Phys 115 1st Edition Lecture 12Outline of Last Lecture I. Adding VoltageII. Equivalent ResistanceIII. Ideal vs. RealOutline of Current Lecture IV. CapacitanceCurrent Lectureo Electric Field of Large Plate It can be shown that the magnitude of the electric field due to a large charged plate is equal to: Remember that the electric field points away from positive charges and toward negative charges. The quantity σ represents the surface charge density on the plate. And ε0 represents the permittivity of free space.o Capacitance The ratio Q/ΔV is equal to the capacitance of the plates. We have shown that it is only dependent on the physical parameters of the plates. o Dielectrics An insulator consists of vast numbers of atoms. When an insulator is placed in an electric field, each atom polarizes. Polarization occurs when an atom’s negative electron cloud and positive nucleus shift very slightly in opposite directions in response to an applied electric field. An induced positive charge builds up on one surface of the insulator, and an induced negative charge builds up on the other surface. When a dielectric is placed between two charged plates, the electric field between the plates decreases. When a dielectric is inserted, the electric field between the plates decreases, which implies that the potential difference decreases as well. The presence of a dielectric results in an increased capacitance.o RC CircuitsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. The current and the capacitor voltage decay to zero after the switch closes, but not linearly. The decays of the voltage and the current are exponential decays. The time constant τ is a characteristic time for a circuit. A long time constant implies a slow decay; a short time constant, a rapid decay:- RC The voltage across a charging of a capacitor is also described by an exponential
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