The Plan

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The Plan - PowerPoint PPT Presentation

The Plan. dielectrics, quickly current &amp; resistance HW3 hints after the break (&amp; in lab) time permitting, some of dc circuits. Exam 1. Right here, 3pm Thursday electric forces &amp; fields electrical potential &amp; energy current &amp; resistance NO RELATIVITY study quick quizzes, example probs

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Presentation Transcript
The Plan
• dielectrics, quickly
• current & resistance
• HW3 hints after the break (& in lab)
• time permitting, some of dc circuits
Exam 1
• Right here, 3pm Thursday
• electric forces & fields
• electrical potential & energy
• current & resistance
• NO RELATIVITY
• study quick quizzes, example probs
• our HW and quizzes
• problems in notes

relax.

it will be ok.

battery = pump

voltage = pressure

current = flow

resistor = constriction

capacitor = diaphragm / flexible reservoir

diode = check valve

with the time left ...

let’s get a jump on dc circuits

series resistors: conservation of energy

Req

R1

R2

a

b

c

a

c

I

I

-

+

-

+

∆V

∆V

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

R2

voltage divider

Vout =

Vin

R1+R2

R1

signal

in

Vin

Vout

R1

signal

out

R2

volume

R2

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

1

1

1

parallel resistors: conservation of charge

=

+

Req R1 R2

∆V1=∆V2=∆V

R2

Req

I1

R1

a

b

I2

I

-

+

-

+

∆V

∆V

current divider

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

more complex arrangements

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

V (open circuit)

Thévenin equivalents

Rth =

I (closed circuit)

Vth = V (open circuit)

any weird combinations of R’s and V’s

is equivalent

to a

SINGLE R and V

(or a single I source

in parallel with R)

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

so what?

V

E

I

R

R

real sources =

ideal sources + R

real meter =

ideal meter with R

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

V

r

I

V source wants R high

one easy solution:

large resistor in parallel with load

source

like a battery

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

r

r+R

r

I

Ip

source

I source wants R low

sourcing currents at high Rload is hard

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

R

I

I

measuring the meter

V

1+R/r

r

Req

V

real

meter

ideal

meter

I

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

summary

voltmeter wants R low!

can use a buffer/follower

I source wants R low

transformer pre-amp

consider sourcing V

V source wants R high

large series + parallel resistors

present large R

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

Sourcing current

This is what a hand meter does.

Why is it no good?

Rwires

I

V

Rthing

Rwires

Vmeter = I(Rthing + 2Rwires)

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

Sourcing current, properly

Rwires

V

I

Rthing

Rthing≪ Rinternal

Rwires

No problem.

You just need four wires.

What is still wrong?

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

Sourcing voltage

Rwires

V

Rthing

Vs

Rwires

Still have to measure voltage on device

the wires still use up some of V

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

Sourcing voltage II

V

Rwires

Vs

Rthing

Rs

Rpar

Ι

Ι

Rwires

=

R = ∆V / I

Note we need 4 wires again

current meter - not hard

still problems?

Rprecise

V

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

source/meter resistances

voltmeter wants R low

but V source wants R high

need buffer/amp on V meter

resistor in parallel with source

if V source is problem, R is too low

consider sourcing I

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

what if I want to measure a *really* high R?

V

Ι

Rwires

Vs

Rthing

Rp

Ι

Rwires

=

=

source voltage

Rp has same voltage as Rthing

Rs has same current

have done >1010 Ohm

Rs≪ Rthing

V

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

what if I want to measure a *really* low R?

Rwires

V

I

Rthing

Rwires

this works just fine ...

so long as your V meter is good

v. good amp / part of a bridge

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

what if I want to measure a small change in R?

R2 =

R1

R3

≈ R3

V

Vs

R2

Rx+dR

balance bridge to V=0

detect small changes from null

trimming resistor on R2 = dR

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center

THE UNIVERSITY OF ALABAMA

CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY

An NSF Science and Engineering Center