Lectures

Basic AC Circuits

by Jared Rovny
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    00:01 Finally, the last thing we´ll do here is briefly discuss some basic properties of AC circuits which are alternating current circuits.

    00:08 In an alternating current circuit, all we have is instead of the current flowing just in one direction and keeping going in that same direction the entire time, it switches direction. So, we have some voltage source we call it, pushing the current one way and then pushing the current the other way, and going back and forth and so in a circuit like this it´s not so simple to define the current flowing into the circuit. The reason it´s not so simple is because if I graph the current as a function of time, I will have something like this.

    00:38 The current would increase and go to the right and then stop and then slow down, and then go back to the left and then stop and go down, and go back to the right, and keep doing this back and forth. So, if I showed you a graph like this showing you the current going up and down and back and forth, and I asked you, 'How do we measure the current?' How do I talk to a fellow engineer to build a circuit if we don´t have a way to talk about how much current this is in the system? So, we do a clever thing. If I were just talking about for example the average current in the original set up that I just showed you, it would be zero because the current spent just as much time being in one direction as the other.

    01:13 So, we can´t just average of the current but we can do something clever which is to first, square the value of the current which makes the entire thing positive since a negative number squared is a positive number and then take the average.

    01:25 So, we can first square, find the mean of this entire new graph for the current and then we could take the square root to get back where we started.

    01:34 In this case, we have what´s called the root mean square current or the RMS current because what we´re doing is taking the square root of the mean of the square of the current and this is just a clever way to get away from the average current being zero because it?s not a very useful number to define when we´re talking about the current going back and forth.

    01:54 The last important thing to know about the RMS or the root mean square current or voltage, since we can apply the same idea to the voltage, is that the root mean square current or the root mean square voltage, both of which are going to be oscillating back and forth, are proportional to the maximum value for the current or the maximum value of the voltage where the maximum value is defined as showing in this picture here.

    02:17 It´s the peak or the greatest value that your voltage or your current attains.

    02:21 The root means square value of your current and the root means square value of your voltage, are simply the maximum values divided by the square root of 2 and you don´t have to worry about how this value is derived but it is very important and useful to know what the root mean square current is and how we define it and derive it, and how also if we know the maximum current, say in the outlet of your wall, that you can find the root mean square current and understand the relationship between those two.

    02:49 So, this wraps up both our brief discussion about AC circuits as well as our final discussion about how to apply Kirchhoff´s law to find things like power and resistance at different parts of more complicated circuits.

    03:01 So, we´re now ready to go on to magnetism.

    03:04 Thanks for watching.


    About the Lecture

    The lecture Basic AC Circuits by Jared Rovny is from the course Circuit Elements.


    Included Quiz Questions

    1. AC circuits have a current which alternates directions
    2. AC circuits have no resistance
    3. AC circuits have an unknown voltage source
    4. AC circuits are the realization of the DC circuits of theory
    5. AC circuits have no voltage drops
    1. The actual average value would be zero
    2. There is no actual voltage or current in AC circuits
    3. The actual values are always unknown
    4. The current and voltage switch places with each other too rapidly
    5. The actual values have square roots of negatives, so are undefined
    1. It is 1/sqrt(2) times the maximum current
    2. It is the square root of 2 times the maximum current
    3. It is half the maximum current
    4. It is twice the maximum current
    5. It is equal to the maximum current

    Author of lecture Basic AC Circuits

     Jared Rovny

    Jared Rovny


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