like charges repel.

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Physics 212Charge

Why positive and negative?

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

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Good and EvilLeft and RightJack and Jill ?21 =+ Evil Good

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Physics 212Charge

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

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

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Physics 212Charge

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

The fundamental unit of charge isThe fundamental unit of charge is

(C) Coulombs 1910602.1 −×=e

Each quark carries a fraction of this h

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

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charge.

Electrons have a charge of -e

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Physics 212Charge

Fundamental Charge

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

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

9A 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)

Physics 212Charge

Charge found on most objects comes typically from one place…

The object has either too many or too few electrons.

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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.

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Physics 212Charge

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 = eElectrons 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 either

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g g yQ or q.

( ) ( )eNeNQ electronsprotons −++=

Physics 212Charge

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

felectrons did the balloon receive from Dr. Mike?

Answer

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

Q 6

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electronsQNe

=−

6

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6.3 10 C1.602 10 C

−

−

− ×=− ×

133.93 10= ×

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Physics 212Charge

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∆ =

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electrons electron

( ) ( )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 − −∆ = ∆ = × = ×

Physics 212Charge

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= + −

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( )( )

16 19

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3.5 10 protons 1.602 10 C

0 electrons 1 602 10 C-- .

−= × × +

×35.6 10 C 5.6 mC−= × =

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Physics 212Charge

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

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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.

Physics 212Charge

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

Examplep

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

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chemistry textbook), we see that there are 8 electrons in on oxygen atom. And there is –e charge in each electron. So we have…

( )

×−

×= −

electronC10602.1

atomoxygen electrons8

moleatoms1002.6moles 3 1923Q

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

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Physics 212Charge

In an insulator, all of the electrons are bound to a nucleus.

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

18These electrons stay on the surface of a conductor.

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Physics 212Charge

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

They are insulators when the energy is low.

19 and conductors when the energy is high

Physics 212Charge

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

They are insulators when the light energy is low.

20 and conductors when the light energy is high

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Physics 212Charge

Dipoles

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

-q

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.)

L

q

+qpr

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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.

qLp =

Physics 212Charge

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.

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Physics 212Charge

Inductors and conductors sometimes exchange charge by rubbing them together.

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

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Physics 212Charge

1. Touching a charged rod to a conducting sphere transfers

In conductors, electrons are free to move…

There are two ways to add charge to a conductor.

conducting sphere transfers electrons to the sphere.

A ti

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

24This is called charging by contact.

A negative total charge remains

begin with?

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Physics 212Charge

2. If a charged rod is simply brought near a conducting

In conductors, electrons are free to move…

There are two ways to add charge to a conductor.

brought near a conducting sphere, the electrons will begin to move. And, some will go into the ground.

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

25This is called charging by induction.

A positive total charge remains

begin with?

Physics 212Charge

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

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

1 A negative conductor repels 2 A positive conductor attracts1. 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.

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Physics 212Charge

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.

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Physics 212Charge

Properties of Objects

Fundamental ParticlesFundamental Particles

Atoms and Ions

Quantization of Charge

Conservation of Charge

C d t I l t d S i d t

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Conductors, Insulators and Semiconductors

How to Put Charge on an Object

Polarization