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Physics 17: Electric Potential & Electric Energy

Physics 17: Electric Potential & Electric Energy. Christopher Chui. Electric Potential & Potential Difference. Electric potential = potential energy/charge =PE/q Potential difference, V ab = V a – V b = - W ba /q

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Physics 17: Electric Potential & Electric Energy

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  1. Physics 17: Electric Potential & Electric Energy Christopher Chui Physics 17: Electric Potential & Electric Energy - Christopher Chui

  2. Electric Potential & Potential Difference • Electric potential = potential energy/charge =PE/q • Potential difference, Vab = Va – Vb = - Wba/q • = negative work done by the electric force to move the charge from point b  point a • 1 V = 1J/C in honor of Volta (1745-1827) = p.d. • The ground is chosen to have 0 V • Change in PE of a charge q between a and b=q Vba • Typical voltages: Thundercloud 108V, HV Pwrline106V,TV tube 104V,nerve 10-1 V,EKG 10–4 V • W = Fd = qEd=qVba Vba = Ed • An equipotential surface is perpendicular to E • Conductors are equipotential surfaces Physics 17: Electric Potential & Electric Energy - Christopher Chui

  3. Electric Potential due a Point Charge • Electric potential of point charge (V=0 at infinity) = kQ/r, where k = 1/4peo = 9 x 109 N.m2/C2 • Potentials add as scalars, but fields add as vectors • Two equal point charges Q, of opposite sign, separated by a distance l, are called an electric dipole • V = kQl cos q / r2 for dipole with r >> l • Dipole moment p = Q l  V = kp cos q / r2 • Capacitance is a device that can store electric charge • For a given capacitor, the amount of charge Q acquired is proportional to the potential difference V • Q = CV where C in the capacitance in Farad, 1F=1C/V • C= eo A/d for //plate capacitor Physics 17: Electric Potential & Electric Energy - Christopher Chui

  4. Dielectrics and Storage of E Energy • C = Keo A/d where K is dielectric constant for //plate • C = e A/d where e = Keo = permittivity of the material • Energy stored in cap U = ½ QV = ½ CV2 = ½ Q2/C • Energy stored / unit volume in E field = ½ eoE2 • CRT (cathode ray tube) is used in TV, computer monitors, and oscilloscopes • ECG is a record of the potential changes of a person’s heart. The amount of charge depends on the size of the cell, but is approximately 10-3 C/m2 of surface. For a cell with surface area 10-5 m2, the total charge on either side ~ 10-8 C Physics 17: Electric Potential & Electric Energy - Christopher Chui

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