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AC/DC Circuits

  1. Aug 10, 2009 #1
    Out of curiousity, I am wondering what happens, theoretically of course, if we place a 1.5V AC light bulb (which do not exist) onto a DC circuit with a 1.5V battery? What would happen if we connect this battery backwards on the circuit (positive to the negative terminal and vice versa). This question arises from my confusion of positive and negative voltages in AC and DC circuits.
     
  2. jcsd
  3. Aug 10, 2009 #2

    chroot

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    A light bulb is just a resistor that glows.

    If you put 1.5 VDC across it, it will glow steadily. If you put 1.5 VAC across it, it will glow and then darken and then glow again, at the frequency of your AC. If this frequency is high enough (60 Hz is plenty), the variation is brightness will not be perceptible.

    Like a resistor, it does not matter which way you connect your light bulb to your battery. Either way, the same amount of current will flow -- just in the opposite direction. The light bulb is not sensitive to the direction of current flow, though, so you won't see any difference.

    - Warren
     
  4. Aug 10, 2009 #3
    If a light bulb does not care which direction the current is coming from, then why must everyday appliances like a flashlight have positive and negative terminals that we must position our batteries to comply or else the device doesn't work?
     
  5. Aug 10, 2009 #4

    russ_watters

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    Try it and find out if that's really true...

    [note: don't try it on an LED flashlight!]
     
  6. Aug 11, 2009 #5

    Danger

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    With a regular incandescent bulb, it doesn't matter which way you put the batteries in as long as they're both polarized correctly in relation to each other. Battery cases, however, are usually set for the negative (non-nippled) end of the battery to impinge upon a spring terminal.
     
  7. Aug 11, 2009 #6

    mheslep

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    No harm in reversing the batteries, it just won't work. Diode breakdown voltage is typically >20VDC. Application of a 60Hz AC (low voltage) source to the LED will cause it flash on/off, appearing to the eye as a lower than normal intensity.
     
    Last edited: Aug 11, 2009
  8. Aug 11, 2009 #7
    Oh okay; so if the cases don't exist then I can put a battery (assuming a flashlight with only one battery) whichever way I want and the flashlight will still work?

    But if what I said is true then what's the advantage of adding the cases?
     
  9. Aug 11, 2009 #8
    Take a flashlight that requires two batteries.

    Have the users put them in however they want.

    Disconnect your technical support when case after case of unhappy customers are mad their batteries exploded on you.
     
  10. Aug 11, 2009 #9

    Danger

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    Yes, it will still work. What I meant by 'case' is simply the receptacle into which the batteries are inserted. Most have one solid contact and one spring-loaded one. The positive (nipple) end of the battery contacts the solid piece, and the negative (flat) end contacts the spring. In a side-by-side arrangement, the directions are usually reversed so that one goes in backwards in reference to the other. That's the easy way to put them in series without excess wiring. Most cases these days are also marked with '+' and '-' signs in the appropriate locations.
     
  11. Aug 11, 2009 #10

    russ_watters

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    I know, I just didn't want to confuse the OP.
     
  12. Aug 11, 2009 #11

    russ_watters

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    Also, as was noted earlier, the terminals may or may not be mechanically interchangeable and batteries are not all exactly the same shape.
     
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