Force of B on A ~ q A x q B

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1 Static electricity Source of electricity: Electric charges Everything (earth, you, the table etc) made of tiny particles called atoms toms are made up of 3 even tinier particles: Electrons, neutrons and protons The atom Particle Charge Mass Electron e kg Proton e kg Neutron kg e = Coulombs Coulomb (C) is the unit of charge Try this for a home experiment this weekend yes I m kidding Lecture 20 : Static electricity Why does this person survive? Reminders: HW 8 due Today Reading for Tuesday: 10.3 Static electricity: What happens when charges are stationary Electricity (and electric currents): What happens when charges (usually electrons) are moving Reading uiz 1. Coulomb s law describes a. The force between electrical point charges b. How dry your socks get in the dryer c. The friction force between items of clothing d. The operation of a conveyor belt 2. Electric charges a. Can never move from one object to another b. Can never move inside an object c. Can move inside an object but not from one object to another d. Can move both inside an object and between objects, depending on what the objects are. 3. n object that is electrically neutral a. Contains more positively charged particles (protons) than negatively charged ones (electrons) b. Contains more electrons than protons c. Contains no charged particles d. Contains eual numbers of protons and electrons. e. Is unable to conduct electricity Reading uiz 1. Coulomb s law describes a. The force between electrical point charges b. How dry your socks get in the dryer c. The friction force between items of clothing d. The operation of a conveyor belt 2. Electric charges a. Can never move from one object to another b. Can never move inside an object c. Can move inside an object but not from one object to another d. Can move both inside an object and between objects, depending on what the objects are. 3. n object that is electrically neutral a. Contains more positively charged particles (protons) than negatively charged ones (electrons) b. Contains more electrons than protons c. Contains no charged particles d. Contains eual numbers of protons and electrons. e. Is unable to conduct electricity Sorting out how charges interact like charges repel opposite charges attract Electrostatic force between charged particles like charges repel opposite charges attract Conside point charges, and. What force does charge feel? What if I have twice the charge as before? a) Half the force as before b) Same force as before c) Twice the force as before d) our times the force Twice the charges twice the force Three times the charge, 3x the force Ten times the charge, 10x the force orce of on ~ x note: if s are the same sign, is positive: pushing away if s are the opposite sign, is negative: attracting together Observed behavior: orce depends on and : More charge, more force 1

2 Sorting out how charges interact Sorting out how charges interact like charges repel opposite charges attract like charges repel opposite charges attract What about the distance, what if they are closer? What about the distance, what if they are closer? Closer means more force! Closer means more force! What math relationship shows this? a) Less force b) Same force as before c) More force a) Less force b) Same force as before c) More force ~ 1/ r (is a good guess) ut it turns out to be stronger! ~ 1 / Putting this together: Electric orce like charges repel opposite charges attract Conside point charges, and. What force does charge feel? r comment on units. on from = k or discussion of units we use brackets: units r To find the units on k [?] we can use the fact that we need the thing on the right to have units N Observed behavior: orce depends on and : More charge, more force orce depends on distance between them (r) : less distance, more force on from = k Describes the force between 2 point charges k is Coulomb constant = 9 x 10 9 N m 2 /C 2 and are amount of charge in coulombs (C ) r is separation in m 1 Coulomb = 6 x electron charges! So N =? C 2 m 2 This means that? = [ N m2 C 2 ] Units are a good way to make sure you ve done your algebra correctly. UT they are difficult to carry around. n electron and a proton are 1 mm apart electron m proton What is force between them? Calculate and write down answer. on from = k electron m proton What is force between them? Calculate and write down answer. on from = k k is Coulomb constant = 9 x 10 9 N m 2 /C 2 e = 1.6 x C Proton and electron have charges of same magnitude (e), opposite sign Nm2 9 = (9 10 C 2 ) C C m 2 = = 1.6 x C = 1.6 x C N = N orce between particles = 2.3 x N What does the minus sign mean? a) orce on electron points to left b) orce on electron points to right =

3 electron m so orce = 2.3 x N proton What does the minus sign mean? b) orce on electron points to right What is force between them? Calculate and write down answer. on from = k Remember force is a vector and so reuires both a magnitude and direction Plugging numbers into Coulomb s Law give the magnitude of the force What about the direction? The force between 2 point charges always acts along the line of centers If Coulomb s law gives a positive number, the charges repel If Coulomb s law gives a negative number the charges attract lternatively: Use Coulomb s law to find the magnitude of the force Look at diagram to find direction alloon demo: Rub a balloon on sweater and stick it to the wall. What attracts the balloon to the wall? fter I have rubbed the balloon on my sweater, predict what charges will be on the balloon and on sweater a. oth have extra charges. b. oth have extra charges c. alloon has extra or charges, sweater neutral, d. Sweater has extra or charges, balloon neutral e. Either sweater has extra and balloon extra or balloon extra and sweater extra. Look at Phet and find out.. nswer is e: Remember atoms are initially neutral Electrons in sweater atoms are bound to atom less strongly than in balloon atoms Rubbing allows balloon atoms to steal electrons, making balloon negatively charged (excess of electrons) and sweater positively charged alloon Sim Rub a second balloon on the sweater. The two balloons will.. a. attract, b. repel, c. not exert a force on each other b. repel: alloons made of same material so must pick up same sign of charge from sweater Like charges repel Move charged balloon close to wall. What will happen? a. Wall is neutral (no extra or ) so will not be affected. b. charges in wall will move away, towards balloon c. charges in wall will move away, towards balloon. d. charges in wall will move away, don t move. Rub a second balloon on the sweater. The two balloons will.. a. attract, b. repel, c. not exert a force on each other b. repel: alloons made of same material so must pick up same sign of charge from sweater Like charges repel Move charged balloon close to wall. What will happen? d. charges in wall will move away, don t move.. Negative charge on balloon repels charge and attracted to charge in the wall toms in wall become stretched (or polarized) Remember force between charges = k alloon is closer to charges in wall than charges, so force of attraction is stronger than force of repulsion normal polarized Electrostatic dust rag (think Swiffer TM ). Rub it on surface, it s very good at attracting electrons and so becomes negatively charged. What kind of dust will this negatively charged rag pick up best? a. Only dust with positive charges. b. Only dust with negative charges. c. The rag will pick up all dust eually. d. The rag will pick up dust with positive charges, and also neutral dust particles, just not as well. e. The rag will pick up dust with negative charges, and also neutral dust particles, just not as well. 3

4 Electrostatic dust rag (think Swiffer TM ). Rub it on surface, it s very good at attracting electrons and so becomes negatively charged. What kind of dust will this negatively charged rag pick up best? a. Only dust with positive charges. b. Only dust with negative charges. c. The rag will pick up all dust eually. d. The rag will pick up dust with positive charges, and also neutral dust particles, just not as well. e. The rag will pick up dust with negative charges, and also neutral dust particles, just not as well. an de Graaff generator. This generator collects charges on the large sphere. It does this by repeatedly rubbing balloons on a sweater and transferring the excess electrons up to the top of the support post The balloon is a rubber belt that is circulated by a motor brush similar to a sweater gives the belt electrons different brush (that really likes electrons) us used at the top to pull off the electrons and charge up the generator an de Graaff generator. ring uncharged metalized mylar balloon up to an de Graaff. If I bring a pompom up to the generator and touch it what will happen? a) Nothing. The electrons won t go anywhere. b) The electrons will charge up the pompom but you won t be able to see anything c) The electrons will charge up the pompom and then the strands will repel each other. d) I don t know Predict what will happen: 1. efore it touches 2. fter it touches 1. efore touching a. Not affected by dg b. ttracted to it c. Repelled 2. fter touching a. Not affected by dg b. ttracted to it c. Repelled ring uncharged metalized mylar balloon up to andergraff. ring uncharged metalized mylar balloon up to andergraff. Predict what will happen: 1. efore it touches 2. fter it touches 1. efore touching a. Not affected by dg b. ttracted to it c. Repelled Predict what will happen: 1. efore it touches 2. fter it touches 2. fter touching a. Not affected by dg b. ttracted to it c. Repelled Charges in neutral ball are polarized by charge on dg charges move closer to dg, charges move away orce between charges = k charges on ball are closer to dg, so force of attraction is strongest When balloon touches, charges are pushed onto it by the other charges on the dg Now balloon is negatively charged like dg Like charges repel 4

5 Discussion of Energy 3 Types of energy (there are others): Kinetic energy of motion rock rolling down hill Potential ability to do work in future rock at the top of a hill or a fluid has it under pressure. Thermal energy that dissipates as heat (e.g. friction, or smashing into a wall) How would this apply to charges? Think of the analogy of a hill pulling away from valley Mechanical work vs. Electrical Think, climbing up a hill stores energy in system Same thing, separating charges We just have to be more careful because unlike mass (all masses attract each other) some charges attract while others repel Hill nalogy for energy Hill nalogy for energy Effort (push) to get ball (at constant speed): a) harder at first b) easier at first up the hill c) takes same effort / push a b Potential Energy of same ball a) PEa > PEb b) PEb > PEa c) PEa = PEb The force reuired is related to the steepness or slope of the potential energy curve. Steeper means more force. Hill nalogy for energy oltage a) b) Energy of ball (Potential Energy, PE) a) PEa > PEb b) PEb < PEa c) PEa = PEb Gravitational PE = mgh Notice: it takes less force to go up further at a) but it has more potential energy. orce is steepness not magnitude of potential energy 5

6 oltage has to do with conditions of system mount of charge Distance from that charge Like the electric force (related but different) oltage is like the height of mountain Think Topographical maps Topo practice oltage tells you what a charge will do Which figure goes with #4 nswer C is like top of hill Is like valley Where is 0? What will a do at 0: a) e attracted to higher voltage b) e attracted to lower voltage c) Not be impacted b) Just like a ball rolls down hill oltage is like the height of mountain oltage Charge, Electrical Potential Energy EPE = D Height (and gravity) Mass, m Gravitational potential Energy GPE = m g Dh Electrostatic potential energy and voltage New force: Electrostatic force between charges New PE: Electric Potential Energy (EPE) orces and PE go in pairs Remember gravitational force: Do work against gravitational force (mg) to raise an object s GPE (mgh) Similarly, do work against electric force to raise an object s EPE EPE = D, where = charge of object and D is voltage difference Like GPE with m and D gdh oltage () tells you EPE of any charge at that location in space Tells you work reuired to bring a unit charge from = 0 to that location Determined by surrounding charges. Closer you are to charge the more the voltage grounded object is always at = 0. Grounded means attached to the ground! lways interested in D: voltage difference between 2 locations like GPE where only Dh mattered est understood by doing practice uestions! 6

7 Conservation of energy and EPE Remember conservation of energy euation: Work Done on object = Change in Energy of object W ext W friction = DPE DKE Now we can add another PE term to this euation: W ext W friction = DGPE DEPE DKE = mgdh D D(1/2mv 2 ) Two metal plates connected by a battery. attery maintains a voltage difference of between the plates Plate is grounded (set to zero ) What is the voltage of plate? a) 0 b) c) d) Can t determine from information given Two metal plates connected by a battery. attery maintains a voltage difference of between the plates Plate is grounded (set to zero ) What is the voltage of plate? a) 0 b) c) d) Can t determine from information given Note: Conductors (bits of metal, wires etc) are a constant voltage all over What will happen to the charge if we let go of it (ignore gravity)? a) Nothing b) It will fly over to plate c) Sparks will fly d) Something else EPE = D What will happen to the charge if we let go of it (ignore gravity)? a) Nothing b) It will fly over to plate c) Sparks will fly d) Something else has a charge. It will be repelled by charges on plates and attracted to charges on plate What is the change in EPE of charge as it flies from plate to plate? a) 0 b) c) d) Can t determine 7

8 EPE = D Conservation of energy: W ext W friction = DGPE DEPE DKE What is the change in EPE as charge flies from plate to plate? c) t : EPE = t : EPE = 0 = 0 Change in EPE = EPE f EPE i = 0 = Plug in numbers: = 2e = C = C = 100 DEPE = J Charged particle loses EPE as it flies from to. What form has this energy turned into just before it hits plate a) KE b) Thermal energy c) GPE d) PPE Conservation of energy: Charged particle loses EPE as it flies from to. What form has this energy turned into just before it hits plate a) KE b) Thermal energy c) GPE d) PPE W ext W friction = DGPE DEPE DKE CT 29.4b positive charge is released from position i to position f between the charged plates. Did the electric potential energy (PE) increase or decrease? Did the voltage () at the position of the test charge increase or decrease? : PE, : PE, C: PE, D: PE, E: None of these. Just before it hits plate: = 1/2mv 2 Just after it hits plate, all this energy has turned into thermal energy University of Colorado, oulder (2008) CT 29.4b positive charge is released from position i to position f between the charged plates. Did the electric potential energy (PE) increase or decrease? Did the voltage () at the position of the test charge increase or decrease? : PE, : PE, C: PE, D: PE, E: None of these. CT 29.4 negative charge is released from position i to position f between the charged plates of a charged capacitor. Did the potential energy (PE) increase or decrease? Did the voltage () at the position of the test charge increase or decrease? : PE, : PE, C: PE, D: PE, E: None of these. Hint: EPE = D University of Colorado, oulder (2008) University of Colorado, oulder (2008) 8

9 CT 29.4 negative charge is released from position i to position f between the charged plates as shown. Did the potential energy (PE) increase or decrease? Did the voltage () at the position of the test charge increase or decrease? EPE = D f > i D is bigger! EPE is D but is negative EPE gets smaller! : PE, : PE, C: PE, D: PE, E: None of these. University of Colorado, oulder (2008) Electrostatics Summary Positive and negative charge: Like charges repel, opposites attract Coulombs law for point charges: = k orce acts along line joining particles oltage: Determines EPE of charge at that location in space EPE: = D Close to charges voltage is more and vice versa Grounded object is at 0 (lso called Electric Potential) New form of potential energy Lots of analogies to GPE (D Dh, m) lashlights, circuits, batteries, and power light bulb circuit = Given batteries, light bulbs, and wire, how can we design a light bulb circuit a) that will burn brightest, b) that will last longer, c) that will be dim, d) that will turn on and off. How can you control and predict current and power in light bulbs? ll this basic circuit stuff applies to home wiring, home electronics, heaters etc. Thursday lecture help save lives physics of dangers of electrocution. uilds on electrostatics (like charges repel, opposite charges attract, voltage, EPE) but now electrons are moving. need to start thinking like an electron! Start by looking at a really simple circuit containing light bulb attery Wires Each element made of metal containing electrons that are free to move What are the electrons doing in each element to ensure that the light bulb lights up? Using each of these three elements can you draw a picture where the bulb lights up? light bulb circuit: Wiring light bulb circuit: Wiring What will happen when hook up positive end of battery () to flashlight bulb with one wire? a. light up b. barely light up c. not light up What will happen when hook up positive end of battery () to flashlight bulb with one wire? a. light up b. barely light up c. not light up Light reuires steady flow of electrons through bulb ut nowhere for electrons to flow!!! Electrons in wire initially attracted to positive end of battery This makes wire and filament positive Electron flow stops when attractions to end of battery and positively charged wire are eual. (technically they are at the same voltage!) 9

10 See this in action! 10