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# UNT PHYS 1420 - Exam 1 Study Guide

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PHYS 1420Exam # 1 Study Guide Lectures: 1 - 3Lecture 1 (January 20)Electric Forces and Electric Fields Equations and constants to know: Magnitude of the charge of an electron: 1.60 x10^-19 CN= q/e, N=number of electrons, e= electric charge, e= charge of electronCoulombs law: F=k|q1|q2|\r^2 F=electric forceq1q2= magnitudes of the charges r^2= distance k=constant, 8.99x10^9 N•m^2/C^2When calculating coulombs law, the magnitude cannot be negative so the values for q1 and q2 are written as positive. Permittivity of free space e0, k=1/4πq0The electric field: E=F/q0 SI unit for electric field: N/C, the surrounding charges are what give the electric field at any given point. q0= test charge, has to be very small, and must not alter other charges.Magnitude of the electric field: E=kq/r^2q=point charge, if q is positive E is directed away from q, if q is negative E is directed toward q. Parallel plate capacitor: E=σ/e0In parallel plate capacitor the positive plate always moves toward the negative plate and is perpendicular to it. Sigma σ= charge per unit areaParallel plate capacitor is used to find the magnitude of the electric field between the plates. Law of conservation of electric charge- during any process the net electric charge of an isolated system remains constant (conserved). Like charges will always repel each other, unlike charges always attract each other. (Opposites attract) Conductors- readily conduct electricity, (metals, gold, copper, and aluminum) Insulators- poorly conduct electricity (rubber, plastic, and wood) Heat transfers from hotter to cooler. Charging can be done by contact, such as rubbing to transfer a negative charge. Charging can be done without contact, this sis known as induction, when an object is brought near another negative and positive charges separate. Electric field lines- if you have a positive point, electric field lines would direct radially outward (away)from the positive point. But If you have a negative point, electric field lines would direct radially inward (towards) the point. In a parallel plate capacitor electric field lines are parallel and evenly spaced out. The number of lines leaving a positive charge or entering a negative charge is proportional to the magnitude of the charge. Electric field inside a conductor: shielding, at equilibrium under electrostatic conditions any excess charge resides on the surface of a conductor. The electric field is zero at any point within a conducting material. The conductor shields any charge within it from electric fields created outsidethe conductor. The electric field just outside the surface of a conductor is perpendicular to the surface at equilibrium under electrostatic conditions. Lecture 2 (January 27) Electric potential energy and electric potential Equations to know: Potential energy: Wab= EPEa –EPEbWab= work done by the electric force The electric force is conserved and moves along a path from A to B Electric potential difference=V=EPE/q0 V= electric potential, q0= small test charge SI unit= joule/coulomb (J/C) Vb –Va= EPEb/q0 – EPEa/q0= -Wab/q0 A positive charge accelerates from a region of higher potential toward a region of lower potential. A negative charge accelerates from a region of lower potential toward a region of higher potential. One electron volt is the magnitude of the amount by which the potential energy of an electron changes when the electron moves through a potential difference of one volt.Total energy of a system E is the sum of its translational (1/2mv^2) and rotational (1/2Iw^2) kinetic energies, gravitational potential energy (mgh), elastic potential energy (1/2kx^2) and electric potential work (EPE) If external non-conservative forces like friction do no net work, the total energy of the system is conserved. Final total energy= E0; Ef=E0 Electric potential difference by point charges: V=kq/rK= 8.99x10^9 N•m^2/C^2, r= distance, q= point charge Equipotential surface- a surface on which the electric potential is the same everywhere. The electric force does no work as a charge moves on an equipotential surface, because the force is always perpendicular to the displacement of the charge. E= -change V/change sChange of s= displacement, change of V= potential difference between the surfacesCapacitor- a device that stores charge and energy. It consist of 2 conductors or plates that are near each other, but not touching. q= CV. q=magnitude of charge, C= capacitance, V= magnitude of potential difference. SI unit for capacitance= C/V Dielectric- insulating material between the plates of a capacitor. Dielectric constant: k=E0/E. E0 and E are the magnitudes of the electric fields between the plates without and with a dielectric, assuming charges are fixed. Capacitance of a parallel capacitor: C=ke0A/dE0= 8.85x10^-12, permittivity of free space, A= area of each plate, d Is the distance between theplates. Electric potential energy stored in a capacitor: Energy=1/2qV=1/2CV^2=q^2/(2C)Energy density- energy stored per unit volume and is related to the magnitude E of the electric field. Energy density=1/2ke0E^2Lecture 3 (February 3)Electric circuitsEquations to know: I=change(q)/change(t) (when rate is constant) Change q= magnitude of the charge crossing a surface, change t=time SI unit for current= C/s, one ampere A Direct current- charges flow in one directionAlternating current- charges direction alternates Conventional current- hypothetical flow of positive charges that would have the same effect in acircuit as the movement of negative charges that actually does occur. There must be at least one source or generator of electric energy in an electric circuit. The electromotive force (emf) of a generator such as a battery, is the max potential difference in volts that exist between the terminals of the generator. Electric current- rate of flow of charge Ohms law- R= V/I or V=IR R= resistance V= voltage I= currentResistance is measured In volts per ampere, ohm (Ω).If the ratio of the voltage to the current is constant for all values of voltage and current, the resistance is constant.Resistance and resistivity: R= ρL/A ρ = resistivity of materialL= length A= cross sectional areaThe resistivity of a material depends on the temperature. Temperature dependence= ρ=ρ[1+α(T-T0)] ρ and ρ0=resistivity, T and T0= temperatures, α= temperature coefficient of resistivityTemperature dependence of resistance= R=R0[1 +α(T –T0)]Electric power- when electric charge flows

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