Physics 102-002 Announcements

• WebAssign – – No assignment for Chapter 11

• Exam #2 corrections not graded yet

Photo: A test shot at the Z Pinch device at Sandia National Laboratory, generating a voltage of 8 million volts.

Class Schedule

3/11 – 3/18

Spring Recess

3/19 Chapter 10 Projectile and Satellite Motion (Pg 184-191)

3/21 Chapter 11 Atomic Nature of Matter (Pg 211-223)

3/26 Chapter 22 Electrostatics, Part 1 (Pg 410-419)

3/28 Chapter 22 Electrostatics, Part 2 (Pg 419-430)

4/2 Chapter 23 Electric Current, Part 1 (Pg 436-443)

4/4 Chapter 23 Electric Current, Part 2 (Pg 444-452)

Chapter 22Electrostatics, Part 1

• Electrical Forces• Electric Charges• Conservation of Charge• Coulomb’s Law• Conductors and Insulators

– Semiconductors– Superconductors

• Charging• Charge Polarization• Electric Field• Electric Potential• Electric Energy Storage

Next time

Electrical Forces • There are 2 types of electrical charges

• A positive (+) charge• A negative (-) charge

• A force always exists between 2 charged objects:

• Like charges repel

• Opposite charges attract

Sign of the charge applethttp://kingfish.coastal.edu/physics/physlets/Electrostatics/sign_of_charge.html

Physics Place videos

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+ -

(c) repulsion

(b) attraction

(a) attraction

Electric Force Applethttp://www.colorado.edu/physics/2000/waves_particles/wavpart2.html

Electric ChargesElectrons have a negative (-) charge

The Greeks were the first to discover electricity about 2500 years ago.

When a piece of amber was rubbed with other materials it would attract small objects such as dried leaves, or straw.

The Greeks word for amber is electron.

The word electric was derived from it and meant "to be like amber."

Protons have a positive (+) charge

Facts about atoms:

All matter is made up of atoms, which are made up of protons and electrons. The positively charged nucleus is surrounded by negatively charged electrons. The electrons of all atoms are identical. Each has the same amount of (-) charge and mass.

Protons and neutrons compose the nucleus. Protons and electrons have an equal and opposite charge. Neutrons have no net charge.

In the natural state, all atoms have the same number of protons and electrons, so they are electrically “neutral”. (They have no NET charge).

A helium atom has 2 protons and 2

electrons

So why don’t atoms fly apart or collapse? The answer is in quantum mechanics.

Question 1How do the charges of an electron and a proton differ?

A. They are the same.

B. They are equal in magnitude, but opposite in sign.

C. The electron charge is 3 times that of the proton.

D. The proton charge is much more colorful.

Question 1 Answer

• How do the charges of an electron and a proton differ?

A. They are the same.

B. They are equal in magnitude, but opposite in sign.

C. The electron charge is 3 times that of the proton.

D. The proton charge is much more colorful.

Conservation of ChargeCharge conservation is the principle that electric charge can neither be created nor destroyed. The quantity of electric charge is always conserved.

Neutral atom

+Charged

“ion”+ e-

(an electron)

You can move charges around (like by rubbing a glass rod with a silk cloth – stripping electrons off atoms in the glass).

Physically add energy

After you do this, the glass rod has a net “positive” charge and the silk has a net “negative” charge.

NOTE: In this case, like all cases, we’re not “creating” charges, we’re just moving them around. No new charges can be created, and none can be destroyed.

The smallest quantity of charge is the charge of an electron (or a proton) … can’t get any smaller amount than that.

Charge is “quantized”. There is a smallest possible amount of charge.

Coulomb’s LawSpecifies the relationship between interacting charges and the force between them.

2

21

d

qqkF

q1 = the charge of first objectq2 = the charge of second objectk = the proportionality constantd = the distance between the two objects

q1 q2

dExample: Two charged objects separated by a distance d

An inverse square law, like gravity …. Remember? But the “Coulomb Force” is MUCH stronger than gravity.

K = 9,000,000,000 Nm2/C2

C is a “Coulomb” – a unit of charge.

An electron has a charge of1.6 x 10-19 C

Conductors and Insulators

Conductors"Conductor" implies that the outer electrons of the atoms are loosely bound and free to move through the material. Examples: metals like copper, gold, aluminum, etc.

Insulators

Most atoms hold on to their electrons tightly and are insulators. In most materials even the outermost electrons are so tightly bound that there is essentially zero electron flow through them. Examples: rubber, plastics, glass, air.

These words describe a material’s ability to permit the flow of charges within the material. In other words, their ability to conduct an “electric current”.

Semiconductors/SuperconductorsSemiconductors

Conduction properties are in between conductors and insulators. Adding trace amounts of impuritities to a semiconductor material can give it good conduction properties. It can be sometimes a conductor and sometimes an insulator.

A semiconductor can be made to conduct when light hits it (photovoltaic cells) or to emit light when a current flows through it (a Light Emitting Diode or LED).

(Don’t have to know this)

Superconductors

Materials in which all resistance to current flow disappears. Usually at very low temperature. Right now the highest temperature for this to happen is at about 100 Kelvin (about -173 Celsius)

ChargingThere are many different ways to cause a NET charge to accumulate on an object.

Charging by Friction and Contact:

Charging by Induction:

An electroscope

Friction:Shoes against a carpetBaloon against your hairRub silk against a glass rod

Contact:

Physics Place Figures

Question 2

A thin stream of water bends toward a negatively charged rod. When a positively charged rod is placed near the stream, it will bend in the

1. opposite direction.2. same direction.3. … but it won’t bend at all.

Question 2 Answer

1. opposite direction.2. same direction.3. … but it won’t bend at all.

A thin stream of water bends toward a negatively charged rod. When a positively charged rod is placed near the stream, it will bend in the