Download Muddiest points Week 2

ECE 1250 Muddiest Points Week 2:
Question: How to assign positive and negative to current and devices when working with
Kirchoffs Laws.
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Voltages show the +/- in their schematic symbol. We know which way to put the +/- on
the schematic, because we bought (or designed) them such as battery with + and –
terminals.
When determining a KVL equation, as you loop around the circuit elements in the loop,
you assign a positive or negative value to the voltages depending on where you enter the
device (within your loop).
For currents through a resistor, you use your current arrows, with a + value at the tail of
the arrow and a - at the tip. For example, when looping through a resistor, the voltage is
positive if the current arrow runs in the same direction that you are traveling through your
loop, otherwise negative.
(We didn’t talk about this a lot yet) For unknown voltage through any other element, you
can ‘guess’ the unknown voltage, solve for it as part of the matrix equation, and then you
know you either guessed right (if your answer is +) or wrong (if your answer is -).
Question: The hardest things for me this week were KCL and KVL. My question would be what is
the difference between the two?
KVL is the sum of voltages around a closed path and KCL is the sum of the current that is either
entering or leaving a node.
Question: How do I know which direction to draw the current arrows?
In practice, you will often NOT be able to tell which direction the current is flowing until after
you complete your calculations. Like the example we did in class, there may be several sources,
and the current can flow in any direction depending on how strong the various sources are. So,
we just GUESS which direction the current is flowing, draw arrows matching these guesses,
calculate the currents, and if we get a - number, we know we were wrong (but we don't usually
go back and change the arrow directions, we just leave the - number with the arrow in the wrong
direction). My personal preference for guessing is drawing currents left to right and top to
bottom. But, any other guess is equally valid.
Question: The muddiest point for me last week was matrices.
I’m uploading an appendix on matrices. Check the MATH link at the top of the home page.
Question: The muddiest point of the week for me has been to remember the sign on the voltages
and direction of currents. For example, when I work on examples where we switch the reference
node. I can do the potential differences easily if I don't worry about the sign. The sign is what
gets me.
From Lecture 1-5:
Node voltages are Va and Vb and Vc. They DO change when we move the reference. (You
measure these by putting the red probe of the voltmeter at the node and black probe at the
reference/ground.)
Voltage difference is Vab. (and the voltages across each resistor). They do NOT change when
we move the reference.
Vab = Va – Vb (so the sign is determined by the math). You measure it by putting the red probe
of the voltmeter at Va and black at Vb.
We did not know about KVL when we did these first problems. Now you can do KVL loops, and
solve for any unknown voltages, and that will give you the signs correctly also.
Question: The Muddiest point of the week for me was on Ohm's Law. I was somewhat confused
on how everything (conductivity, resistivity, resistance) is related.
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Conductivity tells you how EASY it is for current to move through a material.
(meter/ohm).
Resistivity (inverse of conductivity) tells you how HARD it is for current to move
through a material. (ohms/meter)
Resistance (ohms) = Resistivity * Area/Length. You can think of it as a TOTAL
difficulty for moving current through a specific size/length of the material.
Ohm’s Law is V=IR. If we have a constant voltage, increasing resistance decreases
current. If we have a constant current, increasing resistance increases voltage.
Question: What does current DO? WHAT is really flowing?
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When it flows through a resistance, it creates heat. It also creates a voltage difference
across the resistance.
It creates a magnetic field around it (right hand rule … thumb in the direction of the
current, fingers will curl in the direction of the magnetic field).
Charges are flowing. The direction of the current (direction the arrow is pointing) tells
you which direction the positive charges are flowing. This is equivalent to negative
charges flowing the opposite direction.