Current

• Current and charge must be conserved item.
• The current is the amount of charge per second  [latex]I = \frac{ \Delta Q}{ \Delta t}[/latex]
• 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

Batteries

• The potential drop across a battery is fixed

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

[latex]\Delta V = IR[/latex]

• The resistance is fixed by the material, units:

[latex]\frac{V}{A} = \Omega[/latex]

• The potential drop across a wire is zero ([latex]R = 0[/latex])

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:

[latex]C = \frac{Q}{\Delta V}[/latex]

• For parallel plates

[latex]C_{\parallel} = \varepsilon \frac{A}{d}[/latex]

• Units

[latex]\frac{C}{V} = F[/latex]

Power

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

[latex]P = IV[/latex]

• Must do element-by-element