20.2 Resistors in Series or Resistors in Parallel

1. 20.2 Resistors in Series or Resistors in Parallel pp 736- 745

2. Essential Questions • Based on the orientation of resistors in a circuit, how is the distribution of current and voltage affected? • What are the advantages or disadvantages of arranging resistors in certain ways?

3. Objective(s): Students will be able to… • Calculate the equivalent resistance for a circuit of resistors in series, and find the current in and potential difference across each resistor in the circuit. • Calculate the equivalent resistance for a circuit of resistors in parallel, and find the current in and potential difference across each resistor in the circuit.

4. Agenda: • The plan through May 31. • Check and review HW from 20.1 p 735 # 1-5 • Introduction to series and parallel circuits. • Notes: • Resistors in Series • Resistors in Parallel

5. The Plan • Final is Thursday, May 31. We have 10 class days before then, including today. • 5/16-5/17: 20.2 Resistors in Series and Parallel. • 5/18: 20.3 Complex resistor combinations • 5/21: Lab • 5/22: Chapter 20 Review/ Lab Worksheet • 5/23: Chapter 20 Test • 5/25, 5/26, 5/29, 5/30: Review for Final Exam

6. Resistors in Series • When there exists a circuit or portion of a circuit that provides a single conducting path without junctions, that circuit or portion of a circuit is connected in series.

7. Current in Series • Think of current like cars (or sheep if you’d prefer) on a track. • If the cars are bumper-to-bumper (like electrons), the current can’t build up or lessen anywhere in the track. • This means the current is the same everywhere in the circuit (track). • In other words, the current flowing through each resistor in a series is equal.

8. Equivalent Resistance in Series • It is easier to consider multiple resistors as an equivalent single resistor. • Each resistor in a series acts like a dam for the current, so the equivalent resistance is the sum of the individual resistances. • Note: the equivalent resistance in series is always larger than any individual resistance.

9. Calculations in Series • When the equivalent resistance is known, current and voltage can be calculated as if the entire circuit contains just one resistor. • I = ΔV/Req • You try: A 9.0 V battery is connect to four light bulbs in series with resistances of 4.0, 7.0, 2.0, and 5.0 Ω respectively. What is the current in the circuit? • I = 0.50 A

10. Voltage Drop • Each resistor in a circuit uses some of the voltage (potential difference) supplied by the power source. • The amount of potential difference across each resistor is called the voltage drop. • To calculate the voltage drop, use ΔV=IR. • The voltage used by each resistor is equal to the current running through it multiplied by its individual resistance.

11. Series Circuits • Because series circuits allow only one pathway for current, no current flows if an element breaks. Think of cheap seasonal lights. • Advantages to series circuits: • Each element helps to control current, to avoid overheating. • Some circuits succeed if they fail. Like burglar alarms.

12. Resistors in Parallel • When two or more elements in a circuit are connected across common points or junctions, providing multiple pathways for current, these elements are inparallel.

13. Voltage Drop in Parallel • Parallel circuits behave such that each element is directly connected to the incoming power source. • It is almost like each element is in its own circuit. • Thus, the voltage drop across each branch of a parallel circuit is the same.

14. Current in a Parallel Circuit • Because each branch acts like its own circuit… • The total current drawn in a parallel circuit is the sum of the current in each of the branches.

15. Equivalent Resistance in Parallel • Each element in parallel adds another pathway for current to flow. Like adding another lane for traffic. • As a result, when a resistor is added to a parallel circuit, overall resistance DECREASES! • The equivalent resistance is always smaller than any individual resistance.

16. Calculations in Parallel • Try the same elements as the previous example, but now arranged in parallel instead of in series. • A 9.0 V battery is connected to four light bulbs in parallel with resistances of 4.0, 7.0, 2.0, and 5.0 Ω respectively. What is the current in the circuit? • I = 9.8 A

17. Parallel Circuits • Each branch of a parallel circuit provides a separate path for current, so if one branch fails, the rest can keep going. • Think not-cheap seasonal lights. • Household wiring is on parallel circuits because: • We want to be able to turn things on an off independently. • We have a standard input voltage 120V to every device, and can design those devices accordingly.

18. Recap: Series • In series circuits: • The equivalent resistance is equal to the sum of the individual resistances. • Resistance increases and current decreases as elements are added. • Circuits fail if one element fails.

19. Recap: Parallel • In parallel circuits: • The reciprocal of the equivalent resistance is equal to the sum of the reciprocals of the individual resistances. • Resistance decreases and current increases as elements are added. • The rest of the circuit still conducts electricity if one device fails.

20. Recap Activity • Clear your desks. • Organize the slips of paper into the 2 columns in which they belong: series or parallel. • You have 5 minutes.

21. Answers

22. Homework • Due Wed: p739 #1-4 • Due Thurs: p745 #1-6