Unit 4: Electromagnetism

Announcements• No test corrections - Lowest test grade

will be dropped instead (exams will be returned on Wednesday)

• HW 4-1 will be assigned on Wednesday• If you missed the exam, show me your

medical note or travel after class so we can schedule a makeup

Unit 4: Electromagnetism

Charging Things Up

4.1.1 Elementary Charge

Atomic Structure ReviewMatter is made up of small structures called ATOMSAt the center of each atom is a NUCLEUSWithin the NUCLEUS are two types of NUCLEONS

PROTONS NEUTRONSPOSITIVE CHARGE

DO NOT MOVEWITHIN MATTER

NO CHARGE – NEUTRAL

DO NOT MOVEWITHIN MATTER

Orbiting the NUCLEUS are ELECTRONS

ELECTRONSNEGATIVE charge

Orbit nucleus

Definitions• charge: a fundamental property of matter

– measured in COULOMBS or ELEMENTARY CHARGE

• charge on an object is carried by particles– Electron

• -1.6 x 10-19 coulombs• -1e

– Proton•+1.6 x 10-19 coulombs•+1e

Charged ObjectsObjects are normally electrically NEUTRAL.

They have the same number of protons and electronsIf an object loses electrons, it

becomesPOSITIVELY charged

+ -

If an object gains electrons, it becomes

NEGATIVELY charged

LACK OF ELECTRONS EXCESS ELECTRONS

Charged Objects

This plate is neutral – number of protons = number of electrons

If the plate loses one electron

Its charge becomes…

+1eOr

+1.6 x 10-19 Coulombs

+1 Elementary Charge

If the plate loses two electrons

+2 eOr

+3.2 x 10-19 Coulombs

+2 Elementary Charges

Its charge becomes…

If the plate loses three electrons

+3 eOr

+4.8 x 10-19 Coulombs

Its charge becomes…

+3 Elementary Charges

Because objects can only gain or lose electrons, the only charges that are allowed are multiples of the

ELEMENTARY CHARGE

The plate has a LACK OF ELECTRONS

• An object has three excess electrons.– What is its �elementary charge�?

– What is its charge in coulombs (converting)?

-3e

q = -3e x (1.6 x 10-19 C)/1e = -4.8 x 10-19 C

Example #1

• An object has 75 protons and 65 electrons– What is its �elementary charge�?

– What is its charge in coulombs?

+10 e

q = +10e x (1.6 x 10-19 C)/1e = +1.6 x 10-18 C

Example #2

Law of Conservation of Charge

• The total amount of charge in a closed system remains constant – charge is not created or destroyed, it only moves from one object to another

• Charge �moves� as a result of ELECTRONmovement ONLY!!!

• A metal sphere with two excess electrons touches a neutral metal sphere and then taken away.

– What do you predict the charge on each sphere will be after they make contact?

– What is the total charge on both spheres after they come in contact?

-1e or -1.6 x 10-19 C

-2e or -3.2 x 10-19 C

The TOTAL CHARGE remains the SAME!!!

Example #3

Which charges could NOT exist in a real-world object?(a) 3.2 x 10-19 C (e) 0.5 x 10-19 C(b) 6.5 C (f) 3.2 x 10-21 C (c) -20 e (g) 3.5 e(d) 1.2 x 102 e (h) 1.6 x 10-19 e

Object A begins with a charge of -3.0 coulombs; object B begins with a charge of -7.0 coulombs; and object C begins with no charge. Object A is touched to object B then removed. Object B is then touched to object C and removed. What is the final charge on each of the three objects?

Practice

Which charges could NOT exist in a real-world object?(a) 3.2 x 10-19 C (e) 0.5 x 10-19 C(b) 6.5 C (f) 3.2 x 10-21 C (c) -20 e (g) 3.5 e(d) 1.2 x 102 e (h) 1.6 x 10-19 e[e, f, g, h – easiest way is to convert to elementary charge first]

Object A begins with a charge of -3.0 coulombs; object B begins with a charge of -7.0 coulombs; and object C begins with no charge. Object A is touched to object B then removed. Object B is then touched to object C and removed. What is the final charge on each of the three objects? [A = -5C, B = -2.5C, C = -2.5C]

Practice

HW 4-1