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Electricity II

Electricity II. 普通物理學甲下 (202 101A2) General Physics(A)(2) 台大物理　吳俊輝. Flux (used in Gauss Law). The concept of flux – fluid flow across a surface. ( 通量 : 單位時間通過的量 )

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Electricity II

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  1. Electricity II 普通物理學甲下(202 101A2) General Physics(A)(2) 台大物理　吳俊輝

  2. 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

  3. 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 方向為由Gaussian surface內部朝外

  4. Gauss’s Law where is the sum of all charges enclosed by the Gaussian surface.

  5. Examples: choosing a Gaussian surface • Gauss’ Law can be applied to any closed surface: • e.g.: • 高斯定律的用處：求電場，及研究電荷系統的性質…

  6.  Coulomb’s Law 1. Spherical Symmetry • 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 • The electric field points away from the charge center as we know from the area integral.

  7. 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: (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!

  8. 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). • Therefore, Again, the field points away from the plane from the area integration.

  9. 4. Cylindrical symmetry • A very long plastic wire with uniform charge per unit length l • So the electric field with 1/r away from a long wire.

  10. 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： • Electrical potential: • The unit of V: joule/coulomb (J/C) = volt (V).

  11. Equipotential Surface • Surfaces of constant V (electric potential)

  12. 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

  13. 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

  14. 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

  15. 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

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