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Chapter 18

Chapter 18. Direct Current Circuits. Chapter 18. emf Resistors in Circuits Kirchhoff’s Rules RC circuits. Sources of emf. What is emf ? What is the SI units? What is terminal voltage?. Internal Resistance. What is internal resistance? How do I calculate terminal voltage?

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Chapter 18

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  1. Chapter 18 Direct Current Circuits

  2. Chapter 18 • emf • Resistors in Circuits • Kirchhoff’s Rules • RC circuits

  3. Sources of emf • What is emf? • What is the SI units? • What is terminal voltage?

  4. Internal Resistance • What is internal resistance? • How do I calculate terminal voltage? • What about the circuit? • What is an open circuit? • What is a load resistance? • The case for R>>r

  5. Example A battery having an emf of 9 V delivers 117 mA when connected to a 72 ohms load. Determine the internal resistance of the battery.

  6. Resistors in Series • What is a series connection? • What happens to current? • What about voltage? • What about equivalent resistance?

  7. Equivalent Resistance – Series: An Example • Four resistors are replaced with their equivalent resistance

  8. Resistors in Parallel • What is parallel connection? • What happens to current? • What about voltage? • How do I calculate equivalent resistance?

  9. Equivalent Resistance – Parallel Example

  10. Equivalent Resistance – Complex Circuit

  11. Parallel Circuits • What is the advantage?

  12. Gustav Kirchhoff • 1824 – 1887 • Invented spectroscopy with Robert Bunsen • Formulated rules about radiation • Formulated the Kirchhoff’s rules

  13. Kirchhoff’s Rules • Why do you need kirchhoff’s rules?

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

  15. More About the Junction Rule

  16. Setting Up Kirchhoff’s Rules • 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 • When applying the loop rule, choose a direction for transversing the loop • Record voltage drops and rises as they occur

  17. Convention for loop rule

  18. Examples

  19. RC Circuits • What is an RC circuit? • What is a time constant?

  20. Charging Capacitor in an RC Circuit

  21. Discharging Capacitor in an RC Circuit • Charge

  22. Example

  23. 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

  24. 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

  25. Ground Wire • Electrical equipment manufacturers use electrical cords that have a third wire, called a case ground • Prevents shocks

  26. Ground Fault Interrupts (GFI) • Special power outlets • Used in hazardous areas • Designed to protect people from electrical shock • Senses currents (of about 5 mA or greater) leaking to ground • Shuts off the current when above this level

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