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Review

Review. An electric circuit is composed of what 3 elements? Conductor, potential difference, closed loop. What are the units of voltage? Joules / Coulomb Electric potential energy / unit charge Current? Amperes / second A flow of charged particles Resistance? Ohms. Current.

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Review

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  1. Review • An electric circuit is composed of what 3 elements? • Conductor, potential difference, closed loop • What are the units of voltage? • Joules / Coulomb • Electric potential energy / unit charge • Current? • Amperes / second • A flow of charged particles • Resistance? • Ohms

  2. Current • Electric current is the rate of flow of charge • I = Δq/ Δt • Ampere = Coulomb per second (passing by a point in a circuit)

  3. Ohm’s Law: Ohm's Law …says that, for many materials under a wide range of conditions, the voltage, V, and current, I, are linearly related, which implies resistance, R, is independent of V and I.

  4. Ohm’s Law Mnemonic

  5. Ohm’s Law: Ohm's Law …says that, for many materials under a wide range of conditions, the voltage, V, and current, I, are linearly related, which implies resistance, R, is independent of V and I. When does it not apply? (Most important case) • Changing Temperature

  6. Questions What is the effect on the current through a system if the voltage is doubled? Resistance doubled? Both? A device connected to a 1.5V battery conducts a drives a 45x10-6 A current, while the same device connected to a 3V battery conducts a 90x10-3 A current. Is the device ohmic?

  7. Electrical Power • Power is the rate at which work is done or energy is expended. • Unit is Watt = Joule / second • (James Watt worked on steam engines and launched the industrial revolution) • Power = Work / time • Electrical Energy = Power * Time = IV * t

  8. Electrical Power • P= IV “Poison Ivy”

  9. Electrical Power • P= IV = I2R = V2/R

  10. Electrical Power • Electrical Power is priced in kWh • One kiloWatt = 1000 Watts • One kWh = One kW for one hour

  11. Find Resistance of a material • Depends on four factors • R ∞ Temp • R ∞ RHO (ρ) resitivity • R ∞ Length • R ∞ A-1 (cross-sectional Area)

  12. Low resistance • Short • Fat • cold

  13. High Resistance • Long • Thin • Hot

  14. From Reference tables

  15. Diagramming circuits

  16. Kirchoff’s Junction Rule • Conservation of charge

  17. Kirchoff’s Loop Rule • Conservation of Energy • The sum of the changes in potential around any closed path (loop) of a circuit must be zero. A A V + R1 R2

  18. Series Circuit • Current : One path for the flow of charge • It=I1=I2=I3 • Resistance must increase because length increases • Rt-=R1+R2+R3 • Voltage must add to Zero • Vt-V1-V2-V3=0 Vt=V1+V2+V3

  19. Series and Parallel Circuits • What does it mean to go in series? • Single file • What does it mean to go in parallel? • Multiple paths

  20. Series Circuit: • ammeters are always in the circuit, they measure flow of charge • Ammeters should always have low resistance • Otherwise they will affect the measurement by adding resistance A A V + R1 R2

  21. IT + VT V V Series Circuit Example R1 V1, I1 A A V2, I2 V V R2 V V Given: Vt= 10V R1 = 5W I2 = 1A P3 = 2W R3 V3, I3

  22. IT + VT V V Series Circuit Example R1 V1, I1 A A V2, I2 V V R2 V V Given: Vt= 10V R1 = 5W I2 = 1A P3 = 2W R3 V3, I3

  23. Series and Parallel Circuits L Series: Single Path, high effective resistance a L A Parallel: Multiple Paths, low effective resistance

  24. Parallel Circuit • More than one path for the flow of charge • More room for the flow of charge so Resistance goes down • Voltage must stay the same since all charges have the same drop

  25. Parallel Circuit: • Voltmeters are parallel to the circuit, they measure the potential • Voltmeters should always have high resistance • Otherwise they will allow current to flow through them and reduce effective resistance IT = I1 + I2 + I3 by Conservation of charge (conservation of matter) A IT IT A I2 + I3 I1 R3 R1 R2 V1 V2 V3 V V V V V V IT

  26. Parallel Equations • It = I1 + I2 + I3 + … So • It/Vt = I1/Vt + I2/Vt + I3/Vt … But • Vt = V1 = V2 = V3 =, implying • It/Vt = I1/V1 + I2/V2 + I3/V3 … Or, by Ohm’s law, • 1/Rt = 1/R1 + 1/R2 + 1/R3 + …

  27. Parallel Equations

  28. Parallel Circuit: • Follow the current • If R1 > R2 > R3, what will the relationship between I1, I2, and I3 be? V1, V2, V3? A IT A V2, I2 V3, I3 V1, I1 R3 R1 R2 IT Circuit 1

  29. Parallel Circuit: • Given I1 = 3A, R1= 3A, P2 = 18W, and R3 = 2W, find the other values associated with this circuit. A VT V2, I2 V3, I3 V1, I1 R3 R1 R2 IT

  30. Parallel Circuit: • Given I1 = 3A, R1= 3A, P2 = 18W, and R3 = 2W, find the other values associated with this circuit. A VT V2, I2 V3, I3 V1, I1 R3 R1 R2 IT

  31. Series Circuit Example: Voltage Divider IT A A Given: Vt= 24V Choose values of R1 and R2 to produce 20V across R2 + VT V1, I1 V V R1 V2, I2 V V R2

  32. Series Circuit Example: Voltage Divider IT A A Given: Vt= 24V Choose values of R1 and R2 to produce 20V across R2 + VT V1, I1 V V R1 V2, I2 V V R2

  33. Series / Parallel Circuit: • Given V4=12, I1 = 3A, R2= 2, R3=3, P3 = 18W. Find the other values. V4, I4 A R4 VT V2, I2 V3, I3 V1, I1 R3 R2 R1 IT

  34. Series / Parallel Circuit: • Given V4=12, I1 = 3A, R2= 2, R3=3, P3 = 18W. Find the other values. V4, I4 A R4 VT V2, I2 V3, I3 V1, I1 R3 R2 R1 IT

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