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 • 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

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

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.

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.

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.

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

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

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.

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

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.

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, …

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

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.

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.

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.

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

Electric Potential and Electric Energy; Capacitance