Electrostatics: Understanding Charges, Coulomb's Law, and Electric Fields, Slides of Physics

Download Electrostatics: Understanding Charges, Coulomb's Law, and Electric Fields and more Slides Physics in PDF only on Docsity! TODAY Finish Ch. 20 on Sound Start Ch. 22 on Electrostatics docsity.com Chapter 22: Electrostatics docsity.com More on charge • Note that in everyday charging processes (like rubbing objects), it is the electrons that transfer (not the protons). A negatively charged object has an excess of e’s, whereas positively charged one has deficiency (by same amount) • Which object gains the electrons depends on their electron affinity: Eg. Rod has greater affinity than fur, so rod becomes –, fur + Eg. Silk has greater affinity than rod  when rubbed together, rod becomes +, silk - Eg. Combing hair  Comb becomes –, hair + (e’s go from hair to comb) • Charge is quantized: cannot divide up charge into smaller units than that of electron (or proton) i.e. all objects have a charge that is a whole- number multiple of charge of a single e. docsity.com Question Compare the gravitational force between an electron and proton in an H atom with the electrical force between them. Use: Average radius of H atom = 0.5 x 10-10 m Mass of proton = 1.67 x 10-27 kg Mass of electron = proton mass/2000 = 8.35 x 10-31 kg Felec = kqeqp/d2 = (9x109)(1.6 x 10-19)(1.6 x 10-19)/(0.5x10-10)2 = 9.2 x 10-8 N Fgrav = Gm1m2/d2 =(6.67 x 10-11)(1.67 x 10-27)(8.35 x 10-31 kg)/(0.5x10-10)2 = 3.7 x 10-47 N -- far smaller! docsity.com Clicker Question So the electrical attraction is by far dominant in providing the centripetal force that keeps the electron in orbit around the proton. How about the force the electron exerts on the proton? A) it’s larger than the force on the electron B) It’s the same C) It’s smaller D) It depends Answer: B, Newton’s 3rd law The electrical force is an interaction (as any force is) and each pulls on the other equally. docsity.com Superconductors • Have zero resistance, infinite conductivity below a critical temperature • Not common! Have to cool to very very low temperatures. • Current passes without losing energy, no heat loss. • Discovered in 1911 in metals near absolute zero (recall this is 0oK, -273oC) • Discovered in 1987 in non-metallic compound (ceramic) at “high” temperature around 100 K, (-173oC) • Under intense research! Many useful applications eg transmission of power without loss, magnetically-levitated trains… docsity.com Charging (1) Charging by friction and contact Already discussed a lot (rubbing materials together, see earlier slide on charge). Often can see or hear the sparks when the charges move. eg. Walk across a rug – feel tingle when touch door knob: electrons transferred from rug to your feet, then to the door knob. charging by friction charging by contact – simply touch docsity.com Clicker Question Consider a negatively charged rod that touches a long conductor, transferring its charge to the conductor. What is the difference between how the electrons become arranged on the conductor, versus how they would be arranged if it had been an insulator? A) There is no difference – evenly distributed on both B) There is no difference – localized on both, near the contact point with the rod. C) They are evenly distributed on the conductor, but localized on the insulator near the contact point with the rod D) They are evenly distributed on the insulator, but localized on the conductor near the contact point with the rod Answer: C Would spread out evenly on a good conductor, because the transferred e’s repel each other. But on insulator, or poor conductor, would be more localized at where the rod touched. docsity.com Eg Thunderstorms Negative charge at bottom of cloud induces positive charge on ground below. Charge flows most readily to and fro sharp metal points - hence lightning rods. Place rod above a building, and connect it to ground. Then the point of the rod picks up e’s from the air (“leakage”), so prevents large build up of + on the building, hence decreasing chance of a lightning strike. But even if there is a lightning strike (if leakage not enough), the electricity goes through rod to ground, rather than through building. docsity.com Charge polarization Instead, if bring a charged object near an insulator, electrons are not free to migrate throughout material. Instead, they redistribute within the atoms/molecules themselves: their “centers of charge” move Here, usual atom, with center of electron cloud at positive nucleus When a –ve charge is brought near the right, electron cloud shifts to the left. Centers of + and – charges no longer coincide. Atom is electrically polarized Surfaces of material look like this. A – charge induced on left, and + on the right. (Zero net charge on whole object) docsity.com Charge polarization continued • DEMO: Rub balloon on your hair – it will then stick to the wall ! Why? Balloon becomes charged (by friction) when rub on hair, picking up electrons. It then induces opposite charge on the wall’s surface closest to it (+ve), and the same charge as itself (-) on side of wall furthest away. So balloon is attracted to + charges and repelled by – charges in wall – but the – charges are further away so repulsive force is weaker and attraction wins. (Argument applies generally – key thing is difference in distance btn + and -) • Charge polarization is why a charged object can attract a neutral one : • Eg. Charge a comb by rubbing it through your hair, and then see it attracts bits of paper and fluff… docsity.com Electric Field • Just like we defined grav field, we’ll define electric field: both forces act on objects they are not in contact with. i.e. think of the force as interaction between one body and field set up by the other. The orbiting bodies interact with the force fields (grav for planet, electric for proton). Electric field, E= F q And field lines have arrow indicating direction a positive test charge would be pushed. So always point away from +charges, towards – charges… Eg. For a – charge: and for a (larger) + charge: docsity.com Electric field cont. Eg. Field for some other charge configurations: Eg. Field lines shown by small pieces of thread in an oil bath surrounding charged objects: Equal & opp. charges Equal and same sign Opp. charged plates Opp charged cyl & plate • Note: Field concept useful when dealing with motion of charges – creates a disturbance of the field that propagates at the speed of light, affecting other charges via this wave (more later..) (non-examinable) EXTRA READING: docsity.com Clicker Question Say the electric field from an isolated point charge has a certain value at a distance of 1m. How will the electric field strength compare at a distance of 2 m from the charge? A) It’s the same B) It is half as much at a distance 2 m compared to at 1m. C) It is ¼ as much D) It is twice as much E) It is 4x as much Answer: C It will be ¼ as much – inverse square law for force between two charges carries over to the electric field from a point charge. docsity.com Electric Potential • A charged object has potential energy (PE) from its location in E- field (c.f. grav. PE in Ch. 9) • Work is required to push charge against an E-field – this work changes the electric PE of the charged particle. • Compare with a spring: Do work in pushing it in, this work is stored as mechanical PE of spring. • Similarly, push two like charges together, working against the electrical force, increasing its energy. This work is stored as electrical PE. If push a particle with twice the charge, do twice as much work. So, define electric potential = electric potential energy charge docsity.com Electric potential cont. electric potential = electric potential energy charge Units: potential is measured in voltage, or volts, V. 1 volt = 1 Joule/Coulomb Eg. 12-V battery in your car, means that one terminal is 12 V higher in potential than the other. Will use terms “electric potential” and “voltage” interchangeably. • Often useful to think of what the electric potential is at various locations without actually having charge there. (See also Ch 23) • Note important difference between energy and potential: Both the small charged objects are at the same electric potential, but the one with more charge on it has higher electric potential energy. docsity.com Clicker Question When you rub a balloon on your hair, the balloon can get charged to about -5000 V. The charge on it is less than a millionth of a Coulomb. Should you be worried about touching it? A) yes B) no Answer: B No, since very little energy is involved, despite the high voltage. (If, instead the charge was 1 Coulomb, then it would be 5000J of energy, dangerous) docsity.com Van de Graaff generator Is a common device for building up high voltages: EXTRA READING: Needles maintained at large negative potential w.r.t. ground. They discharge electrons continuously onto the rubber belt which then carry them up into hollow conductor. Electrons end up on the outer sphere because there has to be 0 E-field inside – picked up by metal points (acting like lightning rods). Inside remains uncharged so more electrons keep coming up – end up with huge voltage on the dome. Can get as high as 20 million volts! Can raise your hair with this !! Charges go into your hair, causing hairs to repel each other. docsity.com Clicker Question In next chapter we’ll study how charges tend to move from regions of high potential to low potential. In which case here does current flow? A) In tube on the right: when one end of the fluorescent tube is held closer to the charged Van de Graaff generator B) In the tube on the left: when both ends are equidistant C) In both tubes D) In neither tube . fluorescent tubes Answer: A The close end is in a stronger part of the field than the far end. More energy per charge means more voltage at the near end. With a voltage difference across the tube, you get a current. When both ends are equidistant, there is no voltage difference across the tube, and no current - - - - - - - - docsity.com