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Electric Potential and Electric Energy; Capacitance

Electric Potential and Electric Energy; Capacitance. Chapter 17. 17.1 Electrical Potential and Potential Difference. Electric potential- the potential energy per unit charge SI unit is Volts Named after Alessandro Volta who is best known for inventing the electric battery 1V = 1J/C

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Electric Potential and Electric Energy; Capacitance

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  1. Electric Potential and Electric Energy; Capacitance Chapter 17

  2. 17.1 Electrical Potential and Potential Difference • Electric potential- the potential energy per unit charge • SI unit is Volts • Named after Alessandro Volta who is best known for inventing the electric battery • 1V = 1J/C • A form of potential energy • Positively charged objects move naturally from a high potential to a low potential • Negatively charged objects naturally more in the opposite direction

  3. Potential difference- is the measurable difference between two charges. • Ex. The potential difference between a rock on the cliff and on the ground is equal to the change in work for that object

  4. ΔPE = qV • PE = potential energy • q = charge in ___ units • V = potential difference in ___ units • If an object with charge q moves through a potential difference of V is the change in potential energy.

  5. Sample Problem #1 • Suppose an electron in the picture tube of a television set is accelerated from rest through a potential difference of +5000V. • A) What is the change in potential energy of the electron? • B) What is the speed of the electron as a result of this acceleration? • C) Work the same problem only use data for a proton.

  6. 17.2 Relation Between Electric Potential and Electric Field • Every conducting material that has charge moving through it has an electric field. • Charge moves from + to – because of the electric field.

  7. For our discussion we will look only at the electric field between two parallel plates.

  8. There is a direct relationship between an electric field and the potential differnce. • W = qV • W = qEd • qV = qEd • V = Ed

  9. Sample Problem #2 • Two parallel plates are charged to a voltage of 50V. If the separation between the plates is 0.050m, calculate the electric field between them.

  10. 17.4 The Electron Volt • The electron volt is a unit of energy • Joules is a very large unit for dealing with energies of electrons, atoms, or molecules. • Therefore the electron volt is used to describe smaller amounts of energy. • 1eV = 1.6* 10-19J

  11. 17.6 Electric Dipoles • Two equal point charges, Q, of opposite sign and separated by a distance of l. • Some molecules, such a water, naturally have electric dipoles and are called polar molecules.

  12. 17.7 Capacitance • Capacitors are sometimes called a condenser, or a device that can store electric charge • Parallel plates separated by a distance and insulation. • Used for storage of charge for later use, such as: • Camera flash, energy back ups, block surges of charge, RAM in computers, …

  13. Can calculate the amount of charge per volt that may be stored in a capacitor (aka capacitance) • Q = CV • SI unit is Farad (F) • 1F = C/V

  14. Capacitance for a given capacitor is constant and depends on the Area and Distance the plates are separated by. • Larger area means that for a given number of charges, there will be less repulsion between them b/c they are further apart. • Therefore, more charge can be held in each plate.

  15. 17.8 Dielectrics • The insulated material between capacitor plates is known as a dielectric. • Dielectrics are useful b/c it can allow higher voltages to be applied without allowing it to pass from one plate to another AND allow plates to be closer together. • Keyboards operate b/c of dielectrics.

  16. Storage of Electric Energy • Energy stored in a capacitor is equal to the amount o work done to charge it. • To charge a capacitor, charge must be moved from one plate to another. • The more charge already on the plate, the more work is required to add more.

  17. The voltage across the capacitor is proportional to how much charge it already has accumulated. • We calculate the energy stored in a capacitor: • U = ½ QV = ½ CV2 = ½ Q2/C

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