Parts of An Electric Circuit

• Recall: a circuit is a closed path• Electric circuit: closed path that

flowing charge follows• Constructing an electric circuit:• Three key components needed:• “Source”: a source of voltage or

current• Component: a device that

requires electrical energy• Connectors: something to

connect source to component

Images obtained from: http://topdiysolarpanels.com/images/Battery.jpg, http://img6a.flixcart.com/image/coffee-maker/g/9/f/philips-hd7450-hd-7450-400x400-imadbgg73hcavgem.jpeg, http://upload.wikimedia.org/wikipedia/commons/e/e2/Jumper_Wires_with_Crocodile_Clips.jpg

Circuit Diagrams

• Circuit diagram: standardized method of illustrating the parts of a circuit• Components, sources have

specific symbols• Many components– what we’ll

use is the tip of the iceberg

Image obtained from: http://4.bp.blogspot.com/-ECAHR2CPhuk/VCIN8cSmS6I/AAAAAAAAABA/83QAT5nuZAI/s1600/100W%2BLM3886%2BParallel%2BStereo%2BPower%2BAmplifier.png

Circuit Sources

• Battery• Provides potential difference for

circuit• Electrons will flow from high

voltage to low voltage in circuit

Positive end

Negative end

+

-

Circuit Components

• Switch• Controls path of current

within circuit• On: ends of switch

connected, closes circuit• Off: ends of switch not

connected, circuit open

Circuit Components

• Resistor• Creates resistance in circuit• Serve to reduce amount of

voltage remaining in circuit• Causes energy to be released

from it– often thermal energy• Example: incandescent light

bulb

Circuit Components

• Rheostat/Potentiometer• Variable resistor: can change

its resistance

As You Come In…

• Find the voltage drop in this part of a circuit:

5.0 Ω

0.15 A

Circuit Components

• Capacitor• Stores charge in the circuit• Acts like temporary battery• Builds up charge when

connected to source until full• Discharges charge when

disconnected from source until empty

Circuit Components

• Inductor• Resists changes in current• If connected to source,

keeps current from flowing for a while• If disconnected from

source, keeps current flowing for a while (how???)

Circuit Components

• Ammeter• Measures current running

through part of circuit

A

Circuit Components

• Voltmeter• Measures voltage running

through part of circuit

V

Circuit Components

• Generic Device• Appliance or general

electrical device that is part of circuit Name

Circuit Connectors

• Conductor/wire• Connects sources, components• Assumed to have negligible

resistance• Junctions• Sometimes connectors cross

paths or intersect• Node: conductors connect• No node: conductors do not

connect

Node

No node

Circuit Connectors

• Ground• Connects circuit to “ground”• “Ground” has electrical potential

of 0 V• Prevents short circuits– more on

those later!

Circuit Diagrams: Determining Current• Given the following circuit diagram:• Want to know I• Magnitude of I is simple: R = V / I I = V / R• What about direction?Electron flow notation: electrons flow from (-) to (+) of a voltage sourceCurrent flowing CCW

3.0

Ω

1.5 V

⨡⨡

-+

I = 0.50 A

= 0.50 A

Circuit Diagrams: Series Circuits

• Given the following circuit diagram:• Resistors are in a series– one after

another along one path• Called a “series circuit”

R 2 = 3

.0 Ω

1.5 VR1 = 5.0 Ω

R3 = 1.0 Ω

Circuit Diagrams: Series Circuits

• What do we know about circuit?• Only one path for electrons to

flow• Current through each resistor

must be the same• I1 = I2 = I3

• Voltage drop by end of path must equal voltage of source• V1 + V2 + V3 = Vtotal

R 2 = 3

.0 Ω

1.5 VR1 = 5.0 Ω

R3 = 1.0 Ω

Circuit Diagrams: Series Circuits

• Resistors in Series:• I1 = I2 = I3

• V1 + V2 + V3 = Vtotal

• Since all the currents are the same, can rewrite above as:

• Because R = , this simplifies to R1 + R2 + R3 = Rtotal

R 2 = 3

.0 Ω

1.5 VR1 = 5.0 Ω

R3 = 1.0 Ω

= I

Circuit Diagrams: Series Circuits

• Resistors in Series:• I1 = I2 = I3

• V1 + V2 + V3 = Vtotal

• Since all the currents are the same, can rewrite above as:

• Because R = , this simplifies to R1 + R2 + R3 = Rtotal

R tota

l = 9

.0 Ω

1.5 V

Equivalent Circuit

= I

Circuit Diagrams: Series Circuits

• Resistors in Series:• I1 = I2 = I3

• V1 + V2 + V3 = Vtotal

• Since all the currents are the same, can rewrite above as:

• Because R = , this simplifies to R1 + R2 + R3 = Rtotal

R tota

l = 9

.0 Ω

1.5 V

Equivalent Circuit

= I

Practice Problem: Series Circuit

• Given the following circuit diagram, what is the resistance of R2?• R1 + R2 = Rtotal

• Rtotal = • 1.5 Ω + R2 = 2.0 Ω So R2 = 0.5 ΩR 2 =

?6.0 V

R1 = 1.5 Ω

3.0 A

= = 2.0 Ω

About Series Circuits

• Advantages:• Easy to set up (cheap)• Batteries in series: voltages additive,

increases current• Less connectors needed

• Disadvantages:• Voltage divided between components–

more components, less voltage for each• One path for current– if one component

fails, circuit fails• Resistance increases– decreases current

within circuit

R 2 = 3

.0 Ω

1.5 VR1 = 5.0 Ω

R3 = 1.0 Ω

Circuit Diagrams: Parallel Circuits

• Given the following circuit diagram:• Resistors along multiple, different,

parallel paths• Called a “parallel circuit”

1.5 V

R1 = 5.0 Ω

R2 = 3.0 Ω

R3 = 1.0 Ω

Circuit Diagrams: Parallel Circuits

• What do we know about circuit?• Multiple paths for e- to flow• Total current of circuit equal to

current through each resistor• I1 + I2 + I3 = Itotal

• Voltage drop the same across each resistor– equals voltage of source• V1 = V2 = V3 = Vtotal

1.5 V

R1 = 5.0 Ω

R2 = 3.0 Ω

R3 = 1.0 Ω

Circuit Diagrams: Parallel Circuits

• Resistors in Parallel:• V1 = V2 = V3

• I1 + I2 + I3 = Itotal

• Since all the voltages are thesame, can rewrite above as:

• Since R = , that means = ; thus,

1.5 V

R1 = 5.0 Ω

R2 = 3.0 Ω

R3 = 1.0 Ω

= V

Circuit Diagrams: Parallel Circuits

• Resistors in Parallel:• V1 = V2 = V3

• I1 + I2 + I3 = Itotal

• Since all the voltages are thesame, can rewrite above as:

• Since R = , that means = ; thus,

1.5 V Rtotal = 1.8 Ω

Equivalent Circuit

= V

Practice Problem: Parallel Circuit

6.0 V

R1 = 15 Ω

R2 = 5.0 Ω

R3 = 7.5 Ω

• Given the following circuit diagram, what would be the reading on the ammeter?

• 0.067 + 0.20 + 0.13 = = 0.40 , therefore R = 2.5 Ω• I = V / Rtotal

???

= 6.0 V / 2.5 Ω = 2.4 A

About Parallel Circuits

1.5 V

R1 = 5.0 Ω

R2 = 3.0 Ω

R3 = 1.0 Ω

• Advantages:• Voltage the same across each component• Total resistance decreases compared to

each component’s resistance• Batteries in parallel make batteries last

longer• Multiple paths for current– can be

redirected if one part of circuit fails

• Disadvantages:• More connectors needed• Batteries in parallel do not add to the

voltage of the circuit

Circuit Diagrams: More About Series Circuits

• Capacitors in Series:• Recall: V1 + V2 + V3 = Vtotal for series

circuit• Capacitors in series act like one big

capacitor

C 2 = 0

.8 F

1.5 VC1 = 1.2 F

C3 = 0.4 F

Circuit Diagrams: More About Series Circuits

• Capacitors in Series:• Recall: V1 + V2 + V3 = Vtotal for series

circuit• Capacitors in series act like one big

capacitor– one amount of charge (Q)

• Since C = , that means = ; thus,

C tota

l = ?

??1.5 V

Equivalent Circuit

Circuit Diagrams: More About Series Circuits

• Capacitors in Series:• Recall: V1 + V2 + V3 = Vtotal for series

circuit• Capacitors in series act like one big

capacitor– one amount of charge (Q)

• Since C = , that means = ; thus,

C tota

l = 0

.2 F

1.5 V

Equivalent Circuit

Circuit Diagrams: More About Parallel Circuits• Capacitors in Parallel:• Recall: V1 = V2 = V3 = V for parallel

circuit• Capacitors in parallel are

independent of one another– each contain their own charge• Q1 + Q2 + Q3 = Qtotal

• Because C = , this simplifies to• C1 + C2 + C3 = Ctotal

1.5 V

C1 = 1.2 F

C2 = 0.8 F

C3 = 0.4 F

Circuit Diagrams: More About Parallel Circuits• Capacitors in Parallel:• Recall: V1 = V2 = V3 = V for parallel

circuit• Capacitors in parallel are

independent of one another– each contain their own charge• Q1 + Q2 + Q3 = Qtotal

• Because C = , this simplifies to• C1 + C2 + C3 = Ctotal

1.5 V Ctotal = 2.4 F

Equivalent Circuit

Circuits in Both Series and Parallel

• Many circuits utilize both series and parallel circuit properties within a circuit• Case in point:• How do you find the

equivalent resistance of this circuit?• Recommend: drawing

equivalent circuits

Equivalent Circuits

• Strategy: equivalent circuits• Pick out parts that are

exclusively in series or in parallel• Simplify that part of circuit• Repeat as needed until only

one equivalent circuit component remains• Question: What part should

be simplified first for this problem?

Equivalent Circuits

• Step 1: Parallel Circuit

• R = = 0.5

Equivalent Circuits

• Step 1: Parallel Circuit

• R = = 0.5 • So what’s the next step?

Equivalent Circuits

• Step 2: Series Circuit• R1 + Reqv + R5 = Rtotal

• 1.0 Ω + 0.5 Ω + 3.5 Ω = Rtotal

• 5.0 Ω = Rtotal

Equivalent Circuits

• Step 2: Series Circuit• R1 + Reqv + R5 = Rtotal

• 1.0 Ω + 0.5 Ω + 3.5 Ω = Rtotal

• 5.0 Ω = Rtotal

Equivalent Circuits

• Try this one on your own!

Step 1Step 2

Step 3

As You Come In…

• How should a voltmeter be inserted into a circuit?

• How should an ammeter be inserted into a circuit?

VParallel: V equal for both branches

Very high R so very little current comes through

As You Come In…

• How should a voltmeter be inserted into a circuit?

• How should an ammeter be inserted into a circuit?

VParallel: V equal for both branches

Very high R so very little current comes through

ASeries: I equal since along same path

Very small R so very little voltage lost to device