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Predicting CURRENT and Voltage

Predicting CURRENT and Voltage. March 7, 2012. Questions. Two bulbs in series; is second one always less bright ? What is an “element”? What does it mean for voltage to be “across” an element? What does it mean for current to be “through” an element?

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Predicting CURRENT and Voltage

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  1. PHYS132 Spring 2012 Predicting CURRENT and Voltage March 7, 2012

  2. PHYS132 Spring 2012 Questions • Two bulbs in series; is second one always less bright? • What is an “element”? • What does it mean for voltage to be “across” an element? • What does it mean for current to be “through” an element? • What is the difference between the “open circuit” and “terminal” voltage?

  3. PHYS132 Spring 2012 Resistance, voltage and current • Definition of resistance: R = V/I • Unit of resistance is Ω, which stands for ohm. • Some objects have a “constant” resistance (Ohm’s law) • Represented by jagged line in circuit diagrams V = IR

  4. PHYS132 Spring 2012 Measuring resistance • The ohmmeter

  5. PHYS132 Spring 2012 Equations • Where does P=I2R come from? • What does “V = E– I r” mean?

  6. PHYS132 Spring 2012 Three “rules” Charge is conserved (#1)Current flowing into a location = current flowing out V=IR (#2)Voltage across object is related to current through it(definition of resistance) Energy is conserved (#3)For every complete path,total voltage provided by batteries = total voltage dissipated by other elements

  7. PHYS132 Spring 2012 N equivalent bulbs in series Charge is conserved (#1)Current must be same through each bulb Energy is conserved (#3)Voltage across each bulb must be 1/N battery voltage V=IR (#2)With 1/N the voltage, the current must likewise be 1/N what it would be with only one bulb

  8. PHYS132 Spring 2012 N equivalent bulbs in parallel Energy is conserved (#3)Voltage across each bulb must be equal to the battery voltage V=IR (#2)With the same voltage, the current must likewise be the same as what it would be with only one bulb Charge is conserved (#1)Total current must be N times what it was with just one bulb

  9. PHYS132 Spring 2012 Equivalent resistance N equivalent bulbs in series • Current is 1/N times what it would be with only one bulb. • The total resistance is N times the resistance of any single bulb N equivalent bulbs in parallel • Current is N times what it would be with only one bulb. • The total resistance is 1/N times the resistance of any single bulb

  10. PHYS132 Spring 2012 Mixed circuit • Order the currents (Itotal, I1, I2, I3) from largest to smallest. • Why?

  11. PHYS132 Spring 2012 Resistors in series and parallel Resistors in series: Total resistance is GREATER than the largest resistance present Resistors in parallel: Total resistance is LESS than the least resistance present What does “total resistance” mean?

  12. PHYS132 Spring 2012 Measuring total resistance

  13. PHYS132 Spring 2012 Examples • Three resistors are in series: 1 Ω, 2 Ω and 3 Ω. What is total resistance? • What if they were placed in parallel?

  14. PHYS132 Spring 2012 Resistors in series and parallel Resistors in series: • Current is the same through each • Voltage across all is sum of voltages across each • The total resistance is the sum of the individual resistances Resistors in parallel: • Voltage across all is same as voltages across each • Total current is sum of currents through each • The total INVERTED resistance is the sum of the individual INVERTED resistances

  15. PHYS132 Spring 2012 Activity #5 Measuring resistance

  16. PHYS132 Spring 2012 For Friday • Read chapter 17 • Submit chapter 17 reading review • Lab report #4 (for activity #5)

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