Unit IV On-a-Page



  • Current and charge must be conserved item.
  • 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

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)
  • The changes in potential around any closed loop is zero

Circuit Elements


  • The potential drop across a battery is fixed


  • 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)


  • 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


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

P = IV

  • Must do element-by-element


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Physics 132: What is an Electron? What is Light? 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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