Download muddiest points Week 1

Muddiest Points (selected) Week 1:
Anyone else want to add comments or answers? Please do!
Current, Charge & Voltage and their polarities (+/-):
Question 1a: The most confusing concept this week is understanding the difference
between current and voltage.
Question 1b: The most confusing concept for me is what actually happens to electrons
in a circuit when they hit a resistance and how this produces the difference in potential
(voltage) across the component. I thought the electrons were slowed or "held up" by the
resistance, but in this case how is the current the same (in series) after the component
slows the electrons? The internet was also confusing on this topic, and I learned that
electrons don't actually move all the way through a circuit like we imagine them to.
However, I still don't understand how there can be a voltage difference across a load if
voltage is caused by an imbalance of electrons and whatever is receiving electrons yet
the current, or charge flowing through the circuit, remains the same.
Question 1c: Through the lecture about passive sign convention I initially got confused
with the polarity of the circuit and direction of current flow. Some of the arrows indicating
current flow were inconsistent from what I learned previously. It is my understanding
that because electrons hold a negative charge, therefore current flows from the negative
terminal to the positive terminal. It can be confusing when comparing relative voltages
because a high positive voltage is actually a drain for electrons. I’m not sure if there’s
really a good solution to this since the convention itself, as I know it, is inherently
counter to the common understanding of relative values.
Question 1d: I'd have to say how to set up the positive and negative sides on the
resistors and also how to compute voltage differences.
Attempted Answer 1:
Polarity: Current is the flow (movement) of electrical charge. The direction of the
current is defined as the direction POSITIVE charge is flowing. Electrons have negative
charge. So current is defined in the OPPOSITE direction of the flow of electrons. When
we define a current direction (arrow), we ALWAYS put + on the tail, - on tip of the arrow.
The current will physically flow from high to low voltage, but we don’t always know
which voltage in the circuit will be higher. So we guess (I define all currents from left to
right and top to bottom, for convenience), calculate our values, some will be + (correct
guess for current direction!), others will be – (guessed wrong, ok now I know).
When will you care about the electrons? Most of the time, you won’t. Forevermore, EEs
consider current as the flow of positive charge and don’t do a lot of thinking about
electrons. But, when you get to semiconductor physics, then you will care about
positive and negative charge carriers, electrons and holes. Then you will care again.
Voltage: A build up of (positive) charges produces a (positive) voltage. We also call
voltage = ‘potential’. We can think of this as the ‘potential to release charges.’ Current
comes out of the + terminal of a voltage and flows to the – part of the voltage. Current
flows from high to low voltage. (Ok, answering the question above, electrons flow into
the + and out of the -.) This is drawn in the slide below.
So, in an engineering problem, how do you define the polarities (+/-) of voltages and
currents? (1) Anything that is already ‘given’ has a pre-defined polarity, use it. (2) Draw
current arrows in any direction of your choosing. I choose to draw them (almost) always
from left to right and top to bottom, because it is consistent and easy. If I REALLY know
the current is going the other way, ok, I’ll draw it that direction, but otherwise, just guess
and don’t worry about it. When you do your calculations, you might find some currents
have negative values, meaning they are actually flowing in the opposite direction from
your initial guess. That’s ok. Just leave the arrows the way they are, with negative
currents associated with them. (3) For voltages, define a ground (V=0). Then label all
other node voltages, and consider them positive values. If you get negative numbers
from your calculations, that is ok. Leave them as they are, with negative values
associated with them. (4) To find voltage differences, see picture above. Voltage
differences can be positive or negative. That’s ok. Leave them defined as they are,
associated with + or – values.
Let’s do a little example with a resistor (Note the +/- on the voltage (battery) and on the
current (+ on tail, - on tip ALWAYS). Note Vb=Vgnd (because they are the same node)
= 0.
Color Code/ Series / Parallel
Question 2a: The most confusing concept of the week was series and parallel of
resistors.
Attempted Answer 2:
Ordinary nodes have only two components connected to them. Extraordinary nodes
have more than 2. Color code a node between elements, following all of the wires until
you get to (all of) the next element(s).
Series means the components are connected by ordinary nodes. These will have the
same current going through them, because there is no place else for it to go…
Parallel means the components share two identical nodes. This means they will both
have the same colors on either end. They will have the same voltage across them, too.
Short Circuit: When an element has the same node (same color ) on both ends, it is
short circuited. It can be replaced by a wire (R=0). If nothing else is in series with it, a
short circuit has infinite current and zero voltage.
Open circuit: When an element is not connected in the circuit (‘hanging in space’,
‘switch is open’ ‘wire is broken’), it is an open circuit. That element can be removed
from the circuit (replaced by an open circuit). An open circuit has voltage but zero
current.
Dependent and Independent ideal/non-ideal voltage /current sources
Question 3a: I was also a little confused on dependent and independent ideal/non-ideal
voltages.
Attempted answer 3:
Ideal voltage/current sources have no resistance.
Non-ideal voltage source has a resistor in series. Non-ideal current sources has a
resistance in parallel.
Independent voltage/current source is NOT dependent on anything else in the circuit. It
is a constant (voltage or current) source, no matter what the load is. (Shown using a
circle.)
Dependent voltage/current source IS dependent on something else (normally another
voltage or current) in the circuit. A dependent source is used to model other nonlinear
circuit elements (like op amps and other things we haven't learned about yet). (Shown
using a diamond.)
Who, What, Where, When?
Question 4: What is due when?
Attempted answer 4:
To see when something is assigned today, look at the home page (the table below the
picture). To see when it is due today, look at the calendar view. General:
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QOD / watch video lectures are due at start of the class.
Homework is due Mondays (except holiday weeks, when it is due Wednesdays).
Prelab is due at start of lab for which it is ‘pre’. Lab writeup is due the next time
you go to lab. Lab start date is marked on home page (for spring 2015 this is3rd
week of class)
Extra credit (except muddiest points and feedback, which I need right away), is
accepted through end of semester.
(Here is a summary borrowed from another student, thank you!)
General Class prep: When, Where, What, and Who
When:
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Class: MWF 9:40-10:30
Lab: None this week or next, but starts the 3rd week. Lab assignments are all due on the
FOLLOWING lab
Homework: Due each Monday (Wednesday next week)
Lectures: At home PRIOR to class, to be done with the QOD's
Where:
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Class: New room WEB 1250
Lab: 2555 but not this week or next (turn in keycard access paperwork for access)
Homework: Worked on in class, finished at home
Lectures: At home prior to class
What:
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Class: time to work with professor on homework and example problems
Lab: practical circuit building and data mapping
Homework: QOD's and HW's (Assessments, in class handouts, and practice quizzes are
NOT to be turned in. Yes, you may use these notes along with your QODs for the
exams.)
Lectures: Youtube videos with ppt, outside reading
Who:
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Class: Cynthia Furse, full professor
Lab: Lab TA
Homework: Yourself (Check your answers) and a TA (Check for completeness)
Lectures: Cynthia Furse (Youtube) and independent sources