AC ?

[1123581321]

I was wondering, with AC, if the charges are constantly changing direction, wouldn't that mean the current would move slowly, create vast amounts of heat energy and overall wouldn't be very effective/efficient...

 

So how exactly is it better than DC and how does a step-up transformer with AC allow it to be transfered long distances without loosing as much energy ?

 

Also, do power lines have periodically set up step-down transformers which aid the transformation of AC and if so why can't it be done with as much efficiency with DC ?

 

thanks.

[Danijel Gorupec]

(this post might suit better to the Engineering forum)

 

 

The ‘speed of current’ doesn’t have much meaning. (The strength of current does.) You would be surprised how slowly charges move in the wire.

 

 

A wire is all ‘stuffed’ with charges. If you somehow ‘push’ a charge at one end of the wire, the effect (like wave) is rapidly transported to the other end (at speeds comparable to the speed of light). Charges themselves are not moved much.

 

 

In AC you shake charges at one place of the wire. These charges push other charges near them and so on... the wave spreads fast. In AC, charges actually don’t travel – they only ‘shake in place’.... In DC, they do travel, but at incredible slow pace.

 

 

AC does produce a bit more loses in a wire (skin effect), but for low frequency (60Hz) theses loses are not that much.

 

 

AC might be better for power distribution because it makes things simpler (and possibly cheaper)

 

- AC is easily produced by power generators (no need to commutate high currents)

 

- AC voltage can be simple stepped up/down by means of transformers (transformer do not work with DC). By stepping voltage up, we can reduce power losses during transmission

 

- AC can be directly used by asynchronous motors.

 

 

(Today, we have systems that can efficiently convert AC to DC and vice versa, so sometimes you will see large power lines carry DC instead of AC. This is because transferring DC is a bit more efficient, as I already said.)

 

 

DC cannot be used in transformers. More complicated systems (DC/DC converters) are needed to transform DC voltage level.

 

 

A transformer is a simple magnetic machine. The primary winding produces magnetic flux. This magnetic flux goes through the secondary winding. The fact is that a voltage/current will only be created in the secondary winding if the magnetic flux changes with time (Faraday’s law). Therefore, the current through the primary winding must be AC – this creates ever changeable magnetic flux, that creates voltage in the secondary winding.

 

 

Sorry, long post.

 

 

 

 

[Janus]

 

 

So how exactly is it better than DC and how does a step-up transformer with AC allow it to be transfered long distances without loosing as much energy ?

 

 

The power delivered by a line is determined by Voltage x Amperage, so you can provide the same power with a high voltage/low amperage or low voltage/high amperage.

 

The power lost via transmission through the line due to resistive heating is found by Amperage^2 x resistance of the line. Increasing the voltage in the line decreases the amperage needed to provide the same power, and decreasing the amperage decreases the amount of power wasted.