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Understanding Electric Flux in Physics

Explore the concept of electric flux, Gauss's Law, Gaussian surfaces, and practical applications in electricity. Gain insights on charge distribution, potential energy, work, capacitance, and medical devices like defibrillators.

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Understanding Electric Flux in Physics

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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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