After learning the basics of circuitry its time to dive into what a circuit is as well as things we use to measure elements in a circuit. Any closed loop with electricity running through it will have a voltage and current, and fundamentally have a resistance, which denotes a circuit. In order to test for things like amps and volts, we can set up meters in our circuit which measure the current or voltage of the electrons running through the circuit. Along with the math, diagramming with circuit diagrams, as well as knowing how to read circuit diagrams is the electrical equivalent of FBDs in mechanics. For now weve dealt with straightforward circuits, but later on well dive into larger circuits that arent so easy to read.
Electrical circuits are closed-loop paths through which current can flow.
Conventional current flows from high potential to low potential.
Circuit schematics are two-dimensional representations of three-dimensional circuits.
A source of potential difference is required for current to flow.
Voltmeters measure the potential difference between two points in a circuit. Voltmeters are connected in parallel, and have very high resistance.
Ammeters measure the current flowing through a circuit element. Ammeters are connected in series, and have very low resistance.
Circuits & Electrical Meters
Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.
Our objectives are going to be to identify the path and direction of current flow in a circuit, drawing and interpreting schematic diagrams of circuits and effectively using and analyzing voltmeters and ammeters. 0009
Electrical circuit is a closed-loop path through which current can flow. 0022
An electrical circuit can be made of most any materials, but practically speaking, circuits are typically comprised of electrical devices. 0026
This includes things like wires, batteries, resistors, and switches. 0033
However, if you are not careful, even humans can be part of an electrical circuit, so let us try to avoid that. 0036
Now, conventional current flows from high potential to low potential, but if you ever want to talk about electron flow, electrons are going to flow in the opposite direction. 0043
So electrons flow from low potential to high potential, but conventional current flows from high potential to low potential. 0054
So, circuit schematics are two-dimensional representations of three-dimensional physical circuits. 0070
Looking at a physical circuit, things can get complicated in a hurry. 0077
A circuit schematic is a way of representing that on paper or on a computer. 0080
It allows us to much more neatly organize what we are going to do with that circuit. 0085
Symbols represent these circuit elements and lines represent wires, conductive wires. 0090
Now sources for potential difference are required for current to flow and those sources for potential difference can be things like voltaic cells, batteries, or power supplies. 0096
And typically we are going to use a symbol for a battery for all of them, where the longer side is positive and the shorter side is negative. 0105
Here is how I remember that -- when you draw the battery schematic symbol, you have to use more ink over on the longer side -- you also need to use more ink to make a plus, than you do a minus. 0115
So the long side of a battery is the positive side or voltage sources and current sources can also be used. 0126
Here is a single cell. A battery is actually a combination of cells, which is why you get this symbol. 0134
A switch is often times shown opening and closing that way. 0140
A voltmeter with a v in it measures potential difference or voltage. 0144
An ammeter measures current flow in amps. A resistor kind of looks like the Charlie Brown shirt. 0148
Variable resistor just means you can change the resistance and a lamp is a resistor that gives off light. 0155
Over here on this side, we have the positive side of our cell, the negative side of our cell -- we have a resistor -- a light bulb here, but we do not have anything happening. 0166
We do not have a completely closed path, therefore current cannot flow. 0175
On the right hand side, though, once we have closed that switch, we now have a complete loop -- current is going to flow this way from positive potential to negative -- from high to low potential. 0179
Now keep in mind though, that means that the actual electrons are flowing in the opposite direction. 0191
Voltmeters measure the potential difference between two points in a circuit. 0200
They are connected in parallel with the element that you want to measure. 0205
And if a voltmeter is connected correctly, you can remove it from the circuit without breaking the circuit. 0210
For example, as we look at this circuit over here, the voltmeter would measure the potential difference from one side of our light bulb to the other side of our light bulb. 0215
And if we want to make sure we have it installed correctly, imagine what would happen if you cut the circuit there and there. 0223
When you do that, the circuit would still operate -- the voltmeter would not measure anything, but you would still have an operating circuit. 0230
If that is the case, you have done it correctly. 0236
They have that high resistance so that very little current flows through them and they do not impact your circuit performance much at all -- just a tiny amount, a negligible amount. 0242
You want it as little as possible so that you get an accurate reading of how the circuit would function without the voltmeter in there. 0251
Ammeters, on the other hand, measure the current that is flowing through an element of a circuit. 0260
They are connected in series with the circuit, so that whatever current is flowing has to flow through the ammeters so that you can measure it. 0264
You want all of the current that you want to measure to flow through the ammeter. 0271
The ammeter must be broken to correctly insert an ammeter. 0275
If you can pull the ammeter out of the circuit and the circuit still functions, that portion of the circuit, then you have done something wrong. 0279
Ammeters have extremely low resistance, therefore they do not have much potential drop and they minimize their effect on the circuit as a whole as well. 0285
So let us talk about ammeter and voltmeter placement in the circuit. 0296
In the electric circuit diagram here, possible locations of an ammeter and voltmeter are indicated by the circles 1, 2, 3, and 4. 0299
Where should we place an ammeter in order to measure the total current and where should a voltmeter be located to measure the total voltage? 0307
Well, the ammeter -- if we want to measure the total current -- if we put it here at 2, we would only measure the current through this resistor and if we put it here at 3, we would only measure the current going through here -- that would not be a good idea. 0315
If we put it here at 4, we would measure current through here, but really we want our current to be flowing this way and that way and then coming back. 0328
So at the one spot is where we could place our ammeter where we would get the total current flow. 0339
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