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Unit IV On-a-Page

Note: the video below says “Unit V,” but it is really Unit IV. The numbering difference comes from the fact that there used to be a unit between what is now Unit II  (Geometric optics) and Unit III (Electrostatics) which has since been dropped.

 

Current

  • Current and charge must be conserved items.
  • The current is the amount of charge per second  I = \frac{ \Delta Q}{ \Delta t}
  • units: C/s = A
  • Direction of current is direction of positive charges’ motion
Figure 1.

Kirchhoff’s Rules (Or “How to analyze a circuit”)

  • The amount of current going into a junction must equal the amount going out (otherwise it would pile up)
Figure 2.
  • The changes in potential around any closed loop are zero

Circuit Elements

Batteries

  • The potential drop across a battery is fixed
Figure 3.

Resistors

  • Resistors dissipate electrical potential energy into something else (heat, motion, light, etc.)
  • The potential drop across a resistor is related to the current and resistance

\Delta V = IR

  • The resistance is fixed by the material, units:

\frac{V}{A} = \Omega

  • The potential drop across a wire is zero (R = 0)

Capacitors

  • A capacitor is two pieces of metal that don’t touch
  • Capacitors store charges separated. Therefore, they store energy
  • The capacitance is a property of the geometry of the plates
  • The potential drop across a capacitor is related to the charge and the capacitance:

C = \frac{Q}{\Delta V}

  • For parallel plates

C_{\parallel} = \varepsilon \frac{A}{d}

  • Units

\frac{C}{V} = F

Power

  • The power provided/stored/dissipated by a circuit element is

P = IV

  • Must do element-by-element

License

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Physics 132: What is an Electron? What is Light? Copyright © 2020 by Roger Hinrichs; Paul Peter Urone; Paul Flowers; Edward J. Neth; William R. Robinson; Klaus Theopold; Richard Langley; Julianne Zedalis; John Eggebrecht; and E.F. Redish is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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