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Circuits

Cutnell & Johnson, Wiley Publishing , Physics 5 th Ed. Cutnell & Johnson, Wiley Publishing , Physics 5 th Ed. Copywrited by Holt, Rinehart, & Winston. Circuits. Any path along which electrons can flow is a circuit .

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Circuits

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  1. Cutnell & Johnson, Wiley Publishing, Physics 5th Ed. Cutnell & Johnson, Wiley Publishing, Physics 5th Ed. Copywrited by Holt, Rinehart, & Winston Circuits • Any path along which electrons can flow is a circuit. • For continuous flow to occur, there must be a complete circuit with NO GAPS! • Diagram that represents the arrangement of electrical components.

  2. Copywrited by Holt, Rinehart, & Winston Circuit Schematics

  3. Copywrited by Holt, Rinehart, & Winston Copywrited by Holt, Rinehart, & Winston Series and Parallel Circuits Most electric circuits have multiple devices that receive electrical energy. These devices can be connected in two ways. Series Parallel

  4. Copywrited by Holt, Rinehart, & Winston Series Circuits • In a series circuit, bulbs (or any other load) are wired together one after the other. • Any break in the path means the flow of electrons stops and the bulbs won’t light. This could be caused by: • Burning out of one lamp filament • Opening a switch

  5. http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l4c.htmlhttp://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l4c.html Copywrited by Holt, Rinehart, & Winston Series Characteristics • There is only one pathway for electron flow between the terminals of the battery.

  6. Copywrited by Holt, Rinehart, & Winston Series Characteristics • CURRENT is the SAME through each device!

  7. Series Characteristics • RESISTANCEadds up because resistance is encountered as the current flows through each device in the circuit. Copywrited by Holt, Rinehart, & Winston

  8. http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l4c.htmlhttp://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l4c.html Resistance Adds Up • For example: Each lamp filament is like a gravel filled section of pipe. • Now the current has three times as much gravel to flow through. Three times the resistance! • There is an inverse relationship between resistance and flow of current, so tripling the resistance means that only 1/3 of the current can flow

  9. Copywrited by Holt, Rinehart, & Winston Series Characteristics • The current can be calculated using Ohm’s Law when voltage and resistance are known.

  10. Copywrited by Holt, Rinehart, & Winston http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l4c.html Series Characteristics • VOLTAGE • across each device depends directly on the resistance of each device • voltage is therefore shared between devices • Voltage across each resistoradds upto be the total voltage of the battery.

  11. Series Circuits Disadvantages to series circuits: • If one “bulb” goes, the whole circuit goes out. • As bulbs are added, the light gets dimmer.

  12. Current is the same throughout the circuit. Resistance adds to make the total for the circuit. Voltage adds to make the total for the circuit. Itotal = I1=I2=I3 Resistance adds: RTotal = R1 + R2 + R3 (RTotal is sometimes called equivalent resistance) Voltage adds: VTotal = V1 + V2 + V3 Series Characteristics See table on back of notes

  13. http://courses.science.fau.edu/~rjordan/rev_notes/images/25.2.gifhttp://courses.science.fau.edu/~rjordan/rev_notes/images/25.2.gif Series Example Animation

  14. Copywrited by Holt, Rinehart, & Winston Parallel Circuits • Each lamp must be connected directly with the battery with no other bulbs between. • In a parallel circuit, bulbs (or any other load) are wired together so that they form branches, each with a separate path for electrons to flow.

  15. Copywrited by Holt, Rinehart, & Winston http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l4c.html Parallel Characteristics • There are multiple pathways for electron flow between the terminals of the battery.

  16. Copywrited by Holt, Rinehart, & Winston Parallel Characteristics • VOLTAGE is the SAME throughout the circuit because each device is connected directly to the battery.

  17. Copywrited by Holt, Rinehart, & Winston http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l4c.html Parallel Characteristics • The CURRENT of the circuit must divide between each of the branches.

  18. Copywrited by Holt, Rinehart, & Winston http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l4c.html Parallel Characteristics • The total CURRENT equals the sum of the current of the branches! IT = I1 + I2 + I3

  19. Copywrited by Holt, Rinehart, & Winston Parallel Characteristics • As the number of branchesINCREASES, the overall resistance of the circuit DECREASES. • This means that the overall resistance of the circuit is less thanthe resistance of any one of the branches.

  20. http://www.rockwood.k12.mo.us/greenpines/5thgrade/Subjects/Science/electricity/circui1.jpghttp://www.rockwood.k12.mo.us/greenpines/5thgrade/Subjects/Science/electricity/circui1.jpg Copywrited by Holt, Rinehart, & Winston Parallel Advantages • When one bulb goes out the others still receive current and light. • As more bulbs are added, they bulbs don’t dim.

  21. Copywrited by Holt, Rinehart, & Winston Copywrited by Holt, Rinehart, & Winston Christmas Lights What will happen to a string of Christmas light if a light burns out and it is wired in series?Parallel?

  22. Summary of Circuit Characteristics • Series: • Currentstays the same! • Resistance adds: RTotal = R1 + R2 + R3 • Voltage adds: • VTotal = V1 + V2 + V3 • Parallel: • CurrentADDS! • IT = I1 + I2 + I3 • Resistance is found by: • Voltage stays the same!

  23. Series Parallel Series vs Parallel

  24. Summary

  25. Cutnell & Johnson, Wiley Publishing, Physics 5th Ed. Voltmeter • Connected in parallel • Contains large internal resistance • the voltmeter is assumed to have infinite resistance (a modern digital voltmeter has a resistance of around 107Ω).

  26. Cutnell & Johnson, Wiley Publishing, Physics 5th Ed. Ammeter • Connected in series • Low internal resistance

  27. DC vs AC Electric current can be either DC or AC. Direct Current (dc) - charges move around a circuit in the same direction at all times. • (Produced by batteries) Alternating Current (ac) - charges move first one way and then the opposite way • (Produced by generators)

  28. Direct Current Direct Current (DC) is current that flows in one direction only. • Batteries create electric current because the terminals of the battery are always of the same sign. •  electrons always flow from the negative pole (from which they are repelled) to the positive pole (which attracts them).

  29. Cutnell & Johnson, Wiley Publishing, Physics 5th Ed. Alternating Current Electrons first flow in one direction and then in the opposite direction.

  30. What type of current flows through your house? • AC • DC Most circuits in the U.S. use Alternate Current that cycles back and forth at a frequency of 60 cycles per second (60 hertz).

  31. Power Transmission • Most U.S. voltages are 120 V. It used to be 110-V because early light bulbs couldn’t handle higher voltages. • Most other countries have switched to 220 V, which transmits power more efficiently.

  32. Power Generated at Power Plant Power stepped up in voltage and sent to high voltage transmission lines Power stepped down in voltage through transformers prior to sending it to your home. Cutnell & Johnson, Wiley Publishing, Physics 5th Ed. Power Transmission

  33. Cutnell & Johnson, Wiley Publishing, Physics 5th Ed. Wiring Your Homes • Electricity is fed into your home through two lead wires called lines. • The lines have very low resistance and are connected to wall outlets in each room. • As more devices are added, more pathways are provided for the current.

  34. Cutnell & Johnson, Wiley Publishing, Physics 5th Ed. Cutnell & Johnson, Wiley Publishing, Physics 5th Ed. Added Load What do these additional paths do to resistance? • They lower the resistance, allowing a greateramount of current to flow! • Lines that carry more than the safe amount of current are said to be “overloaded”. • Overloaded circuits can result in heatthat is sufficient to melt the insulation and start a fire!

  35. Example, a toaster, a lamp, and a heater, wired in parallel. How does the circuit overload and what safety mechanism can prevent this from happening? • If only the toaster is running and drawing 8 amps, the total line current is 8 amps. • If both the toaster and heater are running, they draw a total of 18 amps. • When all three are running, the total current running through the circuit is 20 amps.

  36. Cutnell & Johnson, Wiley Publishing, Physics 5th Ed. Protective Devices Most circuits have safety mechanisms that determine the maximum amount of current that can pass through a line. • To prevent overloading in circuits, FUSES are connected in series along a supply line. Why in series? • So the entire line current must pass through the supply line. • Fuses are constructed with a wire ribbon that will heat up and melt at a given current. • 20-amp fuse will pass 20-amps of current (but NO MORE) • Currents above 20-amps melt the fuse, thereby breaking the circuit.

  37. Cutnell & Johnson, Wiley Publishing, Physics 5th Ed. http://www.gcsescience.com/a/Circuit-Breaker.gif Circuit Breakers • Circuit breakers are essentially an update on fuses. • Use magnetsto open the switch. • Used widely because they don’t have to be replaced each time the circuit it opened

  38. http://www.fei.vsb.cz/kat453/www453/soubory/mfp_lab/cz/ukazky/anim/ac_breaker.gifhttp://www.fei.vsb.cz/kat453/www453/soubory/mfp_lab/cz/ukazky/anim/ac_breaker.gif http://www.fei.vsb.cz/kat453/www453/soubory/mfp_lab/cz/ukazky/anim/mccb.gif

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