Electromagnetic Waves

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Electromagnetic Waves

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Lecture Note
The University of Vermont
Phys 012 - Elementary Physics
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Lecture 7 Outline of Last Lecture I. Self inductance constant, L a. Measured in Henrys (H) i. 1 H = 1 (Vs)/A b. Depends on configuration c. For solenoid: L = (N2Aμ0)/l i. εind = -L(ΔI/Δt) ii. N (ΔΦ/Δt) = -L(ΔI/Δt) ; L = (NΦ)/I iii. Φ = Aμ0(N/l)I II. Energy a. Depends on current b. For solenoid, energy = (AlB2)/2μ0 III. Energy density, μB a. μB = B2/2μ0 i. True for energy density of any magnetic field in free space. IV. RL circuits: circuits with a resistor and an inductor a. Back εmf is generated when I is increasing. No εmf is generated when I is constant. i. I = (ε/R)(1-e-t/τ) 1. τ = time constant = L/R ii. When t = 0, I = 0. iii. As t approaches infinity, I approaches ε/R. Econ 101 1st Edition b. Forward εmf is generated when I is decreasing. i. I = I0e-t/τ ii. When t = 0, I = I0. iii. As t approaches infinity, I approaches 0. V. Transformers a. Magnetic field will constantly be switching directions, causing a constantly changing flux which generates a continuous εmf. i. Diagram above shows only one direction the magnetic field can be going. ii. The change in flux will be the same in both the primary and secondary circuits. iii. Ip/Is = Vs/Vp = Ns/Np VI. LC Circuits: circuits containing an inductor and a capacitor a. Capacitor is initially fully-charged, causing I to increase. b. Imax is reached when charge on capacitor is zero.

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