1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A.c or d.c

  1. Aug 12, 2009 #1
    Hello all, I hope you all are fine.
    That’s is my first post
    Questions are

    1. which is first discovered or invented / A.C current or D.C
    2. Which is more convenient to burn a 100watt bulb? A.C/D.C (silly ones )
    3. is there variation when we light a 100watt bulb by A.C current (I mean ON/OF in small time)

    Thanks a lot
  2. jcsd
  3. Aug 12, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Chemical batteries (which are DC) were discovered 100years before anybody invented a way of generating electricity.
    But the story of the first AC andDC power generation is complicated http://en.wikipedia.org/wiki/War_of_Currents

    The bulb doesn't really care. We have AC delivered to our houses because it is easy to change the voltage of AC so you can run everything from an oven to an electric toothbrush from the same supply.

    Yes a light bulb will brighten and dim 50/60 times a second (different countries use different rates) but we normally don't notice it because our eyes can't work that fast.
  4. Aug 12, 2009 #3
    thanks for reply

    can you give me idea why it happened. its because of alternating current frequency.am i right :smile: !
  5. Aug 12, 2009 #4


    User Avatar
    Science Advisor
    Homework Helper

    Yes, the voltage varies like a sin wave. For the first half of the cycle current flows through the bulb one way, heating it up and giving off light, then the voltage drops to zero and the current reverse flowing the other way, the bulb heats again. This repeats 60 times/second.

    So twice in every 1/60 s cycle the bulb brightness drops. Since the filament takes a little time to cool down you don't see much of a change in the light.
  6. Aug 12, 2009 #5
    In the United States, 100-watt incandescent bulbs on 60 Hz flicker at about 120 times per second.
    I once worked around a big cyclotron with a lot of stray magnetic field. The Lorentz force [F = (I x B)] on the AC-powered incandescent bulb filaments literally vibrated the filaments off. So DC was required.
  7. Aug 15, 2009 #6
    While this is a true statement, it's not the reason why AC is delivered to our houses. Bottom line, Nikola Tesla was brilliant and realized that moving electrons back and forth (alternating) was a much more efficient way of delivering power than Edison's DC which forced the electrons to flow the entire length of the wire. If our homes were DC powered we'd need transformers every few hundred feet (average). AC is easier to manipulate, like you said, but is not the reason why we have AC delivered to us today.

    (Just my two cents.. Nikola Tesla has always fascinated me)
  8. Aug 15, 2009 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You are way off the mark with this post. First off transformers only work with AC, so it the ability to change the voltage with a transformer that makes transmission of AC better then DC. Transformers in a DC transmission system would do nothing useful. The only reason you have given is that NT is brilliant, sorry that is not a reason to use AC.
  9. Aug 15, 2009 #8
    I've always felt Tesla was brilliant. I feel no need to change anyone's opinions about him, so you won't get a list of reasons. Truly take a look at his life and you'll understand. I feel he got so many things done in his life, even though he started off at such a disadvantage; having his patents purchased and the purchaser making all the money, etc.

    Anyway that's off topic. I did a little further reading, and my response was based on my knowledge of the AC/DC wars of the late 1800's. In order to provide DC power to homes, generating plants needed to be built very close by. That was what 'transformers' meant in my post. AC allowed freedom of much longer distance transmission, but that is because of the relative ease of being able to change the voltage.

    So, mgb_phys, I apologize for assuming you didn't know the whole story, when in fact what you said was more relevant by today's standards. I also read up on HVDC transmission between two AC stations, and that seems like a pretty swell idea. (Or at least a cost saving one). So Integral, you are correct as well. I did not provide enough reasoning. But I think now between the three of us we have a pretty accurate picture of AC vs. DC.
  10. Aug 15, 2009 #9


    User Avatar
    Science Advisor
    Homework Helper

    It is a bit of a complicated mix between theoretically most efficent and most practical!

    HVDC transmission is more efficient. You have to rate the system for the maximum voltage, in AC this is 1.7 times the average (RMS) voltage so you waste some engineering cost. DC essentially keeps the cable 'full' all the time, although this mean s more cooling.
    The main problem is that it is more difficult/expensive to convert DC to HVDC and back.
    HVDC transmission lines are most often used to avoid having to phase two AC grids together (eg the UK-France link) or link istance links in the USA.

    In the home DC transmission would be more efficent than having to have a PSU for every appliance that needs DC. But you would have to pick a voltage, LEDs and CPUs need 3V, motors need 6-12V, lights and cookers would need much more.
    In telecoms and some computer centers it is common to run everything from 48v DC.

    It's also more difficult to safely switch high power or high voltage DC.
  11. Aug 15, 2009 #10

    Although it would be nice to not have 25 dc converters plugged into surge protectors around the house, (phone chargers, laptop plugs, etc.) you're right. You'd have to pick the voltage, and that would be way over most peoples heads to even begin to ponder. Hopefully this new "smart grid" tech will eventually get to the point where we'll get the best of both worlds, since AC and DC both have particular advantages. I can't wait till "warm superconducting" cables get better and easier to manage, then we could have no power loss. (at least until it gets to the end user, then who knows how wasteful they are).

    Anyhow, I'm enjoying the convo.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook