Pretest A. Mean=43% 2 year college physics profs (US) get 80% (26/32)

Transcription

1 Announcements: Lab schedule will be posted next week. The first labs will start the week after Ask FB questions in real time during lectures Assignment 1 is due January 18 Quiz 3 is due January 15 Tutorials will start next week (Tuesday evenings)

2 Pretest A Mean=43% 2 year college physics profs (US) get 80% (26/32)

3 Pretest A Mean=43% The winners are: Yiting Fan Urvashi Singh

4 Pretest B Mean=29% Phys 342 (McGill) Mean=71%

5 Pretest B Mean=29% Phys 342 (McGill) Mean=71% The winners are: Elias Al Homsi (Id: ) Hongtao Yao (Id: )

6 Pretest comments: Congratulations! The pretest score will only be visible tomorrow: 13/1/2018 The first week (including pretest, quiz 2, clickers) will not count towards your grade (we have to resolve all technical issuses)

7 Did you have technical problems with the pretest A. Yes B. No

8 If you had technical problems, what system were you using: A. Iphone B. Android phone C. Other smart phone D. Android tablet E. Ipad F. Mac laptop G. Windows laptop H. Other

9 What do you feel?

10 How do you feel?

11 A plus charge and a neutral charge A. Repel each other B. Attract each other C. Neither attract nor repel D. None of the above

12 From quiz 2: Proton: +e Neutron: neutral No electrostatic force Proton: +e Neutral object Induced charge Electrostatic attraction

13 Proton: +e Neutron: neutral No electrostatic force But STRONG nuclear force at very small distances (nucleus radius ~ 0.9 fm). The nuclear force keeps the nucleus together. It s much stronger at very small distances than the electrostatic force repelling the protons (so called strong force).

14 I can only feel a charge if I have charges!

15 What is a charge? Rank Responses 1 ELECTRON 2 ELECTRONS 3 A FORCE 4 ELECTRICITY 5 ACCUSATION 6 Other

16 What is a charge?

17 I don t know!

18 I don t know! But I know it s properties

19 I don t know! But I know it s properties: 2 Types: + and -

20 How many types of masses? A. 0 B. 1 C. 2 D. 3 E. 4 F. 5 G. 6

21 What are neutrons composed of? Rank Responses 1 QUARKS 2 MASS 3 QUARTZ 4 QUARK 5 MATTER 6 Other

22 How many types of quarks? A. 0 B. 1 C. 2 D. 3 E. 4 F. 5 G. 6

23 Three important forces!

24 Three important forces! 1. Gravity

25 Three important forces! 1. Gravity 2. Electrostatic

26 Three important forces! 1. Gravity 2. Electrostatic 3. Nuclear

27 Three important forces! 1. Gravity (1 mass) 2. Electrostatic (2 charges) 3. Nuclear (6 colours)

28 Which force is missing? Rank Responses 1 MAGNETIC 2 WEAK NUCLEAR... 3 ELECTROMAGNE... 4 WEAK 5 WEAK NUCLEAR 6 Other

29 There are 4 forces! 1. Gravity 2. Electrostatic 3. Nuclear 4. Radioactivity

30 There are 4 forces! 1. Gravity 2. Electrostatic 3. Nuclear 4. Radioactivity This is the one we will care about in this course

31 Coulomb s law Coulomb s Law: The magnitude of the electric force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. (See the figure at the right.) Mathematically: F = k q 1 q 2 /r 2 = (1/4π 0 ) q 1 q 2 /r 2 Copyright 2012 Pearson Education Inc.

32 Where do charges come from? Rank Responses 1 ELECTRONS 2 ELECTRON 3 ATOMS 4 GOD 5 NATURE 6 Other

33 Where do charges come from?

34 For example: wiki 1 neutron can form an electron and a proton! (called radioactivity)

35 What is the total charge of the Universe? A. Huge B. Close to zero C. Changes over time D. I don t know

36 The sum of all the positive charges = sum of all negative charges. Total charge conservation law: the total charge of the Universe does not change only the total amount of positive (and negative charge) can change.

37 In classical physics (old physics before quantum) not even the total amount of positive (and negative) charge can change. (You need quantum physics to change a neutron into an electron and proton). In this course (we do not do quantum), the total amount of positive charge (and negative charge) will not change: Charge is conserved.

38 Rabbit skin demo

39 Electric charge Two positive or two negative charges repel each other. A positive charge and a negative charge attract each other. Figure 21.1 below shows some experiments in electrostatics. Copyright 2012 Pearson Education Inc.

40 Electric charge We transfer charge from one material to the other but we do not change the total Figure 21.1 below shows some experiments in electrostatics. amount of charge Two positive or two negative charges repel each other. A positive charge and a negative charge attract each other. Copyright 2012 Pearson Education Inc.

41 25% mark in quiz 2 Atoms are composed of Option1 25% Electrons Option2 25% Protrons Option3 25% Neutrons Option4 100% All of the above

42 Electric charge and the structure of matter The particles of the atom are the negative electron, the positive proton, and the uncharged neutron. Protons and neutrons make up the tiny dense nucleus which is surrounded by electrons (see Figure 21.3 at the right). The electric attraction between protons and electrons holds the atom together. Copyright 2012 Pearson Education Inc.

43 25% mark in quiz 2 Atoms are composed of Option1 25% Electrons Option2 25% Protrons Option3 25% Neutrons Option4 100% All of the above The charge of an atom can be Option1 25% positive Option2 25% negative Option3 25% neutral Option4 100% all of the above

44 Atoms and ions A neutral atom has the same number of protons as electrons. A positive ion is an atom with one or more electrons removed. A negative ion has gained one or more electrons. Copyright 2012 Pearson Education Inc.

45 Textbook: The quark charges, and of the electron charge, are probably not observable as isolated charges.) Thus the charge on any macroscopic body is always either zero or an integer multiple (negative or positive) of the electron charge. I disagree: There are several examples of charge fractionalization (quarks, FQHE) Nobel prize when you contradict textbook (sometimes) "the discovery of asymptotic freedom in the theory of strong interaction (quarks) Nobel prize for David Gross, David Politzer, and Frank Wilczek "for their discovery of a new form of quantum fluid with fractionally charged excitations. Nobel prize for Robert B. Laughlin, Horst L. Störmer, and Daniel C. Tsui ( it s a quark! )