Lecture 18

Readings

http://umdberg.pbworks.com/w/page/73085141/Ways%20to%20think%20about%20current%3A%20A%20toolbox%20of%20models%20%282013%29

http://umdberg.pbworks.com/w/page/73085177/Kirchhoff%27s%20principles%20%282013%29

Models of Current

• Rope - the rope itself is the charges

  • Pulling rope with 1 hand, moving through other hand
  • $Current(I)$$Current\ (I)$ is the same in every "circuit"

• Nail board

  • Driving force - high V to low V , like slope

  • Balls collide with nails

    • Resistance to motion
    • Steady state speed

  • Some "machine" needed to get balls back to top

    • Like ski-lift ( = battery )

• Air in straws

  • Long narrow pipe = less flow

    • Like long narrow wire -> higher resistance

  • Pressure difference produces flow:

  

• Water Flow

  • Similar to nail-board, but water wheel means "flow" can do "something useful"

    • Equivalent to an electrical circuit powering a light bulb.

      • The light bulb is like the water wheel

Kirchhoff's Principles

• 1st Principle: Current In = Current Out

• 2nd Principle: Voltage changes across resistor and capacitor

  • Resistor:

  

  • Capacitor

    

    • $V_1-V_2=\frac{Q}{C}$$V_1 - V_2 = \frac{Q}{C}$
    • $Q=$$Q =$ charge on plate

• 3rd Principle: Voltage change around any complete loop must add to $0$$0$

  

  • Rule: no change in V for different points in same wire.

    • Same wire = no element ( R , C , or battery ) between points

  • V only changes across R, C, and batteries

  • So $V_B+V_R+VC=+10+-4+-6=0$$V_B + V_R + VC = +10 + -4 + -6 = 0$