Quiz Preparation

1

Quiz Preparation

Have Quiz sheet ready. Top Center: QUIZ 3 Upper Right:

»Name (L, F, MI), »Today’s date 9/4/12»Lab day , time» section number

Lab Day/Time Section Number

Mon. 1:00 8Mon. 3:00 9Wed. 1:00 10Wed. 3:00 11Fri. 1:00 12

Lab Day/Time Section Number

Mon. 1:00 8Mon. 3:00 9Wed. 1:00 10Wed. 3:00 11Thu. 2:00 29Fri. 1:00 12

Lab Day/Time Section Number

Mon. 1:00 8Mon. 3:00 9Tue. 2:00 36Wed. 1:00 10Wed. 3:00 11Thu. 2:00 35Fri. 1:00 12

2

Announcements

Read Chapter 6 before lab.

Work problems at the end of the Chapter 6 for practice. Solutions on web site.

Be sure to do the Pre-Lab assignment. You will lose in-lab points if you do not have the pre-lab worked when you arrive at lab.

3

Electricity – Water Analogy

Electricity flows through a circuit like water through a pipe. » charge -- # of water molecules in the pipe» current -- water flow rate (gallons per minute)» voltage – pressure (psi = pounds per square

inch)» resistance -- resistance to flow (inverse of pipe

cross-sectional area)» capacitance -- water stored under pressure (e.g.

a bladder tank)

4

Units

charge -- coulombs (C) current -- amperes (A), A=C/s voltage -- volts (V) resistance -- ohms (), =V/A capacitance -- farads (F), F = C/VNote: Farads are huge units -- usually use F(microfarads)

5

Circuit Elements

voltage source (e.g. battery) – like water pump with specified pressure. Units: volts (V)

6

Circuit Elements

resistor -- pipe with small cross-sectional area that impedes water flow. Units: ohms ()

Wire can be viewed as a resistor with very low or zero resistance.

7

Circuit Elements

CAPACITOR – a reservoir or sponge that can store water (charge). Units of capacitance: Farads (F)

8

Circuit Elements

diode -- valve that only allows current flow one direction. (No units)

current

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Quiz

1. Electric current is analogous to:» a. water pressure» b. water flow rate» c. # of water molecules in the pipe

2. Voltage is analogous to: » a. water pressure» b. water flow rate» c. # of water molecules in the pipe

10

Ohm’s Law

The current I flowing through a resistor is proportional to the voltage V across it and inversely proportional to the resistance R:

orI

RV

IRV

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Ohm’s Law - Example

I = 5/100 = 0.05A

I +

5V

-

100 · Label current

directions· Put +/- signs on

voltages

Always:

Passive sign convention: place the positive voltage reference at the same terminal that the current enters

12

Quiz

3. I

+10V

-1000

The current I is(a) 100 A(b) 0.01 A(c) -10,000 A

13

Connection Types

Series (daisy chain)

Parallel (side by side)

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Series Connection

Single branch. Current is the same through all elements.

I

15

Series Connection

Resistors add in series.

Ex: R1=50, R2=100 R=150

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Multiple Voltage Sources

Multiple voltage sources in series add. These circuits are equivalent.

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Wait – I thought they ADDED!

10 + 4 = 6? Have to look at the polarity

1k

10

4

+

+

--

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Parallel Connection

Multiple branches connected at both ends. The voltage is the same across all elements.

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Parallel Connection The reciprocal of resistance adds.

»

Ex: R1=50, R2=100 R=(50)(100)/150=33.33

1 2

1 2

1 1 1 11 1R

R R RR R

For two resistors only = product / sum

20

Series vs. ParallelAre the lights in your house/apartment

wired in series or parallel?

Answer: Parallel!

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Voltage Division

121

22 VRR

RV

• If R1 represents the wire resistance and R2 represents the motor and motor circuit resistance, we want R1 to be as small as possible. This will deliver the maximum voltage to the motor.

(wire)

(motor)

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Voltage Division - Example

20

10

15V

202 15 1010 20

V V

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Resistors in Parallel - Example

What is the current in each branch?

Vo

I1 I2

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Resistors in Parallel - Example

Voltage is the same across each branch– use Ohm’s law:

Vo

I1 I2

0 01 2

1 2

V VI IR R

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Resistors in Parallel - Example

Ohm’s law

Vo

I1 I2

1 215 151.5 0.7510 20

I A I A

201015V

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Series/Parallel Example

What is the current in each branch?

I2I1

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Series/Parallel Example

Combine the two 1kΩ resistors in series.

I2I1

2k

28

Diode Example

What is the current in the circuit (I) and the voltage drop across the resistor (VR)?

IVo

+ VR -

R

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Diode Example

IVo

+ VR -

R VD=0.7V

o D

R

V VI

RV IR

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Diode Example

I9V

+ VR -

1k VD=0.7V

9 0.70.0083 8.3

1000(0.0083)(1000) 8.3R

I A mA

V V

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Breadboards

Used for rapid prototyping » Easier & faster than soldering» Allow for easy alterations

Generally for temporary use Components just plug in

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How a Breadboard Works Connect

components by plugging them in

Components are connected by copper wires underneath holes

33

Connecting Resistors in Series

34

Connecting Resistors in Parallel

35

Quiz

4. Voltage division is used(a) to find the voltage across a single element in a series circuit(b) to find the voltage across a single branch in a parallel circuit(c) to determine the fair allocation of batteries in a team-based design

36

Quiz

5. Current division is used(a) to find the current through a single element in a series circuit(b) to find the current through a single branch in a parallel circuit(c) to determine the fair allocation of batteries in a team-based design