Circuit – Any complete path along

Circuits

Understand the what causes the flow of charge and explain how we

Which of the following bulbs will turn on?

A

B

Circuit

• _{Circuit – Any complete path along which}

Reason for motion

• _{Charges only move in a particular direction if}

there is a voltage.

----

++

++

Motion of Electrons

• _{Recall positive charges are bound in place, so}

Visualizing Current

Current: the rate of flow of electric charge through a particular area.

Direction of Flow

• _{Current is defined as the flow of positive}

charges, which means that…

Cu

rre_nt

Flow o f

electr

Question

• _{You flick a switch to turn on an overhead light.}

Speed of Electrons

• _{Because of the collisions in a wire, electrons}

do not move that quickly through a wire, even if a large voltage is applied.

– _{The speed of electrons in a circuit is called the}

Question

• _{You flick a switch to turn on an overhead light.}

How long does it take for electrons to travel from the switch to the light bulb?

• _{Drift speed: 4 cm / hour}

• _{Distance from switch to lightbulb: 10 m}

Pop Quiz

1. Describe the features necessary for a

working circuit.

2. What causes charges to move? Which

types of charges move in a circuit?

3. In the diagram written on the board,

which direction does the current go?

Which direction the protons?

Devices in a circuit

Understand the basic components that appear in a circuit: wires,

Wires

• _{In ideal circuits, wires are assumed to have}

Resistors

• _{Resistors are objects that oppose the flow of}

electric charge.

• _{Resistors often represent the “load” of a}

Source of Resistance

• _{The source of resistance is that as electrons}

Factors that affect resistance

1st factor

2

factor

3

factor

Factors affecting resistance

As length increases…

As area increases…

Batteries

• _{Basic function is there is chemical reaction the}

forces the electrons out of one side of the

Instructions

• _{Go to Phet “Circuit Construction Kit: DC”}

• _{Build the different circuits and determine the}

patterns for how to predict the voltage and current that would be through resistor

• _{Tip: Complete the following sentences}

– _{When resistors are in a stacked configuration, the}

voltage is…

To the simulation

1. Is there ever a place where the electrons in

DO NOT equal the electrons going out?

2. Why does the resistor warm up?

3. What does negative voltage mean?

Goal: Make the light bulb glow

•

_Materials

–

_{One wire, one battery, one bulb.}

•

_{If it gets hot or you smell smoke,}

Question 1

• _{Draw a picture that shows the correct set-up}

Anatomy of a light bulb

filament

Screw thread contact

Insulator _{Electric foot}

Circuits

– _{A complete path}

• _{Electricity definition of circuit}

Questions to consider

Will the light bulb light if…

– _{An open switch is in the circuit?} – _{A closed switch is in the circuit?}

– _{You reverse the direction of the battery?}

– _{You attach both contacts on the bulb to the}

More Questions to Consider

(+) end to (-) end in the circuit?

• _{What will happen if there are two light bulbs}

Circuits

Components of Simple Circuits

Battery Wire

Recall Ohm’s Law

I = V / R

Practice

• _{Determine the current through the resistor, if}

Nodes

• _{A node is a continuous area in a circuit that is}

at the same voltage.

– _{Basically, it is an area of wire on the circuit.}

• _{The circuit below has two nodes:}

How many nodes?

How many nodes?

How many nodes?

Finally, how many nodes?

Voltage Rule Analogy: Height

Node 1

Rule #1: Voltage Rule

The difference between the voltage of two nodes is constant.

Node 1: V₁

V₁ – V₂ = constant

Charges only move if there is a difference of voltage.

• _{When figuring out the voltage of the nodes,}

you can set one of them to zero and find all other nodes with reference to the zero node.

Example:

• _{If each battery is 2 V, find the voltage drop}

across each resistor.

R_B R_C

Rule #2: Current Rule

Definition: Current entering a node = current going out

Simple example:

Node

I

I

I

Another Example

• _{Three resistors are}

hooked up to a battery.

• _Let

– I₁ = 5 A

– I₂ = 2 A

• Find I₃

9 V

R₃ R₂

I₁ I_{3 I}

2

Fi

n

al

E

xa

m

p

le

• _{Determine the voltage across and}

Analyzing Circuits

Basic Configurations

Objects in series

• _{Objects in series are placed}

in a row, attached end to end.

Objects in parallel

• _{Objects in parallel are}

Equivalent Resistance

• _{No matter the complexity, any network of}

resistors can be analyzed by finding an

Current in series

• _{When objects are in series, the current is the}

same in all the devices.

Current in Series: I

= I

= I

Voltage in Series

Voltage in series: V

= V

+ V

+ V

Equivalent Resistance in Series

yields….

Example

Voltage in Parallel

When objects are in parallel, all objects have

the same voltage.

Voltage in parallel:

V

= V

= V

= V

R1

R₂

R₃

Current in Parallel

When objects are in parallel, theirs currents

will add together.

Current in parallel:

I

= I

+ I

+ I

R1

R₂

R₃

Equivalent Resistance in Parallel

Practice Problem

1. What is the voltage across each resistor?

Final Example

Four identical 5Ω resistors are connected to a

Strategies for Circuits

– _{Look for sets of resistors that are only in parallel or}

only in series and simplify them.

– _{Draw each simplification as a new circuit.} – _{Repeat until there is a single resistor.}

• _{Find the current coming from the battery.}

• _{Use Ohm’s law to figure out Voltage or Current in}

equivalent circuits.

• _{Backtrack through your circuit until all info is}

Po

p

Q

u

iz

• _{For the circuit below, determine the}

current through and the voltage across the 3Ω resistor.

3Ω

9Ω

9 V