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svtfast
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Posted: 6/23/2013 10:04:22 AM EST
This is a multiple choice question and I am confused. I swear that my prof is trying to trick us.

Awnswers:

A) lower power loss than DC
B) easier to convert to any voltage (this is not the answer)
C) tastes great and is less filling

When I read it I figured it was C but I cant find the answer to "Does AC power have less power loss than DC power."

I looked through the classroom and I read by book then reread my book but I am stuck.


Can somepoint me in the righe direction or show me how to figure this out? Google skills didnt help.
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Commando223
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Posted: 6/23/2013 10:11:26 AM EST
[Last Edit: 6/23/2013 10:13:51 AM EST by Commando223]
AC travels further than DC

*Edit from Google*
Thomas Edison was a major proponent for using direct current and he argued it was the only choice for household electrical power. As we also know George Westinghouse (working from the ideas of Nikola Tesla), argued for alternating current.
The big advantage of AC over DC is that AC "travels" better. AC can be stepped up to higher voltages sent along power lines and then stepped down. This allows for the use of thinner power lines and allows AC to travel much farther (further?) than DC (no, not the city). I think direct current can only travel a few miles before it loses sufficeient power to be usable.
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SmilingBandit
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Posted: 6/23/2013 10:14:08 AM EST
I'm confused on why it's not B.
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Posted: 6/23/2013 10:16:25 AM EST
AC is much better for power distribution. High voltage and step-down x-formers provide a stable voltage with little line loss. DC would have to be transmitted at a much higher voltage than the end user would require, and voltage dividers/regulators would provide further losses/inefficiencies.
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Posted: 6/23/2013 10:18:01 AM EST
Originally Posted By SmilingBandit:
I'm confused on why it's not B.


Very true, but "any-voltage" could mean step-up, which would require a pretty high primary current to provide a usable secondary current.
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Posted: 6/23/2013 10:19:19 AM EST
Originally Posted By Commando223:
AC travels further than DC

*Edit from Google*
Thomas Edison was a major proponent for using direct current and he argued it was the only choice for household electrical power. As we also know George Westinghouse (working from the ideas of Nikola Tesla), argued for alternating current.
The big advantage of AC over DC is that AC "travels" better. AC can be stepped up to higher voltages sent along power lines and then stepped down. This allows for the use of thinner power lines and allows AC to travel much farther (further?) than DC (no, not the city). I think direct current can only travel a few miles before it loses sufficeient power to be usable.


I think westinghouse nuked an elephant and lobbied for DC executions to scare people into going to AC.
learath
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Posted: 6/23/2013 10:24:26 AM EST
A is a critical factor, B is a convince factor. Question sucks.
svtfast
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Posted: 6/23/2013 10:42:41 AM EST
Originally Posted By SmilingBandit:
I'm confused on why it's not B.


Because I read in my book that AC IS easier to convery to any voltage. DC requires all kinds of extra crap to convert it
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Posted: 6/23/2013 10:44:56 AM EST
I am not sure if your professor has some hidden reason behind option (C), but the correct answers are (A) & (B) and here is why...

If your professor is referring to the distribution and transmission of power (either AC or DC) then yes, AC has less power loss. There is quite a bit of theory involved here but here goes:

- Due to the nature of AC, it can be very easily transformed from one voltage to another (compared to DC) because of the ever changing direction and amount of current flow. This constant change of direction and magnitude of current flow in an AC circuit creates a magnetic field around the conductor that also increases and decreases with the changing current flow. The magnetic field also changes polarity when the current changes its direction of flow. Transformers are AC devices and require this fluxuating magnetic field to induce a voltage and current into the secondary windings. DC does not change direction of flow and, all else equal, doesn't change the amount of current flow either. There is no such thing as a DC transformer because the magnetic field surrounding a conductor in a DC circuit doesn't have a fluxuating magnetic field.

- Because it can be transformed, a much larger voltage can be produced, and in turn a smaller amount of current is required, that is then used to transmit the same amount of power over long distances. Power equals voltage x current (P=IxE). The equation for power loss in a conductor is I2R (current squared x resistance of the conductor). Using this power loss equation, if the current was reduced by half then the power loss will actually be reduced by 3/4 of its original amount. Also, using the proper size conductor (the R or resistance of the circuit) affects the amount of power loss because it is the actual opposition to this current flow. A larger conductor will have less resistance - which 'uses' power and converts it into heat.

So I hope that helps...and I wouldn't want to taste any AC or DC voltage because it hurts and can do bad things to you like burns and death .
svtfast
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Posted: 6/23/2013 10:45:44 AM EST
Originally Posted By Gspointer:
Originally Posted By Commando223:
AC travels further than DC

*Edit from Google*
Thomas Edison was a major proponent for using direct current and he argued it was the only choice for household electrical power. As we also know George Westinghouse (working from the ideas of Nikola Tesla), argued for alternating current.
The big advantage of AC over DC is that AC "travels" better. AC can be stepped up to higher voltages sent along power lines and then stepped down. This allows for the use of thinner power lines and allows AC to travel much farther (further?) than DC (no, not the city). I think direct current can only travel a few miles before it loses sufficeient power to be usable.


I think westinghouse nuked an elephant and lobbied for DC executions to scare people into going to AC.


That was Edison.
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Posted: 6/23/2013 10:47:51 AM EST
Converting AC to higher voltages takes a transformer and involves a percentage of power lost to heat.

On the other hand, transmitting power at high currents causes increased heat losses to the resistance of the wire.

Of your answers, B is true and A makes sense if you assume step up voltages and transmission over long distances.

Really is a crap question.
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Posted: 6/23/2013 11:13:16 AM EST
Yes a crap question.

However, DC is more efficient for distribution - far less transmission losses.

Several long distance high voltage transmission lines are DC, with rectifiers /converters on each end.

For general distribution around a town/city, AC is much more convenient, as voltage changes are much more easily
handled by simple step up/down transformers.

http://en.wikipedia.org/wiki/HVDC

Description of design, build, operation of a HVDC undersea system
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Merlin
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Posted: 6/23/2013 11:16:29 AM EST
Originally Posted By svtfast:
Originally Posted By Gspointer:
Originally Posted By Commando223:
AC travels further than DC

*Edit from Google*
Thomas Edison was a major proponent for using direct current and he argued it was the only choice for household electrical power. As we also know George Westinghouse (working from the ideas of Nikola Tesla), argued for alternating current.
The big advantage of AC over DC is that AC "travels" better. AC can be stepped up to higher voltages sent along power lines and then stepped down. This allows for the use of thinner power lines and allows AC to travel much farther (further?) than DC (no, not the city). I think direct current can only travel a few miles before it loses sufficeient power to be usable.


I think westinghouse Edison nuked an elephant and lobbied for AC DC executions to scare people into going to DC AC.


That was Edison.


FIF both of you.
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Posted: 6/23/2013 11:20:34 AM EST
If the question is "Does AC (power) have lower power loss than DC (power), then the answer is obviously (A).
While either of the other answers may or may not be true they don't answer the question - (A) does.
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Posted: 6/23/2013 11:27:18 AM EST
(A) is the more correct answer because it directly answers the question...even though (B) is also true.
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Posted: 6/23/2013 11:48:45 AM EST
[Last Edit: 6/23/2013 12:27:23 PM EST by Frank_B]
Sounds like prof has little, if any, real-world experience. Which is usually the case. If you can't learn it, teach it.

Answer is A. AC can be transformed to high voltage for distribution and transmission, then back down to low voltage for customer use. Because the current required for a given power level is reduced by the same ratio that voltage is increased, smaller conductors can be used.
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Posted: 6/23/2013 12:01:08 PM EST

Originally Posted By us-kiwi:
Yes a crap question.

However, DC is more efficient for distribution - far less transmission losses.

Several long distance high voltage transmission lines are DC, with rectifiers /converters on each end.

For general distribution around a town/city, AC is much more convenient, as voltage changes are much more easily
handled by simple step up/down transformers.

http://en.wikipedia.org/wiki/HVDC

Description of design, build, operation of a HVDC undersea system

New York City is fed (partially) with HVDC. Relatively new to the market, and costs are high, but is better then HVAC.
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Posted: 6/23/2013 12:04:54 PM EST
[Last Edit: 6/23/2013 12:07:26 PM EST by GUYwith_theAK]
Well it depends. There are DC circuits that are more efficient than AC circuits and vise versa. A more useful question is which is more efficient, high voltage or high current circuits?
Rumrunner358
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Posted: 6/23/2013 12:04:58 PM EST

Originally Posted By AZ_Sky:
If the question is "Does AC (power) have lower power loss than DC (power), then the answer is obviously (A).
While either of the other answers may or may not be true they don't answer the question - (A) does.

not (always) correct anymore

http://www.energy.siemens.com/hq/en/power-transmission/hvdc/hvdc-classic/#content=Description
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Posted: 6/23/2013 12:07:28 PM EST
resistive or reactive load?
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Posted: 6/23/2013 1:17:59 PM EST
I squared R losses, far greater in DC.

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