20.2 Resistors in Series or Resistors in Parallelpp 736- 745

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?

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.

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

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

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.

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.

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.

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

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.

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.

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 in PARALLEL.

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.

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.

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.

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

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.

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.

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.

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.

AnswersSeries Parallel

The current through each resistor is the same

The voltage through each resistor is the same.

As resistors are added the total resistance increases.

As resistors are added, the total resistance decreases.

As resistors are added, the current in the circuit decreases

As resistors are added, the current in the circuit increases.

If a device fails, current will cease to flow to other devices.

If a device fails, current will continue to flow to other devices.

A flashlight is a good example of this type of circuit.

Household wiring tends to be on this type of circuit.

If you have a 6V battery connected to a 2Ω resistor and a 3Ω resistor, the total current running through the circuit is 1.2 amps.

If you have a 6V battery connected to a 2Ω resistor and a 3Ω resistor, the total current running through the circuit is 5 amps.

Homework

Due Wed: p739 #1-4 Due Thurs: p745 #1-6