Electricity and Magnetism Charge and Conduction Coulomb s Law

Transcription

1 Electricity and Magnetism Charge and Conduction Coulomb s Law Lana Sheridan De Anza College Jan 9, 2018

2 Last time course structure introduced charge

3 Overview conductors insulators induced charge quantization of charge charge conservation force on interacting charges

4 Electric Charge Charge is an intrinsic property of subatomic particles. Charge can be positive or negative. Particles can also be chargeless, ie. have zero net charge. The unit for charge is the Coulomb, written with the symbol C.

5 Electric Charge on larger objects Before there was any knowledge of atoms, charge was imagined as a kind of continuous fluid. A large scale effect: In dry weather, it is easy to get a shock from static electricity. This is due to a charge imbalance.

6 Charge on larger objects Glass Most large objects around us have (approximately) zero net charge. + Glass Objects can become charged when rubbed against one another. F (a) These examples reveal carpets, doorknobs, faucets, amount of electric charge. fundamental particles makin automatically with those par F The vast amount of cha Glass the object contains equal am Glass negative charge. With such a Glass F F to be electrically neutral; th Plastic F charge are not in balance, (a) charged to (b) indicate that it ha 1 Diagrams from Halliday, Resnick, Fig. Walker, 21-29thalways (a) ed. Two much charged smaller rods of the than 2 CHAPTER 21 ELECTRIC CHARGE F

7 Electrostatic force Charged objects exert a force on one another. Charges with the same electrical sign repel each other. Charges with opposite electrical signs attract each other.

8 Some Vocabulary electrically neutral An object is electrically neutral if its net charge is zero. electrically isolated An object is electrically isolated if it cannot exchange charge with its surroundings.

9 Conductors and Insulators Some materials allow charges to flow through them easily, some do not.

10 Conductors and Insulators Some materials allow charges to flow through them easily, some do not. Conductors materials through which charge can move readily Insulators (also called nonconductors) are materials that charge cannot move through freely

11 most loosely held) electrons become free to ving Induced behind Charge positively Polarization charged atoms ( positive ns conduction electrons. There are few (if any) If a conductor is brought close to a charged object, positive and negative charges in the conductor start to separate and we say a charge is induced on one side of the conductor, or the conductor s charge is polarized. emonstrates the mobility of charge in a conducod will attract either end of an isolated neutral lectrically isouspended on a he copper rod re, conduction d to the far end the plastic rod. remaining posiper rod, rotating closer to the Neutral copper F F Charged plastic

12 realignment of charges in Induced Chargethe Polarization molecules of the wall. in th Charged balloon a Wall Induced charge separation b

13 Course Tool Kahoot Allows me to ask multiple choice questions or do surveys, and get real-time feedback. You can remain anonymous. You need a device connected to the internet. Go to Then: Enter the Game PIN.

14 he molecules to emit ultraviolet light. You cannot see this type of er, Question the wintergreen molecules on the surfaces of the candy pieces ltraviolet light and then emit blue light, which you can see it is the ing from your friend s mouth. A, B, and D are charged pieces of plastic. C is an electrically neutral copper plate. OINT 1 shows five s: A, B, and ged plastic A C C D B is an electral copper electrostatic en the pairs shown for B A D A D pairs. For the remaining two pairs, do the plates repel or attract each other? Plates C and D (A) attract each other (B) repel each other 1 Page 564, Halliday, Resnick, Walker, 9th ed.

15 he molecules to emit ultraviolet light. You cannot see this type of er, Question the wintergreen molecules on the surfaces of the candy pieces ltraviolet light and then emit blue light, which you can see it is the ing from your friend s mouth. A, B, and D are charged pieces of plastic. C is an electrically neutral copper plate. OINT 1 shows five s: A, B, and ged plastic A C C D B is an electral copper electrostatic en the pairs shown for B A D A D pairs. For the remaining two pairs, do the plates repel or attract each other? Plates C and D (A) attract each other (B) repel each other 1 Page 564, Halliday, Resnick, Walker, 9th ed.

16 he molecules to emit ultraviolet light. You cannot see this type of er, Question the wintergreen molecules on the surfaces of the candy pieces ltraviolet light and then emit blue light, which you can see it is the ing from your friend s mouth. A, B, and D are charged pieces of plastic. C is an electrically neutral copper plate. OINT 1 shows five s: A, B, and ged plastic A C C D B is an electral copper electrostatic en the pairs shown for B A D A D pairs. For the remaining two pairs, do the plates repel or attract each other? Plates B and D (A) attract each other (B) repel each other 1 Page 564, Halliday, Resnick, Walker, 9th ed.

17 he molecules to emit ultraviolet light. You cannot see this type of er, Question the wintergreen molecules on the surfaces of the candy pieces ltraviolet light and then emit blue light, which you can see it is the ing from your friend s mouth. A, B, and D are charged pieces of plastic. C is an electrically neutral copper plate. OINT 1 shows five s: A, B, and ged plastic A C C D B is an electral copper electrostatic en the pairs shown for B A D A D pairs. For the remaining two pairs, do the plates repel or attract each other? Plates B and D (A) attract each other (B) repel each other 1 Page 564, Halliday, Resnick, Walker, 9th ed.

18 equal numbers of positive Charging by Induction and negative charges. A conductor is initially neutrally charged. Electrons redistribute when a charged rod is brought close. 1 Figures from Serway & Jewett, 9th ed. a is li iden eno I occ 25.7 stan cre me has neu

19 Electrons redistribute when a Charging by Induction charged rod is brought close. A (negatively) charged object is brought close, polarizing the conductor. b Some electrons leave the grounded sphere through the ground wire. 1 Figures from Serway & Jewett, 9th ed. neu Q u W a A o o t

20 Some electrons leave the Charging by Induction grounded sphere through the ground wire. Excess (negative) charge on the far side is drawn off the conductor by grounding it. The excess positive charge is nonuniformly distributed. 1 Figures from Serway & Jewett, 9th ed. c 23. It is c throu Ele ele the bo Mater

21 The excess positive charge is Charging by Induction nonuniformly distributed. The conductor is isolated again. The remaining electrons redistribute uniformly, and there 1 Figures from Serway & Jewett, 9th ed. d b Mat insu bec the I con read S som man

22 is a net uniform distribution of Charging by Induction positive charge on the sphere. Figure 23.3 Charging a metallic object by induction. (a) A neutral The conductor is now (positively) charged. metallic sphere. (b) A charged rub- 1 Figures from Serway & Jewett, 9th ed. e cat hom ove ato T side in F if t is b rep

23 Charge is Quantized quantization A physical quantity is said to be quantized if if can only take discrete values. Originally, charge was thought to be a continuous fluid, but it is not.

24 Charge is Quantized quantization A physical quantity is said to be quantized if if can only take discrete values. Originally, charge was thought to be a continuous fluid, but it is not. Just like water has a smallest unit, the H 2 O molecule, charge has a smallest unit, written e, the elementary charge. Any charge must be e = C q = ne, n Z

25 Question Initially, sphere A has a charge of 50e and sphere B has a charge of 20e. The spheres are made of conducting material and are identical in size. If the spheres then touch, what is the resulting charge on sphere A? (A) 50e (B) 30e (C) 15e (D) 20e

26 Question Initially, sphere A has a charge of 50e and sphere B has a charge of 20e. The spheres are made of conducting material and are identical in size. If the spheres then touch, what is the resulting charge on sphere A? (A) 50e (B) 30e (C) 15e (D) 20e

27 Conservation of Charge Charge can move from one body to another but the net charge of an isolated system never changes. This is called charge conservation.

28 Conservation of Charge Charge can move from one body to another but the net charge of an isolated system never changes. This is called charge conservation. What other quantities are conserved?

29 Conservation of Charge Charge can move from one body to another but the net charge of an isolated system never changes. This is called charge conservation. What other quantities are conserved? Noether s Theorem the corresponding

30 Conservation of Charge One interesting phenomenon that shows the conservation of charge is pair production. A gamma ray (very high energy photon) converts into an electron and a positron (anti-electron): γ e + e + New mass is created out of light, but charge is still conserved!

31 Electrostatic Forces Charged objects interact via the electrostatic force. The force that one charge exerts on another can be attractive or repulsive, depending on the signs of the charges. Charges with the same electrical sign repel each other. Charges with opposite electrical signs attract each other. Charge is written with the symbol q or Q.

32 Electrostatic Forces For a pair of point-particles with charges q 1 and q 2, the magnitude of the force on each particle is given by Coulomb s Law: F 1,2 = k e q 1 q 2 r 2 k e is the electrostatic constant and r is the distance between the two charged particles. k e = 1 4πɛ 0 = N m 2 /C 2

33 Electrostatic Forces: Coulomb s Law F 1,2 = k e q 1 q 2 r 2 Remember however, forces are vectors. The vector version of the law is: F 1 2 = k e q 1 q 2 r 2 ˆr 1 2 where F 1 2 is the force that particle 1 exerts on particle 2, and ˆr 1 2 is a unit vector pointing from particle 1 to particle 2.

34 Summary charge conductors and insulators induced charge quantization of charge charge conservation Coulomb force Quiz Friday, start of class. Homework Get the textbook: Physics for Scientists and Engineers, 9th Edition, Serway & Jewett Read Ch 23. Ch 23, onward from page 716. Objective Qs: 7, 9; Conceptual Qs: 1, 5; Probs: 1, 3, 9, 16, 17