- Slides: 124
Static Electricity 1
Free Associate – the terms • Static • Electricity Senior Physics Conductors and Insulators 9: 45 https: //www. youtube. com/watch? v=q. Uhxm. XZw. Pmg 2
Atomic model positively charged nucleus (protons) negatively charged electrons Neutral objects have same # p+ & e-. Charged objects have net p+ or e- 3
When objects have charge imbalance, can exert electrostatic force. What happens when to objects Fnet applied? 4
When objects have excess or deficit of charge, can exert electrostatic force (Fe). 5
Charged objects can apply a Fnet. Proof? 6
What happens to the forces as the 2 objects separate? Decreases 7
Which graph do you think shows how Fe between 2 objects changes with distance. 8
Charge Notation Amount of Charge Outer Part electrons e. Nucleus Protons p+ neutrons no Elementary Charge – 1 +1 0 9
In solids, Charge transferred by e- only. How can we get positive charge object? Loss of e-. 10
• Bozeman Science 6 min. Good Phet Demos. • https: //www. youtube. com/watch? v=z. HJk. JG Bdvw. E 11
Uncharged objects can feel electrostatic force too: by polarization 12
Polarization Atoms can be polarized by redistributing e-. Polarization is separation of charge not imbalance. 13
Charged balloon causes wall to become polarized. 14
Pith ball polarization 15
Concept Check: If 2 small objects are attracted to each other and move together, which of the following can be said with confidence? • • 1. They have opposite charges. 2. They have the same charge. 3. At least one is charged. 4. None of the above. 16
Conservation Law applies to charge Although charge ( e-) can be transferred, charge cannot be created or destroyed. Sum of charges in system remains the same. For polarization the system is the balloon and the wall. 17
2 types of materials. • Conductors – allow charges to move around – can be polarized. • Insulators – hold excess charge in place – hard to polarize. 18
Conductors – materials that allow e- to move freely often redistribute charge. Metals are good conductors. 19
Metal conductors distribute charge uniformly. 20
Insulators – charges do not move freely. Tend to stay concentrated in one spot on object. 21
What’s happening here? 22
Polarization produces only a surface charge. Try at home. 23
3 ways of Charging Objects: • 1. Friction – rub 2 neutral objects together. • 2. Conduction - Contact with charged object. • 3. Induction – by bringing charged object in vicinity of neutral conductor. 24
Friction Works well for insulators. Do objects get same or opposite charge? Opposite! 25
Conduction • Charges transfer by touching charged object to neutral one. • Good for conductors.
Conduction: touch charged object to neutral object. Do objects get same or opposite charge? SAME! 27
Why you get a shock. • Charge yourself transfer e- either to or from your body to neutralize your charge. Always accompanied by E release. 29 29
Static Electricity 9: 15 min. http: //www. youtube. com/watch? v=A 893_7 FGMHY&feature=relmfu 30
Induction- – no touching of objects. Need to polarize & separate them. 31
Charging by induction conductors only. A ground can serve as an infinite source or sink of e-. Earth, your hand, floor, wall. 32
Charge an Electroscope by Induction 33
Charging By Induction 9: 30 Min. • http: //www. youtube. com/watch? v=p. J 36 Et. A BLAk 34
Sharing of Charge among conductors Conductors will share charge equally if they are in contact. 35
1. The elementary charge of each metal sphere below is shown. If they touch, and are then separated, what will be the resulting charge on each? +3 -6 -9 Total charge 3 – 6 – 9 = - 12 They will share the total charge so divide: - 12/3 spheres = -4.
2. If these 2 spheres touch, what will be the charge on each? Why do the spheres need to touch. Why don’t charges jump from one to the other without them touching? +3 - 7 Total Charge = -4 Each Sphere = -2 Air is an insulator so charge does not easily travel through it. 37
Problem Set. 38
Hwk Read Tx 17 -1 Answer pg 633 #1 -2, 4 -6 pg 654 #1 -10 not 3 Type or write it all including questions. 39
Determining Charge on electron. 40
1909 Robert Millikan measured charge on e- 41
Millikan 1: 15 http: //www. youtube. com/watch? v=X Mf. YHag 7 Liw
Charge is quantized. There is a smallest unit of charge. • Charge can only exist in whole number integers of the charge on 1 e- or p+ • Cannot have fractional numbers. • New Unit - Charge can be measured in Coulombs. • Use your table to find the smallest unit of charge which is 1 elementary charge e. 43
Charge Units of charge = coulombs (C) e- is -1. 6 x 10 -19 C p+ is +1. 6 x 10 -19 C or can consider elementary or fundamental units e- has charge – 1 p+ has charge +1 44
2. Can an object have a charge of 3. 53 x 10 -19 C? • No. • 3. 53 x 10 -19 C ÷ 1. 6 x 10 -19 C = 2. 2. • Charges must be whole number integrals of 1. 6 x 10 -19 C. 45
3. How many electrons carry a charge of 1 -C? • Take the inverse. 46
It takes 6. 25 x 1018 elementary charges (e- or p+) to carry 1 C of charge. Take the inverse of 1. 6 x 10 -19 C. 47
4. What would be the charge on an object with 2. 2 x 1015 excess electrons? 3. 52 x 10 -4 C
5. How many protons does it take to carry 0. 001 C of charge? 6. 25 x 1015 p+
6. What is the total charge (in C) on 6. 2 x 108 electrons? • 9. 9 x 10 -11 C 50
7. A metal sphere with an excess of 2 x 109 electrons is connected to a sphere with a deficit of 1 x 109 electrons. • What is the charge in Coulombs on each sphere before they’re connected? • - 3. 2 x 10 -10 C • What is the charge in Coulombs on each after they’ve been connected? • - 0. 8 x 10 -10 C • + 1. 6 x 10 -10 C
Electrostatic Force Charles Coulomb measured force exerted on one charged object by another. He used torsion balance. 52
Coulomb’s Law Relates Force btw. 2 charged objects. Fe = kq 1 q 2 r 2 k = constant 8. 99 x 109 N m 2/C 2. q charge on obj in Coulombs (C) r is dist in meters. F is force (N) 53
Force vs. Distance Inverse square Fe and Fg 54
8: An alpha particle is a nucleus with 2 protons and 2 neutrons. It is near a proton. 1. What is the charge in Coulombs of each? 2. They are separated by a distance of 3 nm. What is the force between them? 3. Is the force repulsive or attractive?
• nucl = 3. 2 x 10 -19 C. • p+ = 1. 6 x 10 -19 C. • F = 5. 11 x 10 -11 N 56
9. Two protons are 0. 025 m apart. Calculate: • a) the gravitational attraction between • B) the electrostatic force between them. • C) what is the ratio between the forces. • D) What do you think the sign + or – indicates about Fe? them.
• Fg = 3 x 10 -61 N • Fe = 3. 7 x 10 -25 N • Fe is 1036 x stronger than Fg. • pos is repulsive, neg is attractive 58
Hwk: Finish Static Prc 2 • read text 17 -2 and pg 634 – 636 • Do pg 636 #1 -4 and pg 654 #1, 2, 6, 10.
Mech Universe “Static Electricity” 60
Force • 2 objects involved
Gravitational Field, g • The force and direction “felt” by a small mass (N/kg). Same as acceleration but dif units. 62
Electric Fields E region of space around charged object where a “test charge” feels an electrostatic force.
Electric Field (E) defined as: The force and direction a small positive “test” charge feels in presence of field created by a larger charge Q. E = F/q. E = Electric Field (N/C) F is force on test charge (N). q is amt of charge on test charge (C).
Ex 1: A charge of 2 C feels a force of 10 N in an electric field. What is the field strength at that point. E = F/q. = 10 N 2 C E = 5 N/C
Ex 2: How much force does a test charge with + 0. 4 C feel in a field of 8 N/C? • E = Fe/q • Fe = q. E • 0. 4 C x 8 N/C = 3. 2 N. 66
Ex 3. An electron is placed in a field of 100 N/C. a. What is the force on the electron? b. What is the acceleration of the electron? • • q. E = F 1. 6 x 10 -17 N a = Fnet/m 1. 8 x 1013 m/s 2. 67
Electric Field Strength is Inversely Proportional to Distance Around a Point Charge. 68
Field Lines represent electric fields. Electric field lines show the force that a small positive test charge feels in a field created by a much larger charge. They represent the strength and direction of the field.
Phet Charges & Fields. • http: //phet. colorado. edu/en/simulation/charg es-and-fields 70
Sketching E fields. • Suppose you bring a small positive test charge to various points (a, b, c etc) in space around the sphere below. Sketch vector arrows at each point to show the magnitude and direction of the force on the test charge at each point. G D E A B C F J H K I 71
Sketch vectors to show force magnitude & direction on a + test charge at each point. + 72
Field around positive object. 73
Sketch the field around a negatively charges sphere. - - - 74
The denser (close spacing) the field lines are, the stronger the field. Stronger field near charge. 75
What are the field lines now? 76
What if field was formed between opposite charged parallel plates? Sketch it. + + -
Field Between Parallel Plates How would the strength of the field vary if a charge moves from the + to the – plate? 79
Rules: • Fields are vectors, the have strength/intensity and direction. • Lines start on + end on neg. • Direction arrows determined by an imaginary + test charge. • Electric Field lines never touch, cross, or angle sharply. • Density/spacing of lines shows strength. 80
Electric field due to more than one charge. Field is stronger near the larger charge. Density of lines show the increased strength.
E field due to more than one charge. Force due to more than one charge is the vector sum of all the forces on a charged particle.
Electrostatic Equilibrium Fields produced by more that a single charge will have spots where the forces on a charge in the field will be balanced. F net = 0.
Challenge: What would a positive charge feel at z? z small negative • large positive 84
Hwk Watch link “Electric Field Introduction” Handout • Watch Elec Field youtube lessons kahn. • https: //www. youtube. com/watch? v=la. GSICm_ag. Mhttp: //ww w. youtube. com/watch? v=va. DT 4 Gw. AZ 2 I&feature=relmfu • And https: //www. youtube. com/watch? v=pu. TZvh. OFp. RA 85
Work & Energy Electric Potential
Intro to Potential Difference /Voltage Difference. • • • 1. Define gravitational PE. 2. What are the units of PE? 3. How is Energy related to work. Explain. 4. What are the units of work? 5. What is the equation for work? 87
Gravitational Field Every Height associated with dif PEg = mg. Dh Takes work to lift mass to height. h 88
It takes work to push q around in E field. • Each point called potential. • Like a height • Charge has PEelc in E. • Do work to push q in E field, q gains PE. • q released in field, q loses PE, to KE. 89
It takes work to move charges in a field. W = DPE. • Where does a +test charge have more PE – point A or B? • It takes more work to push +q to B. • A is at higher potential, V. • High potential q feels big push. • Voltage, V = work done/C to push charge between points in E field. • Units J/C
Voltage Work done on every coulomb of charge moving it is called electric potential/difference, Voltage. V = W/q. W work in J q is charge in C. V is Volts = J/C. V is the electric potential at P at a point. P is like a particular height in a gravity field. Equals PE gained or lost per C charge, q.
Ex 1. It takes 150 x 10 -6 J to move a 2. 0 m. C charge to point P. What is the electric potential (voltage) at P? V = W/q = 150 x 10 -6 J = 2 x 10 -6 C 75 V or 75 J/C How much PE did every C of charge gain? 75 J
Rearrange to find DPE of charge or work done by E field: W = DPE and V = W/q: PE elc = q. V W = q. V. PE, W – Joules Q – Coulombs V = Volts
Ex 2. The electric potential at point P is 12. 0 V. A 3 C charge is placed at P. What is the PE of q at P? PE = W = q. V (3 C)(12 V) = 36 J
Ex 2 b. If q = -2 C is moved to a point P = 12 V, What is the PE of q? DPE = q. V • (-2 C)(12 V) = -24 J • q lost PE, the field did work on it. Think of the charge as falling. 95
Potential/Voltage Difference DV simply difference between 2 points in field. DV pd = 28 V – 13 V = 15 V. DV = 15 V B = 28 V A = 13 V If q above is 2 C, what is the work done moving it from A to B? W = q. DV = 2 C(15 V) = 30 J
Potential Difference in a Uniform Field. • • • Constant on charge between plates force E, Voltage Work must be done to move charge between plates. Charge gains or loses PE.
Ex 3: What work must be done to move a +5 m. C charge from the – to the + plate in the 250 V pd across plates? 250 0 V 250 V W = q DV = (5 x 10 -6 C)(250 J/C - 0) = 1. 25 x 10 -3 J.
4. What physical value is the slope of the line below? Write the equation. • Voltage. • W = q. V 99
Natural Potential Difference “Lightning”
Lightening caused by p. d. cloud / ground. Cloud bottom becomes neg, polarizes ground, creates E field V push until charges accelerate! 5. Which way will the field go?
Charges set loose in E fields will accelerate! The average lightning bolt contains 5 -10 coulombs 102
Lightening bolt physics 1 min. • http: //www. youtube. com/watch? v=C 3 y 289 F 1 e. Q 4 • Senior Physics Voltage • https: //www. youtube. com/watch? v=YLe. Rb Tl. Kk. JY 103
Label where PE is high and low - + 104
Energy of Moving Charges in Fields. • As a charge moves thru a field, its total E (the SE) is constant. By consv of Energy. • If a charges “falls” toward the oppositely charged plate its PEelc decreases, What increases? KE Hi PEelc Low PEelc 105
• Work done by field will accelerate charge: W = DKE = q. V. • So: Ebefore • E T • PE • q. V = = E after ET KE ½ mv 2. 6. Is acceleration between parallel plates uniform? Explain.
7. An electron is accelerated through a 150 -V pd. What is the maximum speed it can attain? • • • Electrice PE lost = KEgained. q. V = ½ mv 2. (1. 6 x 10 -19 C) (150 V) = ½ (9. 11 x 10 -31 kg)v 2. v = 2. 4 x 10 -17 = 4. 555 x 10 -31 v 2. 7. 3 x 106 m/s 107
• • • Prove that V = Ed for parallel plates V = Voltage E = electric field d = distance between plates. 108
Summery Voltage or Electric Potential • Potential / V = Energy per Coulomb of a charge at a point (potential). • Potential / Voltage difference / pd W/C to move q between 2 points at different potentials. • Charges in field have PEelc. • When charge accelerates in field PE lost = KE gained. 109
Hwk • Watch the following clips. Kahn • http: //www. youtube. com/watch? v=w. T 9 As. Y 79 f 1 k • https: //www. youtube. com/watch? v=Aq 31 mj. WYd. J 8 • Optional but good. • https: //www. youtube. com/watch? v=Lk. Iai_KXGxg 110
New Energy unit electron-volt
How can I calculate PE of a mass in a gravity field? • PEg = mgh How can I calculate PE of a charge q in an Electric field? • PEelc = q. V. 112
The electron-volt: tiny unit of work & E. For very small changes in PEelc (on the order of 10 -19 J) unit e. V is used. The electron-volt, e. V, is the work & E required to push 1 e- (or p+) through a voltage of 1 V. W = q. V = (1. 6 x 10 -19 C)(1 V) = 1. 6 x 10 -19 J = e. V 113
To find e. V given elementary charges: (# e )(# V ) = e. V. If 1 e- is pushed across 1 V then (1 e)(1 V)= 1 e. V of work is done. If a charge of 2 e- is pushed across a 1 V pd then (2 e )(1 V) = 2 e. V. If 2 e- pushed across 6 V then work is 12 e. V. 114
What if 3 e- move across 12 V? 36 e. V To find e. V (# elm charges) (voltage) 115
1. How many joules of energy are represented by 6. 9 x 1029 e. V x 1. 6 x 10 -19 J. = 1. 1 x 1011 J e. V
Ex 2. A field does 3. 3 x 10 -7 J of work on an e-. How many e. V is that? • 3. 3 x 10 -7 J x 1 e. V = 2. 1 x 1012 e. V • 1. 6 x 10 -19 J 117
Ex 3: A proton is accelerated in a 100 V pd. How much work is done in e. V? • W = q. V but if we use elem charge, we can just multiply by the voltage. • (1 p+)(100 V) = 100 e. V 118
Summery Voltage or Electric Potential V = Wk per Coulomb to push a charged particle to point (potential). Potential / Voltage difference Wk per Coulomb to move charge between two points at different potentials. Charges in field have PEelc. High PE charge near point with same charge. Low PE charge near point with opposite charge. 119
Some typical voltages 120
Can calculate acceleration of charges in E fields & through Voltages. Set PE elc = KE 121
Plates with battery d = 1 cm - + AC Delco A 12 volts B d Batteries are meant to maintain the potential difference. 122
Electric PE review youtube. Kahn http: //www. youtube. com/watch? v=w T 9 As. Y 79 f 1 k