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PHYS1041 Notes Week 8

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PHYS 1041
Dr.David Fleming

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Physics 1041 Notes March 24th Electrostatic Energy and Capacitors: • a capacitor: something that stores charge and energy. For example: biological membranes, consumer electronics, defibrillators • capacitors take advantage of energy storage capability by storing charges in a certain location If 2 charges are present, and we want to consider the electric potential at position 2: kq 1 V = to solve for voltage r kq1 2 W = E =V ×q = 2 r to solve for the work/energy to put the second charge there If 3 charges are present, and we want to consider the electric potential at position 3: kq kq V = 1 + 2 to solve for voltage (the sum of the voltage of the 2 other charges) r r kq q kq q W = E =V ×q = 2 1 3 + 2 3 to solve for the work/energy to put the third charge there r r To solve for the entire electric field: kq1q 2 kq 1 3 kq2q 3 E = r + r + r • for 2 parallel plates, if charges are added to one plate, equal and opposite charges will be added to the other • electric field will be uniform between the two plates, and 0V/m outside of the plates • capacitance: ratio of charge stored to potential difference • when C= capacitance, measured in Farads (F) • when Q= charge, measured in Coulombs (C) • when V= voltage between 2 points, measured in volts (V) • then C = Q V Parallel Plate Capacitors: • when C= parallel plate capacitance (F) • when ε = the permitivity of free space constant, equal to 8.85 x 10 F/m O • when A= the area of a single plate (m ) • when d= the distance between the plates (m) • when k= the dielectric constant, assumed to be equal to 1 when parallel plates are in a vacuum or there is only air between them • then C = kεOA d • the electric field within a parallel plate capacitor is constant and uniform • under that situation, the potential difference between the two plates is given by: ΔV = Ed where E is the electric field strength, and d is the distance between 2 plates • electric field can be measured by N/m or V/m Defibrillator: • sends a charge throughout the body so that energy stored becomes energy active to reboot heart and reverse fibrillation • fibrillation is a state in which contraction of ventricles is out of usual rhythm • it is designed to be used before a flat line heart rate March 26th Optics • assumes that all light acts as a ray, traveling in a straight line • optical properties by wave fields • related to microscopes, corrective lenses, human eye Reflection • law of reflection: the angle of incident and the angle of reflection should be equal if light is reflecting in the same medium • angles are measured from light ray to the normal line, which bisects the light ray Refraction • bending of light when changing materials • when θ1 is the angle of incident • when θ2 is the angle of refraction • when n-values are the indices of refraction, which vary with different material (n=c/v, when c= speed of light in vacuum and v= speed of light in given material) • then n sinθ = n sinθ , also known as Snell's Law 1 1 2 2 • if moving from a substance of low index of refraction (n) to high index of refraction, light is
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