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Varying Direct Current and Alternating current

  1. Jul 23, 2015 #1
    1. The problem statement, all variables and given/known data
    I've been struggling to fully understand the difference between Varying Direct Current and Alternating Current. Please explain this to me. Also, how would they differ on a Current-Time graph?

    2. Relevant equations
    E = int (VI dt)

    3. The attempt at a solution
  2. jcsd
  3. Jul 23, 2015 #2
    Additional information:

    Attached Files:

  4. Jul 23, 2015 #3
    Maybe you could think about the 'sign' of the current: which direction it is flowing
    And then think about the magnitude of the current: the size the current - how many amps

    With some graph paper you could plot a line where the magnitude is changing but not the sign. Is this AC or DC?
  5. Jul 23, 2015 #4


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    Staff: Mentor

    The distinctions are somewhat malleable but if the voltage does not change polarity it can be called DC. In some uses, it's sufficient that the voltage has an average value > 0 for it to be consdered DC, so it may indeed spend some time below zero, just so long as for most of the period it is above the x-axis. In cases where the average > 0 and the signal is varying it may be more convenient to regard it as AC having a DC component (or, alternatively, as DC with an AC component).

    * I have restricted my discussion to positive DC, but the same applies to negative DC
  6. Jul 23, 2015 #5


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    Gold Member

    In many ways they are linked. I think of it this way. for the most part, alternating current is a signal that repeats itself. If you can see a pattern arise in the signal, even if that pattern is simply white noise.DC is simply the average, if you need too, think of it as a rolling average. It is the level point of the signal for that span of time.

    keep in mind, you can have AC signals coupled onto a DC signal.
  7. Jul 23, 2015 #6
    So, @William White, according to you, for varying direct current, the current can't be negative basically, but can be in alternating current? And to answer your question, it would be DC I think.
    @NascentOxygen , makes sense, but are you certain the current can go below zero (x-axis), i.e: a change in direction/polarity/sign?
  8. Jul 23, 2015 #7
    @donpacino , I followed you up until you said "DC is simply the average"...I don't quite get what you mean from then on
  9. Jul 23, 2015 #8


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    IF you have a DC current of 5 amps for 10 seconds, that means that your average is 5 amps right? because the current doesnt change.

    now if the current following the following equation I=sin(2*t)+2 for 10 seconds, the DC component of that signal is 2, because that's the average!
  10. Jul 23, 2015 #9


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  11. Jul 23, 2015 #10
    There is a water analogy there! that's going to upset somebody!

    I think, sometimes, the urge not to give a straight answer (because it is against the rules) ends up making a very simple question very difficult. Just saying.

    Best thing is to draw this to convince yourself. Go to
    a very good web site

    and try typing in (on the left hand side)
    y = sin x
    y = 0.5
    y = -0.5

    If the Y axis was measuring current (and its sign) and the x axis is measure of time, what plots are AC and what are DC? What line is changing sign periodically?
  12. Jul 23, 2015 #11
    @William White, thanks for introducing me to desmos, cool website. I know y=sinx would be AC, and the other 2 would be DC, but I'm trying to understand Varying DC
  13. Jul 23, 2015 #12
    You now know that the definition of AC is when the sign changes periodically. This is the frequency (50Hz in the UK).

    So, by defintion, DC does not change sign periodically: only the magnitude of the current can vary, not its sign.

    Of course, there are real world anomolies when current can change sign. But to make it simple, AC is generated specifically to alternate it sign periodically. DC is generated specifically not to.
  14. Jul 23, 2015 #13
    I hope I understand it clearer. AC (most of the time?) is a repeating pattern like sinx or cosx, and has an mean of zero, while Varying DC current would be random mostly, have an average of >0 or <0, and be either positive or negative? (and Varying DC might sometimes go slightly below zero if it's mostly positive, or slightly above zero if it's mostly negative?)
  15. Jul 23, 2015 #14
    AC is always alternating. That is the defintion of AC. The sign changes periodically.

    DC is always direct. That is the defintion of DC. The sign does not change.

    Regarding the current flowing in either an AC or DC circuit. It is certainly NOT random, but depends upon the connected load. As the load demand increases, the amount of current increases.

    Your house is powered by AC currents. If you switch on a lamp, AC current will flow through the lamp. If you switch on another lamp, the load has doubled, so you will draw double the current. The magnitude of the current is not random, it is due to the number of lamps and appliances (the load) that you have switched on. The more you switch on, the more current is flowing, the more energy you use, and the higher your electricity bill!

    Now imagine that your house was powered by a battery that supplied DC current. Exactly the same thing would happen. As you switched on more lamps, more current would flow from the battery. Again it is not random. The current VARIES as the load increases or decreases.

    Simply if the current is VARYING then the magnitude (size) of the current is changing
    if the current is ALTERNATING the sign of the current is changing.

    Do not get hung up on the exceptions to theses rules. They are just confusing you and not helpful at all.
  16. Jul 23, 2015 #15
    Alright, thanks @William White. My bad, the graph for a Varying "DC" over a period of time looked random, without a pattern. It now makes sense why. I'll stick to your suggestion and leave out the exceptions.
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