Physics 212 Lecture 9, Slide 1

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Physics 212 Lecture 9
Today's Concept Electric Current Ohms Law
resistors Resistors in circuits Power in circuits
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Your Thoughts
I'm fine. I think. Therefore I am. Fine.
More about what each variable means and how it is
derived.
Anything to help me get more familiar with
resistors.
I'm confused about v-drift. I know it has to do
with the electrons moving around, but a little
clarification on this topic would definitely help
)
If a car is moving faster than the speed of
light, what happens when you turn on the
headlights?
I would like to see how length and width of a
resistor effect it's resistance.
The prelecture lost me when it started talking
about sigma and roe.
I think that it would help me conceptualize
circuits better if we went over hydraulic analogy
of electricity. Where current of charge and water
are compared, this is particularly helpful to
understand resistance in my opinion. Or maybe
work it into a demo or something that would be
cool.
Hmmm... I would like to talk more on the
production of cheese. Did you know that the first
cheese was made by a traveler who put milk in his
leather flask, rode for several days with it, so
that it was evenly mixed, and POOF! Cheese! The
power...
I would like to discuss why an EMP wave will take
out all electronic devices yet the Terminators
and SKYNET were completely unaffected in
Terminator 3.
A platypus is a small, aquatic, egg-laying
monotreme, Ornithorhynchus anatinus, of Australia
and Tasmania, having webbed feet, a tail like
that of a beaver, a sensitive bill resembling
that of a duck, and, in adult males,
venom-injecting spurs on the ankles of the hind
limbs, used primarily for fighting with other
males during the breeding season.
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How confident are you in your understanding of
the concepts presented in the prelecture? A  I
am confused by all of it. B  I understand a
little but I am confused by most of it. C  I
understand some parts and I am confused by other
parts. D  I understand most of itE  I
understand everything F I didn't view the whole
prelecture so I can't comment.
4
Thanks for your feedback about the course
Based on these, we will try the following
today 1) Summary of the important concepts to
learn in each lecture 2) Make sure we have time
to work out the problem all the way
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Key Concepts

• How resistance depends on A, L, s, r
• How to combine resistors in series and parallel
• Understanding resistors in circuits

Todays Plan

• Review of resistance preflights
• Work out a circuit problem in detail
• Optional Comments on drift velocity average
  velocity
• - How do these charges move anyway?

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I
A
s
L
V
J s E
same as
I V/R
L
R
where
sA
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R
I
V
I V/R
Ohms Law
Aside Q What is a battery ? A A device that
produces a potential difference V between
its two terminals !
(aka EMF)
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This is just like plumbing! I is
like flow rate of water V is like pressure R is
how hard it is for water to flow in a pipe
To make R big, make L long or A small
To make R small, make L short or A big
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A B C
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Preflight 2
VIR and R2 has lower resistance because of its
larger area.
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Preflight 4
Since I is the same in both, R is proportional to
V. Since R2 is bigger, V2 must also be bigger.
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Preflight 6
Current is always the same in series since it has
only one path to travel.
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Preflight 8
R2 is a third of the total resistance, therefore
V2 will be a third of the total voltage E.
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Resistor Summary
Series
Parallel
R1
R2
R1
R2
Each resistor on the same wire.
Each resistor on a different wire.
Wiring
Different for each resistor. Vtotal V1 V2
Same for each resistor. Vtotal V1 V2
Voltage
Same for each resistor Itotal I1 I2
Different for each resistor Itotal I1 I2
Current
Increases Req R1 R2
Decreases 1/Req 1/R1 1/R2
Resistance
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Calculation
R2
R1

• In the circuit shown V 18V,
• R1 1W, R2 2W, R3 3W, and R4 4W.
• What is V2, the voltage across R2?

V
R3
R4

• Conceptual Analysis
• Ohms Law when current I flows through
  resistance R, the potential drop V is given by V
  IR.
• Resistances are combined in series and parallel
  combinations
• Rseries Ra Rb
• (1/Rparallel) (1/Ra) (1/Rb)
• Strategic Analysis
• Combine resistances to form equivalent
  resistances
• Evaluate voltages or currents from Ohms Law
• Expand circuit back using knowledge of voltages
  and currents

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Calculation
R2
R1

• In the circuit shown V 18V,
• R1 1W, R2 2W, R3 3W, and R4 4W.
• What is V2, the voltage across R2?

V
R3
R4

• Combine Resistances

Parallel Can make a loop that contains only
those two resistors Series Every loop with
resistor 1 also has resistor 2.
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Calculation
R2
R1

• In the circuit shown V 18V,
• R1 1W, R2 2W, R3 3W, and R4 4W.
• What is V2, the voltage across R2?

V
R3
R4

• Combine Resistances

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Calculation
R2
R1

• In the circuit shown V 18V,
• R1 1W, R2 2W, R3 3W, and R4 4W.
• What is V2, the voltage across R2?

V
R3
R4

• Combine Resistances

R2 and R4 are connected in series R24 2 4
6 W
Redraw the circuit using the equivalent resistor
R24 series combination of R2 and R4.
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Calculation

• In the circuit shown V 18V,
• R1 1W, R2 2W, R3 3W, and R4 4W.
• R24 6W
• What is V2, the voltage across R2?

• Combine Resistances

R2 and R4 are connected in series R24 R3 and
R24 are connected in parallel R234
(1/Rparallel) (1/Ra) (1/Rb)
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Calculation
R1

• In the circuit shown V 18V,
• R1 1W, R2 2W, R3 3W, and R4 4W.
• R24 6W R234 2W
• What is V2, the voltage across R2?

V
I1 I234
R234
R1 and R234 are in series. R1234 1 2 3 W
V
R1234
I1234
Ohms Law tells us I1234 V/R1234
18 / 3
6 Amps
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Calculation
V

• In the circuit shown V 18V,
• R1 1W, R2 2W, R3 3W, and R4 4W.
• R24 6W R234 2W I1234 6 A
• What is V2, the voltage across R2?

I1234
R1234
R1

I234 I1234 Since R1 in series w/ R234
V
R234
V234 I234 R234 6 x 2
12 Volts
I1 I234

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Calculation

• V 18V
• R1 1W
• R2 2W
• R3 3W
• R4 4W.
• R24 6W
• R234 2W
• I1234 6 Amps
• I234 6 Amps
• V234 12V
• What is V2?

Which of the following are true? A) V234 V24 B)
I234 I24 C) Both AB D) None
Since R3 and R24 where combined in parallel to
get R234 Voltages are same!
Ohms Law I24 V24 / R24 12 / 6
2 Amps
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Calculation

• V 18V
• R1 1W
• R2 2W
• R3 3W
• R4 4W.
• R24 6W
• R234 2W
• I1234 6 Amps
• I234 6 Amps
• V234 12V
• V24 12V
• I24 2 Amps
• What is V2?

Which of the following are true? A) V24 V2 B)
I24 I2 C) Both AB D) None
Since R2 and R4 where combined in series to get
R24 Currents are same!
Ohms Law V2 I2 R2 2 x 2 4
Volts!