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Lecture 24: FRI 13 MAR

Physics 2113 Jonathan Dowling. Lecture 24: FRI 13 MAR. DC circuits III Ch27.4-9. One Battery? Simplify!. Resistors Capacitors Key formula : V=iR Q=CV In series : same current dQ/dt same charge Q R eq = ∑R j 1/C eq = ∑1/C j In parallel : same voltage same voltage

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Lecture 24: FRI 13 MAR

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  1. Physics 2113 Jonathan Dowling Lecture 24: FRI 13 MAR DC circuits III Ch27.4-9

  2. One Battery? Simplify! ResistorsCapacitors Key formula: V=iRQ=CV In series: same current dQ/dtsame charge Q Req=∑Rj1/Ceq=∑1/Cj In parallel: same voltagesame voltage 1/Req=∑1/RjCeq=∑Cj P = iV = i2R = V2/R U = QV/2 = Q2/2C = CV2

  3. Many Batteries? Loop & Junction! Three Batteries: Straight to Loop & Junction One Battery: Simplify First

  4. i(t) CE E/R RC Circuits: Charging a Capacitor In these circuits, current will change for a while, and then stay constant. We want to solve for current as a function of time i(t)=dq/dt. The charge on the capacitor will also be a function of time: q(t). The voltage across the resistor and the capacitor also change with time. To charge the capacitor, close the switch on a. VC=Q/C VR=iR A differential equation for q(t)! The solution is: Time constant: τ = RC Time i drops to 1/e.

  5. Solution: i(t) + C E/R - RC Circuits: Discharging a Capacitor Assume the switch has been closed on a for a long time: the capacitor will be charged with Q=CE. +++ --- Then, close the switch on b: charges find their way across the circuit, establishing a current.

  6. Too Many Batteries!

  7. Fire Hazard: Filling gas can in pickup truck with plastic bed liner. • Safe Practice: Always place gas can on ground before refueling. • Touch can with gas dispenser nozzle before removing can lid. • Keep gas dispenser nozzle in contact with can inlet when filling.

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