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Circuits

Circuits. It’s what we see…. Simple Circuits. A basic circuit contains 3 parts : A source of electric potential ( voltage ) gives charge an electric potential ex. battery, solar panel. Simple Circuits. A resistance load uses up energ y 3 ways: converts to

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Circuits

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  1. Circuits It’s what we see…

  2. Simple Circuits • A basic circuit contains 3 parts: • A source of electric potential (voltage) • gives charge anelectric potential • ex. battery, solar panel

  3. Simple Circuits • A resistance load • uses up energy • 3 ways: converts to • light (ex. lightbulb) • heat (toaster) • motion (motor)

  4. Simple Circuits • most loads do more than one • ex. lightbulbs also give off heat • ex. toaster wires glow (light)

  5. Simple Circuits • Conductors • provides path for flow between source and load • ex. wire

  6. Resistance • All materials give some resistance to flow of charge • exception. superconductors • Materials with high resistance • insulators • ex. rubber, plastic • Materials with low resistance • conductors • ex. metals

  7. Resistance • Amount of resistance depends on: • material • length of wire • thickness of wire (cross-sectional area) • imagine a straw • easier to blow through: • shorter • thicker

  8. Calculating Resistance • resistance (Ω = Ohm) • length of wire (m) • cross sectional area of wire (m2) • resistivity (depends on material Ω•m) • R • L • A • ρ

  9. Calculating Resistance semiconductor

  10. ExampleHow much resistance is in the graphite of a 20. cm long pencil if the graphite has a diameter of 4.0 mm? Given: (from table) Want:

  11. Examplesolve for R

  12. Current rate of electric flow (of charge) • charge (C) • time (s) • current (A) • q • t • I

  13. Current Flow • Conventional Current is the direction positive charges move • Actual electron flow is the other way!

  14. Ohm’s Law • Current depends on two things: • the potential difference, V • theresistance, R • Together, they make Ohm’s Law:

  15. Ohm’s Law Water Circuit Analogy • Potential Difference • amount of water (pressure)

  16. Ohm’s Law Water Circuit Analogy • Switch • turns flow on and off

  17. Ohm’s Law Water Circuit Analogy • Resistance • thickness & length of hose

  18. Ohm’s Law Water Circuit Analogy • Current • rate of flow

  19. Electric Shocks • 1 mA • 10 mA • 100 mA = pain = release current = death

  20. Inside a Lightbulb • Each wire of filament goes to a different part • This allows for the current to flow through

  21. Power and Energy • Power = rate of energy useage • P = Energy/time • New unit  Watts and so,

  22. Combining Equations twinkle, twinkle…

  23. Power and Energy Energy Equations

  24. Units and Variables coulombs (C) seconds (s) volts (V) ohms (Ω) amperes (A) joules (J) Watts (W)

  25. ExampleIn 3.0 minutes an electric pot delivers 48,000 J of energy to the water inside it. The coffee pot is connected to a standard 120-volt source. What is the resistance of the coffee pot? Given: Want:

  26. Examplepick an equation

  27. Example1 kW-hr costs $.50:Let’s find the cost of running a 60 W lightbulb for 24 hours Given: Problem: what is a kW-hr? Want:

  28. ExampleLet’s take a close look at the kilowatt-hour: • kW = P • hr = t • Pt = E • kW-hr is a unit of energy

  29. Example1 kW-hr costs $.50:Let’s find the cost of running a 60 W lightbulb for 24 hours • Let’s solve for energy

  30. Example1 kW-hr costs $.50:Let’s find the cost of running a 60 W lightbulb for 24 hours

  31. Time to Practicego to pg. 260

  32. Measuring Resistance • Symbol • Isolate from circuit • Place leads on each side • Ohmmeter

  33. Measuring Voltage • Symbol • Voltage source must be connected to circuit • Place leads on each side • Voltmeter

  34. Measuring Current • Need to break open circuit • Force current through meter • ammeter

  35. Electrical Circuits Lab Time for lab! Go to page 264

  36. Series Circuit • Let’s start with Current • In order for all of the current to make it around the loop, they all have to flow at the same rate! • So,

  37. Series Circuit • Resistance • The current is forced to pass through each resistor. • So,

  38. Series Circuit • Voltage • The voltage drops a constant amount • The voltage drops are spread out proportionately between each resistor so they add up to the total • So,

  39. Parallel Circuit • Let’s start with Voltage • Imagine parallel as separate serial circuits connected • Circuit 1 • Circuit 2 • Circuit 3 • Each uses the whole voltage • So,

  40. Parallel Circuit • Current • The total flow is split into each branch-off loop • So,

  41. Parallel Circuit • Resistance • Let’s start with the current • now substitute I=V/R • since VT = V1 = V2 = V3

  42. Parallel Circuit • Resistance • Now divide each side by V

  43. Parallel Circuit • Looks weird you say? • let’s try an example: • Find total resistance if a 100 Ω, 200 Ω and a 300 Ω resistor are all in parallel.

  44. Parallel Circuit • Looks weird you say? • let’s try an example: • Find total resistance if a 100 Ω, 200 Ω and a 300 Ω resistor are all in parallel.

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