What does conventional current mean?

[sanjuro]

When I was much younger, my teacher told me that electricity could be defined as "a flow of electrons". So when I turn on a light bulb or television set, what is actually happening inside? What is making the electrons move? A positively charged field perhaps? But how is the field created? Where is it? What does conventional current mean?

Not challenging questions, but physics is new subject for me.

Thanks

 

[chroot]

The current flows in the wires because there is a non-zero electric field throughout them. The battery or generator or other device creates a field in the wires. The electrons move in the wires in response to the field.

In a metal, the electrons are not bound to individual atoms, but are instead quite free and approximate a free electron gas. The electrons are actually moving at very large random speeds in the wires, due to their thermal energy. However, the electric field causes the electrons to preferentially "drift" towards the highest potential at a very slow speed of tens of centimeters per hour.

"Conventional current" is a simply a convention in which positive charge carriers move from high potential to low potential; in reality, the charge carriers are negatively charged and move the opposite direction. This is no importance in most circuits; the results of considering negative or positive charge carriers are the same. Only in the case of semiconductor devices does it matter whether the charge carriers are negative or positive.

- Warren

 

[Chi Meson]

When you turn on your light in a house circuit (let's assume a standard light bulb) you are using "alternating current" electricity. THis means that the electric field (see previous post) is changing directions over 100 times every second (over 50 "cycles" per second).

Since the field is what influences those "free electrons," and the field keeps changing directions the net flow of electrons is zero. But they do vibrate back & forth over extremely short distances and can still transfer energy. In the lightbulb, these vibrating electrons increase the temperature of the filament to thousands of degrees. WHen anything gets so hot, it will begin to glow.


The gernerators from the electric power company create the fields that are set up in the wires. THe field travels at close to the speed of light inside the wires and therefore the influence on the free electrons in your lightbulb is nearly instantaneous.

There are some new, alternative theories about what is actually going on, but this is still the current (ha ha) accepted theory.

 

[sanjuro]

Field and direction

When you say the field is "changing direction", do you
mean that its charge is changing, fluctuating
between positive and negative?

Are there any advantages of "alternating current"
over direct current, and why do we use it?

Sanjuro

 

[Chi Meson]

Disclaimer: THe following is a "classical" description.

The electric field has no "charge." Electric charge is a property of matter, and electrons carry the negative variety of charge.

THe electric field is a "zone" that exists around all charged objects wherein another electric charge will feel a force. When an electric field is created in a wire, the negative electrons will feel a force (protons will feel the force too, but they are not free to move, and only one or two of all the electrons [edit] per one atom of any conductor are free to move)

Generators create an electric field inside the wires by rotating a magnet inside a coil of wire (simplified description). THe phenomenon is known as electromagnetic induction.

Picture a bar magnet on a spindle so that it rotates end over end. Now picture a coil of wire around the magnet . You should be seeing the magnet "sticking through" the coil with the north end poking out the "front" face of the coil.

As the magnet rotates, an electric field is created in the wire, and the field "points" along one direction inside the wire. When the magnet reaches the opposite orientation (North now sticks out the "back" of the coil) and starts the second half of the "flip," the electric field that is created will be in the opposite direction in the wire.

In US generators these magnets are spinning 50-60 times every second.

THe advantage of ac is that the voltage can be stepped up and stepped down. That is, 10,000 volts along the "high tension" power lines can be converted to the 120 volts in your home. THis can only be done when the electric field is constantly changing.

The advantage of dc is that it can be stored in batteries

 

[chroot]

sanjuro,

Keep in mind that the distinction between alternating and direct current is not at all important for the understanding of how electricity "flows" through wires. Electrons drift through wires due to an electric field in the wire; the electric field can be constant (DC) or changing (AC), but the concept is the same.

The reason we use AC rather than DC is because transformers (devices used to convert voltages) have to have varying voltages to work. The electric company can use one transformer to boost the voltage on their transmission lines, and another to lower it down to 110 VAC for your home. The reason we bother raising and lowering it at all is because power dissipation in a wire goes as I^2 R -- if you halve the current on your transmission line (by doubling the voltage), you reduce the power loss by four times.

- Warren

 

[chroot]

Originally posted by Chi Meson
The advantage of dc is that it can be stored in batteries

I just have to comment on this one: you can't store any kind of current in batteries! What you mean, of course, is that batteries produce a constant (DC) potential, and must be charged similarly.

- Warren

 

[Chi Meson]

Originally posted by chroot
I just have to comment on this one: you can't store any kind of current in batteries! What you mean, of course, is that batteries produce a constant (DC) potential, and must be charged similarly.

- Warren

Yep. True. Should have said "dc can be used to store energy in a battery." Man, you got to be careful in this forum!

 

[chroot]

I should also mention that there are ways to convert DC voltages as well, and very efficiently, too -- such converters are called "regulators" and typically use a capacitor or inductor and a switching circuit that rapidly opens and closes switches. Such devices can operate easily at 99% efficiency, and can produce nearly any voltage from nearly any other -- even negative voltages from positive ones! I'm not sure how well these kinds of devices scale up to the power and current on the grid, but you can buy small ones for less than a buck.

These devices are called "boost converters" when they increase voltages and "buck converters" or "switch-mode regulators" when they decrease voltages.

There are some municipalities which use high-voltage DC transmission lines -- I believe Montreal has one, and one is being built in Australia.

If you'd like to know alllllll about HVDC transmission, read this:

http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=330311

- Warren

 

[Canum]

Disclaimer: THe following is a "classical" description

Chi, how would your quantum explanation differ from the one you posted?

 

[Chi Meson]

Originally posted by Canum
Chi, how would your quantum explanation differ from the one you posted?

Well, I would not have posted a quantum explanation. My comment was a bit tongue-in-cheek, expecting that someone would post a "correction" with talk about virtual photons and Maxwell's equations. I just try to keep it in mind that all classical explanations are simplified approximations of what is actually happening in the quantum realm. Quantum is not my forte.

 

[Adrian Baker]

So basically the Electric company sends you a bill for just wiggling your electrons back and forwards 50 times a second!

They don't even supply new ones!