Transcript 普物甲下
普通物理學甲下 (202 101A2) General Physics(A)(2) 台大物理 吳俊輝 Electricity II Flux (used in Gauss Law) The concept of flux – fluid flow across a surface. (通量: 單位時間通過的量) Intuitively, it is the component of the fluid velocity that is normal to the area of interest, i.e. the flow across the area: flux (v cos ) A v A Gaussian surface A Gaussian surface is a closed surface which we use to calculate the flux of E field. Electric flux: the electric field has been given the “properties” of a fluid-flow field. This allows us to determine the properties of electric field more easily. The total net electric flux across a Gaussian surface is E dA dA方向為由Gaussian surface內部朝外 Gauss’s Law E dA q enc where qenc is the sum of all charges enclosed by the Gaussian surface. Examples: choosing a Gaussian surface Gauss’ Law can be applied to any closed surface: E dA qenc e.g.: E dA qenc q1 q2 q3 高斯定律的用處：求電場，及 研究電荷系統的性質… 1. Spherical Symmetry Coulomb’s Law From symmetry, we know that the electric field is the same at a given distance r from the center of the charge. By Gauss Law, we have E dA q enc 左式 E dA EdA sphere E dA E (4 r 2 ) sphere 故 E q 4 r 2 The electric field points away from the charge center as we know from the area integral. 2. Charge on an isolated conductor 將電荷置於絕緣導體上時… 電荷會分佈在導體的外表面!!! By the Gauss Law: E dA qenc (a) electric field must be zero inside a conductor, otherwise the charges would flow. Therefore, excess charges must be on the conductor exterior. (b) electric field inside a conductor must be zero, therefore, no charge is on the inner surface! 3. Planar geometry and Gauss’ Law Excessive + charge on a large planar conductor. Let’s use Gauss’ Law: Again, the symmetry dictates that E must point perpendicular away from the plane ( is charge per unit area). E dA EA E dA EA A Therefore, E Again, the field points away from the plane from the area integration. 4. Cylindrical symmetry A very long plastic wire with uniform charge per unit length l E dA E (2 rdh) E dA E dA E (2 rh) cylinder qenc lh E (2 rh) l E 2 r So the electric field with 1/r away from a long wire. Electrical work (WE), energy (U), and potential (V) 電荷在電場中移動時，電場會對其作功， 造成其位能的改變： f U U f U i WE dWE i f f F dsE q E ds i i Electrical potential: V U q f U V E ds i q The unit of V: joule/coulomb (J/C) = volt (V). Equipotential Surface Surfaces of constant V (electric potential) Equipotential Surface E Electron volt (eV) 能量的單位：joule (J), electron volt (eV). 1 eV = energy needed to move a particle with charge e across a potential difference of 1 V. So 1eV (1.60 1019 C )(1J / C ) 1.60 1019 J Some practical knowledge about electricity Power P=IV Current I=dq/dt : 1. AC: Alternating Current 2. DC: Direct Current Ohm’s Law V=IR Medical device - defibrillator Energy is stored in a capacitor to stop fibrillation of heart attack patients (100kW of pulsed energy can be applied to a patient) + + + - - - + - + - + + - - Apply to patient Exercise 1 (習題1) Ch 21: 25, 60, 83 Ch 22: 22, 41, 65, 87 Ch 23: 21, 29, 47, 81 Deadline: 3/6, 10:20AM