Understanding Electric Potential Energy, Potential, and Capacitors, Lecture notes of Physics

Download Understanding Electric Potential Energy, Potential, and Capacitors and more Lecture notes Physics in PDF only on Docsity! ELECTRIC POTENTIAL AND ENERGY + Electric Potential Energy ● E fields can do work on charged particles – So they must contain energy ● PEelectric similar to PEgravity ( qEd vs. mgd ) – Except it can be attractive or repulsive – General rule: forces push toward lowest possible PE F = qE Work = Fd = qEd E field d + + High PE + + Low PE + – Very Low PE Point Charge Equations F = k q1q2 r2 E = k ∣q∣ r2 U = k qq ' r V = k q r E = F q V = U q ' F =− U  x E =− V  s Storing Electric Energy ● Must separate + and – charges – Like stretching an imaginary rubber band (the E field) ● Batteries – Place 2 different substances in contact with acid – + and – charges separate – ΔV is roughly constant for life of battery ● Capacitors – 2 metal plates with a gap between them – Place + charge on one plate and – charge on the other – Good for releasing energy quickly → e.g. camera flash bulb – ΔV depends on how much Q is on plates Capacitors C = 0 A d Capacitance C = Q V E = V d V = V b−V a0 = 8.85∗10−12 C V⋅m Units: Farad (F) = 1 C/V Dielectrics – Making Capacitors Stronger ● Put an insulator (with “bound electrons”) in an E field – e– are not “free” → shape of the electron cloud is affected ● Atom is now “polarized” → energy is stored like a spring – Can make capacitors stronger by inserting these “dielectrics” ● Too much polarization → electron separates from nucleus – “Dielectric breakdown” – material becomes a conductor + Capacitors with Dielectrics ● Every material has 2 dielectric properties: – “Dielectric constant” K – “Dielectric strength” → E field at which breakdown occurs ● Capacitance with dielectric – Is K times bigger ● Increase in C → change in U – Depends on whether capacitor is connected to battery C = KC0 Energy Stored in E Field ● Can calculate amount of energy E field stores – Using parallel-plate capacitor as example U = 1 2 CV 2 = 1 2  0 A d  Ed 2 = 1 2 0E 2 Ad  U Volume = U Ad = 1 2 0 E 2 = Energy Density = u u = 1 2 0 E 2 This turns out to be a general result for all E fields