1. Chapter 17: Electric Potential 1

2.  As in earlier chapters on mechanics we learned that energy is conserved; it is neither created nor destroyed but is transferred from one object to another or transformed into another type of energy  Energy and its interactions can help us understand nature  Work performed on a charged particle in an electric field can result in the particle gaining electric potential energy (PE), kinetic energy (KE) or both 2

3.  The electric field does work when it moves the charged particle from location a to b 3

4.  Electric potential is defined as the electric potential energy per unit charge and is measured with a voltmeter  V a = Electric Potential: Units=volt ( V ). Named after Alessandro Volta, inventor of the electric battery  PE a = Electric Potential Energy: Unit= joule (J)  q = Charge on particle: Unit=Coulomb (C) 4

5.  Also called voltage  Electric potential difference is the difference in electric potential (V) between the final and initial position Δ  Also the ratio of work needed to move a charge between two points divided by the magnitude of the charge ΔV=W q 5

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7. 17.2 Relation between Electric Potential and Electric Field  A uniform electric field can made by placing two large flat conducting plates of opposite charge parallel to each other  The electric field can be calculated by dividing the potential difference between the plates by the distance between the plates (in meters) 7

8. 17.3 Equipotential Lines  Electric potential can be represented by drawing equipotential lines (green)  An equipotential is a line over which the potential is constant  Equipotential lines are perpendicular to the Electric field (red)  Conductors are equipotential surfaces 8

9. The electric field is strongest where the equipotential lines are closest together. 9

10. 17.4 The Electron Volt, a Unit of Energy  The joule is a large unit to deal with energies of electrons, atoms or molecules  The electron volt (eV) is used  An eV is the energy gained by an electron moving through a potential difference of one volt. 10

11. 17.7 Capacitance  A capacitor consists of two conductors that are close but not touching. A capacitor has the ability to store electric charge  In general capacitance increases as the plates become larger and decreases as the separation between plates increases 11

12.  Capacitors are used widely in  electronic circuits  power failure back ups  Blocking surges of charge and energy 12

13.  (a) Parallel-plate capacitor connected to battery. When connected to a battery the plates become charged; one + and one –  (b) In a circuit diagram the capacitor is represented as seen here 13

14. C is the capacitance and depends on the size, shape, position and separating material of the capacitor Unit of capacitance: farad ( F ) 1 F = 1 C / V (coulomb/volt)  When a capacitor is connected to a battery, the amount of charge (Q) on its plates is proportional to the potential difference (voltage) between them 14

15. 17.8 Dielectrics  Most capacitors have an insulating sheet of between the plates  This insulator is a dielectric  Do not break down and allow charge to flow as easily as air, allowing higher voltages  Allow plates to be closer together  Increase the capacitance by a factor of K; a dielectric constant 15

16. If the electric field in a dielectric becomes too large, it can tear the electrons off the atoms, thereby enabling the material to conduct. This is called dielectric breakdown; the field at which this happens is called the dielectric strength 16

17. 17.9 Storage of Electric Energy  A charged capacitor stores electric energy by separating + and – charges  The energy stored is equal to the work done to charge it  Stored energy in a capacitor can cause burns or shocks, even when the external power is off!  There are many uses for capacitors; a camera flash, a cardiac defibrillator, etc.  An essential part of most electrical devices used today 17

18.  A defibrillator is a capacitor charged to a high voltage. Once charged it sends a brief charge through the heart. This can stop the heart and (hopefully!) allow it to resume normal beating 18

19.  Zitewitz. Physics: Principles and Problems. 2004  Giancoli, Douglas. Physics: Principles with Applications 6th Edition. 2009.  Walker, James. AP Physics, 4 th Edition. 2010  http://commons.wikimedia.org/wiki/File:Ca pacitor_schematic_with_dielectric.svg 19