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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