Unit 4: Electromagnetism

Transcription

1 Unit 4: Electromagnetism

2 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

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6 Unit 4: Electromagnetism

7 Charging Things Up Elementary Charge

8 Atomic Structure Review Matter Within At is Orbiting the made center NUCLEUS up the of NUCLEUS small each are two atom structures are types is ELECTRONS a NUCLEUS of called NUCLEONS ATOMS ELECTRONS NEGATIVE charge Orbit nucleus PROTONS POSITIVE CHARGE DO NOT MOVE WITHIN MATTER NEUTRONS NO CHARGE NEUTRAL DO NOT MOVE WITHIN MATTER

9 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 coulombs -1e Proton +1.6 x coulombs +1e

10 Charged Objects Objects are normally electrically NEUTRAL. They have the same number of protons and electrons If an object loses electrons, it becomes POSITIVELY charged If an object gains electrons, it becomes NEGATIVELY charged + - LACK OF ELECTRONS EXCESS ELECTRONS

11 Charged Objects Because objects can only gain or lose electrons, the only charges that are allowed are multiples of the ELEMENTARY CHARGE This plate is neutral number of protons = number of electrons If the plate loses one two three electrons Its charge becomes The plate has a LACK OF ELECTRONS Elementary Charges e e Or x Coulombs

12 Example #1 An object has three excess electrons. What is its elementary charge? -3e What is its charge in coulombs (converting)? q = -3e x (1.6 x C)/1e = -4.8 x C

13 Example #2 An object has 75 protons and 65 electrons What is its elementary charge? +10 e What is its charge in coulombs? q = +10e x (1.6 x C)/1e = +1.6 x C

14 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 ELECTRON movement ONLY!!!

15 Example #3 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? -1e or -1.6 x C What is the total charge on both spheres after they come in contact? -2e or -3.2 x C The TOTAL CHARGE remains the SAME!!!

16 Practice Which charges could NOT exist in a real-world object? (a) 3.2 x C (e) 0.5 x C (b) 6.5 C (f) 3.2 x C (c) -20 e (g) 3.5 e (d) 1.2 x 10 2 e (h) 1.6 x 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?

17 Practice Which charges could NOT exist in a real-world object? (a) 3.2 x C (e) 0.5 x C (b) 6.5 C (f) 3.2 x C (c) -20 e (g) 3.5 e (d) 1.2 x 10 2 e (h) 1.6 x 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]

18 HW 4-1