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DC Circuits

DC Circuits. Series and parallel rules for resistors Kirchhoff’s circuit rules . “DC” Circuits. “ Direct Current or DC ”: current always flows in one direction. For circuits containing only resistors and emf’s the current is always constant in time. Circuits containing

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DC Circuits

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  1. DC Circuits • Series and parallel rules for resistors • Kirchhoff’s circuit rules

  2. “DC” Circuits “Direct Current or DC”: current always flows in one direction. For circuits containing only resistors and emf’s the current is always constant in time. Circuits containing other elements such as capacitors and inductors as well as resistors will have currents that change with time. Alternating current or AC is current that reverses direction many times (eg: 60 Hz current in Canada) and will not be treated in this course

  3. Resistors in Series R1 R2 R3 I B A We want to replace this combination by a single resistor with resistance Reff I A B Reff

  4. Same current through all resistors • Voltages add: • Veff = V1 + V2 + V3 + … • IReff = IR1 + IR2 + IR3+ … (same current through all) So, Easy way to remember: think of the length of a string: Ltot = L1 + L2 + L3 where L1 etc are the segments

  5. Resistors in Parallel I A I1 I2 I3 R1 R2 R3 B I We want to replace these resistors by a single resistance Reff: A I Reff B

  6. Same voltage across each resistor • Currents add: Ieff = I1 + I2 + I3 + Example: find the effective resistance of resistors of 5  and 10  in series and parallel.

  7. R1 R3 11 V R2 R6 R4 R5 Example 1 All resistors = 1 Ω Find: Effective resistance across the battery

  8. R R R R R R Example 2 Find the effectiveresistance of a network of identical resistors

  9. a 3 Ω 6Ω 9Ω 18 V b Example 3 Find a) the current in each resistor b) the power dissipated by each resistor c) the equivalent resistance of the three resistors

  10. Example 4 A regular “40 watt” bulb and a “60 watt” bulb are connected in SERIES across 120 V. What power does each bulb give? (Assume that the resistances don’t change with temperature—these are special bulbs.)

  11. Kirchhoff’s Circuit Rules Junction Rule: total current in = total current out at each junction (from conservation of charge). Loop Rule: Sum of potential differences around any closed loop is zero (from conservation of energy). Charge q moves through circuit changing its potential energy qV but eventually there is no overall change.

  12. I2 I1 I3 Junction Rule: conservation of charge. I1 = I2 + I3 Sum of currents entering a junction equals the sum of currents leaving the junction Alternately: I2 I1 I1 = I2 + (I1-I2) I1-I2

  13. Loop Rule: conservation of energy. Follow a test charge q around a loop:  around any loop in circuit. R I ΔV = -IR - + ΔV = ΔV = Q/C -Q +Q C loop loop going from left to right

  14. Example 5 c 200Ω 300 Ω a b 12 V 300 Ω 400Ω d a) What is Vab when the switch is open? b) Find the current through the switch when it is closed.

  15. Solution

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