Download Muddiest Points Wk 1

Muddiest Point Week 1 – 2016
1) Node Voltage vs. Voltage Difference
Node Voltage
Measured from one point (a node) to ground.
Put red lead on the node. Put black lead on the
ground.
Marked with one number (V1)
Node voltages do NOT add up.
Voltage Difference
Measured between two nodes. To find the
voltage difference V12 = V1 – V2, put the red
lead on node 1 (V1) and the black lead on node 2
(V2).
Marked with two numbers (V12)
Voltage differences can be added up around a
loop.
2) How to tell if things are in series or parallel.
Series
They have only one ordinary node between them.
(They share only one color, and it is on an ordinary
node.)
Current is equal in series elements.
Voltage is generally NOT equal across series
elements. (It is equal across two resistors only if
the resistors are equal.)
When tracing a path through one element, the
ONLY place for the path to go is through the next
element.
Parallel
They share two colors.
When you connect two hoses together this way,
they are in series. There is only one (ordinary)
When you connect the hoses this way, the two
outgoing hoses will be in parallel. This is an
Voltage is equal across parallel elements.
Current is generally NOT equal in parallel
elements. (It is equal only if the two resistors are
equal.)
node between them. The current in one hose will
be the same as the current in the other hose.
extraordinary node. The current splits when it gets
to the extraordinary node .
Sometimes there are circuits where NOTHING is in either series or parallel.
Which of these will produce hoses in series and which in parallel? (if there are only 2 in-out, they will be
series, 3 or more will create some parallel connections)
3) Voltage, Current, Power, Energy, Resistance
Voltage: the amount of potential energy at any point on a node. Is equal to the work
done (Links to an external site.) by an electric field (Links to an external site.) in carrying a
unit of positive charge from infinity to that point.
Power: A the rate of change electrical energy is is transferred.
Energy: Electrical energy is energy that has been converted from Electrical potential energy.
After it is converted it is found in other areas of energy such as heat, light, or motion.
Resistance: A quantity of how much the flow of electricity is resisted. Similar to the flow of
water through pipes.
4) What do I-V curves tell you?
I is current (on the y axis). V is voltage (on the x axis). These can be theoretical (as in this book)
, specified (my a manufacturer), or measured (as in the case when you don’t know what is in
some black box, and you have to guess or reverse engineer it).
I-V for a Resistor is a straight line (linear), and the slope of the line tells you the resistance.
I-V for an LED or other diode is second order, and the shape depends on the diode parameters.
Different colors of LED have different curves.
5) Adding voltages in a circuit, and which way does the current go when it comes to a T.
To add voltage differences (NOT node voltages), define the + and – point for each voltage
difference. Then start anywhere in the circuit, and if you encounter the + first, add it. If you
encounter the – first, subtract it. They should all add up to be zero.
When a current comes to a T, it will split. Here, and elsewhere in the circuit, you may not really
KNOW which way the current will be going. So, just guess. Draw an arrow guessing a
direction. At every node, the sum of the currents coming into the node = sum of currents leaving
the node. After all the math is done, if you guessed right, the answer for the current will be
positive. If you guessed wrong, it will be negative. This is ok, and you don’t have to go back
and change anything… you just know the current is actually going in the other direction.
Hint: When you are really designing circuits, larger circuits, you are unlikely to be able to guess
well which direction the current is really going. So, get in the habit of just drawing current
arrows from top to bottom and left to right. Sure, if you really know which way it will be going,
sketch it that way, but usually you won’t… so, just guess a standard direction, and go from there.
6) Where to put the ground in your circuit
Ground/reference is where your voltage =0. If you know for sure where that is (like a copper rod
in the ground, or the metal chassis of a car, or the metalized case of your cell phone), put it there.
If you don’t know, just choose a convenient place… such as at the bottom (- point) of one of
your voltage sources. There is no ‘rule’ for this, any place is acceptable, but these are
suggestions for convenient places to define your reference point.
7) Ideal vs. non-ideal sources
Ideal voltage and current sources do not have any resistance 'in' them. Non-ideal voltage
sources have a small series resistance that is naturally part of the voltage source (and
which you can't get rid of). non-ideal current sources have a small parallel resistance.
8) Question: The most confusing thing concept for me this week was how to tell the
difference between an open circuit and a short circuit and how to tell if there is a short
circuit.
An open circuit is a gap, so no current can go through. A short circuit is a wire, so all of
the current can go through.
THANK YOU for your muddiest points! Dr. Furse