physicsa.com
class code: SPS21 teacher: Mr. Elert
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Physics A: Problem Set 9: Electric circuits

recommended reading

High Marks: 3:58–3:60, 3:73–3:76
Barron's Let's Review: 9.7 Series circuits, 9.8 Parallel circuits
physics.info: Resistors in circuits
Wikipedia: Series and parallel circuits
HyperPhysics: Resistance
Khan Academy: Circuits (part 2), Circuits (part 3), Circuits (part 4)
Physics Girl: Circuits, Voltage, Resistance, Current
Mr. Machado: 12 Circuit Diagrams - Ammeter and Voltmeter, 13 Series Circuits, 14 Series Circuits - Sample Problems, 15 Parallel Circuits, 16 Parallel Circuits - Sample Problems, 17 VIRP Tables - Series Circuits, 18 VIRP Tables - Parallel Circuits

in class

  1. Boring series circuit
    Determine the following quantities for the given circuit…
    1. the equivalent resistance
    2. the current from the power supply
    3. the current through each resistor
    4. the voltage drop across each resistor
    5. the power dissipated in each resistor
  2. Boring parallel circuit
    Determine the following quantities for the given circuit…
    1. the equivalent resistance
    2. the current from the power supply
    3. the current through each resistor
    4. the voltage drop across each resistor
    5. the power dissipated in each resistor
  3. Circuit diagram
    A power source and four identical, numbered light bulbs are connected together as shown in the circuit on the right.
    1. Rank the light bulbs in order of brightness.
    2. In a single word, describe the effect that removing any one of the light bulbs has (if any) on the brightness of the other three.
      1. Remove bulb 1
      2. Remove bulb 2
      3. Remove bulb 3
      4. Remove bulb 4

    Compile your answers to this question in a table like the one below.

    R1 R2 R3 R4
    a.        
    b. i. ×      
    b. ii.   ×    
    b. iii.     ×  
    b. iv.       ×

at home

  1. State two reasons why household outlets are not wired together in series.
  2. What is meant by the term "short circuit"? What makes a short circuit dangerous?
  3. What happens to the total current as resistors are added to a circuit…
    1. in series?
    2. in parallel?
  4. A kitchen in North America has three appliances connected to a 120 V circuit with a 15 A circuit breaker: an 850 W coffee maker, a 1200 W microwave oven, and a 900 W toaster.
    1. Draw a schematic diagram of this circuit.
    2. Which of these appliances can be operated simultaneously without tripping the circuit breaker?
  5. Track workers in the New York City subway system don't get their electricity from wall outlets. They use the third rail. Since the voltage of the third rail is different from the voltage of a wall outlet in an American home, they often use modified or specialized electrical devices. For example, the temporary lights the workers use are mounted on strips with five bulbs — never four, never six, always five. This design choice is intentional.

    Photo of subway track work illuminated with five light bulb stripsSource: MTA New York City Transit.

    1. What is the voltage of a typical household outlet in the United States?
    2. What is the voltage of the third rail in the New York City subway system?
    3. Why do the work lights used in the New York City subway come is strips with five light bulbs? Be sure to state the basic wiring pattern (series or parallel) in your answer.