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PH 122 (46)
Lecture

# Capacitor formulas, dielectrics, electric field of the cell membrane

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School
Department
Physics
Course
PH 122
Professor
Brian Jones
Semester
Spring

Description
7 February The electric potential energy of an electron increases when it is moved away from a positive charge. Its speed will lessen. Energies of some atomic and molecular processes Breaking a hydrogen bond between two water molecules: 0.24 eV Energy released in metabolizing one molecular of ATP: 0.32 eV Breaking the bond between atoms in a water molecule: 4.7 eV Ionizing a hydrogen atom: 13.6 eV Capacitor Formulas Q = C ΔV C Charge on a capacitor with potential difference ΔC Unit: farad, F 1 farad = 1 F = 1 C/V ϵ A 0 C = d Capacitance of a parallel-plate capacitor with plate area A, separation d 1 Q 2 1 U C = C (ΔV C2 2 C 2 Electric potential energy of a capacitor with charge Q and potential difference ΔV C The flash unit in a camera creates a very short but very bright flash: 10 W for a time of 10 μs. The flash is produced by a 22 μG capacitor. To what voltage must the capacitor be charged? ΔE = PΔt -6 ΔE = (100,000 W) (10 x 10 sec) ΔE = 1.0 J 1.0 J = ½ (22 x 10 F) (ΔV) 2 V = 300 volts Dielectrics The positive nuclei shift very slightly in the direction of the field. The negative electrons shift very slightly in the direction opposite to the field. Because of polarization, one surface has an excess of positive charge and the other has an excess of negative charge. κ ϵ A 0 C = d Capacitance of a parallel-plate capacitor with a dielectric of dielectric constant κ The net electric field is the vector sum of the applied field and
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