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Direct Current Circuits

Direct Current Circuits. Chapter 18. Introduction. Simple Circuits Batteries Resistors Capacitors Kirchhoff’s rules Conservation of energy Conservation of charge Steady-state circuits. Sources of emf.

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Direct Current Circuits

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  1. Direct Current Circuits Chapter 18

  2. Introduction • Simple Circuits • Batteries • Resistors • Capacitors • Kirchhoff’s rules • Conservation of energy • Conservation of charge • Steady-state circuits

  3. Sources of emf • A source of emf (electromotive force) maintains a constant current in a closed circuit. • Generators • Batteries • Neglect internal resistance (r) • Potential drop equals emf • See Figure 18.1 18.1a, 18.1b

  4. Terminal Voltage • Terminal voltage of the battery • When I equals zero, e equals DV

  5. Joule Heat • Power output of the source is converted into Joule heat

  6. Current In Series Circuits • Only one conducting path for the current • Currents through all resistors are identical • Water analogy 18.2

  7. Resistors In Series

  8. Voltages in a Series Circuit • The potential differences across each resistor add up to the terminal voltage of the source

  9. Total Resistance in a Series Circuit • The equivalent resistance of a series circuit is equal to the sum of the individual resistances

  10. The equivalent resistance of a series circuit is always greater than any of the individual resistances

  11. Currents In Parallel Circuits • More than one conducting path for the current • Currents through all resistors are different • Water analogy 18.6

  12. Resistors In Parallel

  13. Voltages In Parallel Circuits • The potential differences across each resistor are equal to the terminal voltage of the source

  14. Resistors In Parallel Circuits • The equivalent resistance of a parallel circuit can be found by using this formula: There is a shortcut! :-)

  15. If you only have two resistors:

  16. The equivalent resistance of a parallel circuit is always less than any of the individual resistances red, 185, Figuring Physics, 182

  17. QUESTIONS 1 – 7 Pg. 617

  18. Activities Series And Parallel Circuits Resistors In Series And Parallel

  19. Kirchhoff’s Rules And Complex DC Circuits • Kirchhoff’s two rules give us a method for solving more complex circuits.

  20. Kirchhoff’s Junction Rule • The sum of the currents entering a junction must equal the sum of the currents leaving the junction. • Charge is conserved 18.9

  21. Kirchhoff’s Loop rule • The sum of the potential differences around any closed-circuit loop must be zero. • Energy is conserved

  22. Using Kirchoff’s Rules to Solve Complex DC Circuits • Two requirements before you start • Assign symbols and directions to all currents. • Choose a direction for going around the loop.

  23. RC Circuits • RC circuits are direct current circuits involving capacitors and resistors.

  24. Charging and Discharging of Capacitors • Equations: Charging Discharging

  25. The RC Time Constant • The time (t) it takes for a capacitor to charge to 63.2 % of its maximum charge • A large capacitance/resistance combination will take longer to charge. 18.16, 18.17, 52

  26. The RC time constant is also thetime it takes for a capacitor to lose 63.2 % of its maximum charge

  27. Household Circuits • Houses are wired in parallel • All electrical devices in the home are connected in parallel. 181, 183

  28. Adding more devices will increase the current flow through the wires. • Fuses and circuit breakers • Both are heat sensitive devices.

  29. An Electric Service Panel

  30. Household Voltages • 120 volts or 240 volts? • Some appliances require 240 volts • Electric ranges • Hot water heaters • Air conditioners • Clothes dryers

  31. Electrical Grounding • Why are there 3 wires entering your home? • + 120 volts (above ground) • - 120 volts (below ground) • Ground wire 53, 189, 190

  32. Electrical Safety • Electric shock can be dangerous! • Burns • Death • Accidents such as falls

  33. Biological Effects of Currents • 5 mA can be felt • 10 mA causes the muscles to contract • 100 mA can be fatal • 1 A can produce serious burns

  34. Safety Devices • Ground wires on plugs • Tools • Appliances • GFCIs • Kitchens • Bathrooms • Basements • Pools and hot tubs

  35. Conduction of Electrical Signals by Neurons • Neurons form a complex electrical network that receives, processes and transmits information from one part of the body to another.

  36. There are about 1010 interconnected neurons in the human body.

  37. Neurons have a “firing threshold”.

  38. Three Types of Neurons • There are three types of neurons: • Sensory neurons • Motor neurons • Interneurons

  39. The Structure of Neurons • Neurons are made up of the cell body, dendrites and an axon.

  40. Transferring Information • Dendrites are the input ends. • The axon provides the output signal. • Axons are active sources of energy.

  41. Neuron Research • Much of what we know about neurons has come from studies involving giant squids.

  42. Conduction of Electrical Signals by Neurons • Sodium ions and potassium ions play key roles in the operation of the nervous system.

  43. Axons Are Like Capacitors • Axons are somewhat like capacitors because they have a dielectric membrane separating different charges.

  44. QUESTIONS 8 – 14 Pg. 617

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