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Chapter 18

Chapter 18. Direct Current Circuits. Sources of emf. The source that maintains the current in a closed circuit is called a source of emf Any devices that increase the potential energy of charges circulating in circuits are sources of emf (e.g. batteries, generators, etc.)

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Chapter 18

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  1. Chapter 18 Direct Current Circuits

  2. Sources of emf • The source that maintains the current in a closed circuit is called a source of emf • Any devices that increase the potential energy of charges circulating in circuits are sources of emf (e.g. batteries, generators, etc.) • The emf is the work done per unit charge • SI units: Volts

  3. emf and Internal Resistance • A real battery has some internal resistancer; therefore, the terminal voltage is not equal to the emf • The terminal voltage: ΔV = Vb – Va ΔV = ε – Ir • For the entire circuit (R – load resistance): ε = ΔV + Ir = IR + Ir

  4. emf and Internal Resistance ε = ΔV + Ir= IR + Ir • ε is equal to the terminal voltage when the current is zero – open-circuit voltage I = ε / (R + r) • The current depends on both the resistance external to the battery and the internal resistance • When R >> r, r can be ignored • Power relationship: I ε= I2 R + I2 r • When R >> r, most of the power delivered by the battery is transferred to the load resistor

  5. Resistors in Series • When two or more resistors are connected end-to-end, they are said to be in series • The current is the same in all resistors because any charge that flows through one resistor flows through the other • The sum of the potential differences across the resistors is equal to the total potential difference across the combination

  6. Resistors in Series • The equivalent resistance has the effect on the circuit as the original combination of resistors (consequence of conservation of energy) • For more resistors in series: • The equivalent resistance of a series combination of resistors is greater than any of the individual resistors

  7. Resistors in Parallel • The potential difference across each resistor is the same because each is connected directly across the battery terminals • The current, I, that enters a point must be equal to the total current leaving that point (conservation of charge) • The currents are generally not the same

  8. Resistors in Parallel

  9. Resistors in Parallel • For more resistors in parallel: • The inverse of the equivalent resistance of two or more resistors connected in parallel is the algebraic sum of the inverses of the individual resistance • The equivalent is always less than the smallest resistor in the group

  10. Problem-Solving Strategy • Combine all resistors in series • They carry the same current • The potential differences across them are not necessarily the same • The resistors add directly to give the equivalent resistance of the combination: Req = R1 + R2+ …

  11. Problem-Solving Strategy • Combine all resistors in parallel • The potential differences across them are the same • The currents through them are not necessarily the same • The equivalent resistance of a parallel combination is found through reciprocal addition:

  12. Problem-Solving Strategy • A complicated circuit consisting of several resistors and batteries can often be reduced to a simple circuit with only one resistor • Replace resistors in series or in parallel with a single resistor • Sketch the new circuit after these changes have been made • Continue to replace any series or parallel combinations • Continue until one equivalent resistance is found

  13. Problem-Solving Strategy • If the current in or the potential difference across a resistor in the complicated circuit is to be identified, start with the final circuit and gradually work back through the circuits (use formula ΔV = I R and the procedures describe above)

  14. Chapter 18Problem 13 Find the current in the 12-Ω resistor in the figure.

  15. Gustav Kirchhoff 1824 – 1887 Kirchhoff’s Rules • There are ways in which resistors can be connected so that the circuits formed cannot be reduced to a single equivalent resistor • Two rules, called Kirchhoff’s Rules can be used instead: • 1) Junction Rule • 2) Loop Rule

  16. Kirchhoff’s Rules • Junction Rule (A statement of Conservation of Charge): The sum of the currents entering any junction must equal the sum of the currents leaving that junction • Loop Rule (A statement of Conservation of Energy): The sum of the potential differences across all the elements around any closed circuit loop must be zero

  17. Junction Rule I1 = I2 + I3 • Assign symbols and directions to the currents in all branches of the circuit • If a direction is chosen incorrectly, the resulting answer will be negative, but the magnitude will be correct

  18. Loop Rule • When applying the loop rule, choose a direction for transversing the loop • Record voltage drops and rises as they occur • If a resistor is transversed in the direction of the current, the potential across the resistor is – IR • If a resistor is transversed in the direction opposite of the current, the potential across the resistor is +IR

  19. Loop Rule • If a source of emf is transversed in the direction of the emf (from – to +), the change in the electric potential is +ε • If a source of emf is transversed in the direction opposite of the emf (from + to -), the change in the electric potential is – ε

  20. Equations from Kirchhoff’s Rules • Use the junction rule as often as needed, so long as, each time you write an equation, you include in it a current that has not been used in a previous junction rule equation • The number of times the junction rule can be used is one fewer than the number of junction points in the circuit • The loop rule can be used as often as needed so long as a new circuit element (resistor or battery) or a new current appears in each new equation • You need as many independent equations as you have unknowns

  21. Equations from Kirchhoff’s Rules

  22. Problem-Solving Strategy • Draw the circuit diagram and assign labels and symbols to all known and unknown quantities • Assign directions to the currents • Apply the junction rule to any junction in the circuit • Apply the loop rule to as many loops as are needed to solve for the unknowns • Solve the equations simultaneously for the unknown quantities • Check your answers

  23. Chapter 18Problem 23 Using Kirchhoff’s rules, (a) find the current in each resistor shown in the figure and (b) find the potential difference between points c and f.

  24. RC Circuits • If a direct current circuit contains capacitors and resistors, the current will vary with time • When the circuit is completed, the capacitor starts to charge until it reaches its maximum charge (Q = Cε) • Once the capacitor is fully charged, the current in the circuit is zero

  25. Charging Capacitor in an RC Circuit • The charge on the capacitor varies with time q = Q (1 – e -t/RC ) • The time constant,  = RC, represents the time required for the charge to increase from zero to 63.2% of its maximum • In a circuit with a large (small) time constant, the capacitor charges very slowly (quickly) • After t = 10, the capacitor is over 99.99% charged

  26. Discharging Capacitor in an RC Circuit • When a charged capacitor is placed in the circuit, it can be discharged q = Qe -t/RC • The charge decreases exponentially • At t =  = RC, the charge decreases to 0.368 Qmax; i.e., in one time constant, the capacitor loses 63.2% of its initial charge

  27. Chapter 18Problem 54 An emf of 10 V is connected to a series RC circuit consisting of a resistor of 2.0 × 106 Ω and a capacitor of 3.0 μF. Find the time required for the charge on the capacitor to reach 90% of its final value.

  28. Electrical Safety • Electric shock can result in fatal burns • Electric shock can cause the muscles of vital organs (such as the heart) to malfunction • The degree of damage depends on • the magnitude of the current • the length of time it acts • the part of the body through which it passes

  29. Effects of Various Currents • 5 mA or less • Can cause a sensation of shock • Generally little or no damage • 10 mA • Hand muscles contract • May be unable to let go a of live wire • 100 mA • If passes through the body for just a few seconds, can be fatal

  30. Answers to Even Numbered Problems Chapter 18: Problem 2 • 27 Ω • 0.44 A • 3.0 Ω, 1.3 A

  31. Answers to Even Numbered Problems Chapter 18: Problem 6 • 11.7 Ω • 1.00 A in the 12.0-Ω / 8.00-Ω resistors, 2.00 A in the 6.00-Ω / 4.00-Ω resistors, And 3.00 A in the 5.00-Ω resistor

  32. Answers to Even Numbered Problems Chapter 18: Problem 18 • 0.909 A • 1.82 V, point b at the lower potential

  33. Answers to Even Numbered Problems Chapter 18: Problem 22 • 60.0 Ω • 0.20 A • 2.4 W • 0.50 W

  34. Answers to Even Numbered Problems Chapter 18: Problem 36 587 kΩ

  35. Answers to Even Numbered Problems Chapter 18: Problem 38 • 8.0 A • 120 V • 0.80 A • 5.8 × 102 W

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