Series and Parallel Resistor Circuits

Learning Intentions

  • To learn how resistors add together in parallel and in series
  • To learn how to calculate the voltage, current, and resistance of parallel and series circuits


Demonstration and Notes

  1. Recall Ohm's Law: V = IR (voltage in Volts, V, is equal to current in Amps, A, times resistance in Ohms, Ω)
  2. Take a clean straw and scissors from the front of the class.
  3. The air from your breath pushing through a straw is like electric current in a resistor. The pressure developed by your lungs is like the voltage created by a battery.
  4. Breathe through the straw. How hard is it to breathe?
  5. Cut the straw in half. We have now effectively halved the "resistance" of the straw to the "current" of your breath. How hard is it to breathe through one of the half-length straws?
  6. Try breathing through the two half-length straws. This is essentially a parallel circuit with 2 straws. What do you notice?
  7. Cut the two halves in half, resulting in 4 straws. How hard is it to breathe through a single quarter-length straw?
  8. Put two of the quarter-length straws in your mouth, and breathe. How hard is it?
  9. Put a third straw in your mouth and breathe. How hard is it?
  10. Put all four straws in your mouth and breathe. How hard is it?
  11. TODO: Add stuff with resistors and breadboards and ohmmeter here.
  12. When analyzing electronic circuits, multiple resistors can be combined into a single "equivalent" resistor.
    1. What is the rule for finding the equivalent resistance when resistors are in series? A single long straw can be through of as 4 quarter-length straws connected in series.
    2. What is the rule for finding the equivalent resistance when resistors are in parallel?
  13. Any components that are in series will have the same _______________, but will probably have different _______________.
  14. Any components that are in parallel will have the same _______________, but will probably have different _______________.


Questions

  1. A resistor with a resistance of 20 Ω is hooked up to a battery. There is a current of 0.15 A through the resistor. What voltage is the battery?
  2. A 12V truck battery pushes a current of 1500 A through the starter. What is the resistance of the starter?
  3. A 120V circuit powers a 30 Ω incandescent light bulb. What is the current through the light bulb?
  4. If the maximum safe current through a 150 Ω resistor is 0.25 A, what is the largest battery the resistor can be safely connected to?
  5. Draw the following circuit. What is the equivalent resistance of the resistors?
    1. A 200 Ω resistor and a 400 Ω resistor in series
    2. A 200 Ω resistor and a 400 Ω resistor in parallel
    3. A 200 Ω resistor, 400 Ω resistor, and a 800 Ω resistor in series
    4. A 200 Ω resistor, 400 Ω resistor, and a 800 Ω resistor in parallel
    5. A 200 Ω resistor and a 400 Ω resistor in series, both in parallel with a 600 Ω resistor
  6. A 150 Ω resistor is placed in series with a 250 Ω resistor, and connected to a 3.7V Lithium ion battery.
    1. Draw the circuit schematic.
    2. What is the combined resistance of the two resistors?
    3. What is the current through each resistor?
    4. What is the current through the battery?
  7. A 150 Ω resistor is placed in parallel with a 250 Ω resistor, and connected to a 3.7V Lithium ion battery.
    1. Draw the circuit schematic.
    2. What is the combined resistance of the two resistors?
    3. What is the current through each resistor?
    4. What is the current through the battery?
  8. Extending: A 150 Ω resistor is placed in parallel with a 250 Ω resistor. The two resistors are then placed in series with a 100 Ω resistor and a 5V USB battery charger.
    1. Draw the circuit schematic.
    2. What is the combined resistance of the three resistors?
    3. What is the current through each resistor?
    4. What is the current through the battery?