Investigation 17F: Compound circuits

Essential questionsHow can voltage, current, and power be used to predict the behavior of electric circuits?
In the first part of this investigation, you will predict the brightness of bulbs in a circuit. Bulb brightness is a measure of the dissipation of power. In the second part, you will explore the properties of a commonly used kind of circuit called a voltage divider.
Part 1: Power dissipated by elements in a compound circuit

Part 1: Power dissipated by elements in a compound circuit
  1. Open the experiment file 17F_CompoundCircuits, and then power-on the Current and Voltage sensors and connect them to your software.
  2. For the circuit at right, predict the relative brightness of the bulbs when the switch is closed and when it is open.
  3. Construct the circuit.
  4. Note the brightness of the bulbs when the switch is open.
  5. Close the switch and note how each bulb’s brightness changes.
  1. Using Kirchhoff’s laws or an equation for power, justify your prediction for the relative brightness of the bulbs when the switch is closed.
  2. Did the brightness of bulb #1 change when the switch was closed? Provide an explanation.
  3. Add a voltage and current sensor to your circuit. Measure the voltage across and current through bulb #1 with the switch open and closed then calculate the power. How do the measurements justify your explanation?
  4. Did the brightness of bulb #2 change when the switch was closed? Provide an explanation.
  5. Measure the voltage across and current through bulb #2 with the switch open and closed. Calculate the power. How do the measurements justify your explanation?
Part 2: Voltage divider

Part 2: Voltage divider
  1. Construct the circuit shown using the resistor values in the table.
  2. Measure the input voltage and the output voltage for each resistor combination in the table.
  3. Record your measurements in the table.
  1. How does the output voltage vary with R1?
  2. How does it vary with R2?
  3. Create a mathematical model to predict the output voltage using the values of the input voltage and the two resistances.
  4. Why is this circuit called a voltage divider?
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