What is Negative Voltge and Negative Current?

"Current" is nothing but the flow of electrons. The conventional 'flow' of current is a theoretical assumption, and is opposite the flow of electrons. They always flow from anode to cathode (outside the source). The anode and cathode change every half-cycle in AC. Clear ?

If not check wikipedia for a more detaoled analysis.

By the way, what are you doing in an electrical forum? Don't get me wrong, it is never too late to learn

the cathode and anode changes in every half cycle .then wehre these electrons going?is it an oscillation?where these electrons are generated its from ground ?if it is like an oscillation then what is the need of earth?

http://www.google.co.uk/search?q=negative+voltge+and+negative+current&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&ie=&oe=&rlz=1I7DKUK_en-GB&redir_esc=&ei=YjIBTIzjK4yi0gTPqZn3Ag

http://www.google.co.uk/search?q=alternating+current&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&ie=&oe=&rlz=1I7DKUK_en-GB&redir_esc=&ei=rjIBTNPIEY_u0wTu8qDMAw

Dear Jasin,

Negative voltage means that we have connected the system in such a way that now reference is not the earth or cathode but the anode. Also the current in this way flows from cathode to anode is negative current.

Hope the answer of your question is provided.

In ac the voltage follows a sinusoidal wave the voltage goes positive then down the wave to zero voltage and then negative voltage then it rises still negative until the zero line when it changes to positive and so on.

The Links i have provide will give you a better understanding

I hope you've read the previous posts and have a better idea already.

I would only try to simplify the answer, please stick to the electronic theory.

A negative voltage is when the ground or reference point has a lack of electrons (is more +) compared to the point you're meassuring.

A negative current is when you close the mentioned circuit and allow the electrons flow, from the point you're meassuring, to ground.

Yahlasit

? ? ? ? ? ? ? ?

I hope I haven't confussed him more, this is what I meant:

(The signs and needle inside the meter are only for showing polarity, not for denoting the black and red leads of the meter, black goes to ground always)

Yahlasit

It's a frame of reference thing.

First thing: there's no such thing as 'absolute voltage' ... no point has a voltage of a given value. All that exists is difference in voltage BETWEEN two points, just like difference in pressure, location (distance), time, etc.

Second: the - terminal of a voltmeter defines the reference voltage against which other voltages are measured. When a voltmeter has its - terminal connected to ground or another reference point, another point's voltage is deemed 'positive' (which actually mean 'greater than') if that voltage is greater than that of the reference point's. Conversely, a point's voltage is deemed 'negative' if the voltage is less than the reference point's.

The voltage at the + end of the red battery is more positive than that of the reference voltage (the ground voltage) because the battery generates voltage between its own two terminals, and that its + terminal has a higher one than its - terminal's (by definition). In reality, the + terminal connects to an electrode that lacks (and therefore attracts) electrons, but voltage serves as the abstraction needed to make things simpler. Thus, connecting a voltmeter's - terminal to the red battery's - terminal makes the latter the reference point against which the red battery's voltage is measured; and since the battery's + terminal has a voltage greater than the reference point's (the battery's - terminal) by definition, then the voltmeter reads a positive DIFFERENCE in voltage.

The converse is true for the blue battery; connecting a voltmeter's - terminal to the + terminal of the blue battery makes the battery's + the reference point. Since the battery's terminal has a LOWER VOLTAGE THAN THE REFERENCE POINT'S (note: it doesn't have a negative voltage), then the voltmeter reads a less-than-zero (negative) DIFFERENCE in voltage between the reference point (the battery's + terminal) and the battery's - terminal.

The things to under understand: points don't have voltages; and two points can have a DIFFERENCE in voltage; and the - terminal of a voltmeter defines the reference point against which differences in voltage (compared to the reference point) are measured. The trick is to understand that there's no such thing as 'absence of voltage' or 'zero voltage' unless one defines it as such.

Cheers! DZ

Actually, in the case of nagative voltage as defined here, the ground or reference has more electrons than it has positive charges.

Cheers! DZ

I believe it's sound more as a differential of potential, basically. You'll get it here with the CR-4 guys, they good to the bone.

Hang in There,

MC

When the electrical potential is applied across a conducting media, the flow of electrons start.

However the speed of electron is not the speed of light (at which the current is flowing) - electron have a mass, and the energy needed to accelerate it to the speed will be infinite (as per great E). Actually speed (drift speed) of electron as I remember - memory weak- is 3.5mm/s

As soon as the contacts are closed, the Electric field is formed across the whole length. The electrons nearest to the +ve pole are sucked into it and the -ve pole expells equal number (assuming the field is uniformly distributed across the incremental lengths).

When the pole reverses, same thing happens, only +ve is now -ve and vice versa.

Basically for all practical purposes, the electrons oscillate in the AC. The field moves (or is established) at speed c all particles move at a lower finite speed.

There is a lot of confusion here.

In the first place, positive and negative flows exist in an ac circuit. They relate to the positive and negative cycles. These two flows are allways from positive pole to negative pole (defined by convention...) since the polarity is ALTERNATING (100 times for 50Hz supply). You are confused because you are thinking that "how the current is reversing?" Well the speed of the electrons (for the purpose of explaining this issue only), goes at the speed of light... therefore, from source to destination, there is enough time for it to reverse completely, and this is possible as long as the length of the cable (...!) is less than the wave length at 50Hz (6000 km). Well this is maybe hard for you to understand??

This is as far as I can go here. You better take a course or read or consult ... you seem very green...

It was bound to happen. The speed of electricity. Best explanation is from Wikipedia :

"The Speed of electricity refers to the relatively slow movement of free electrons or ions through a conductor in the presence of an electric field, also known as drift velocity. It is often confused with the propagation speed of an electromagnetic wave. It is the electromagnetic wave that usually conveys information (data), not the movement of electrons.

Free electrons in a conductor vibrate randomly, but without the presence of an electric field there is no net velocity. When a DC voltage is applied the electrons will increase in speed proportional to the strength of the electric field. These speeds are on the order of millimeters per second. AC voltages cause no net movement; the electrons oscillate back and forth in response to the alternating electric field.^[1]

In contrast, electromagnetic wave propagation is much faster, and depends on the dielectric constant of the material. In a vacuum the wave travels at the speed of light and almost that fast in air. Propagation speed is affected by insulation, such that in an unshielded copper conductor it is about 96% of the speed of light, while in a typical coaxial cable it is about 66% of the speed of light.^[2]".

My understanding is; all materials have electrons- a metal conductor has more than an insulator- when a source of electromagnetic energy is connected to a conductor, the em energy is transferred via electron movement- with dc the movement is 1 way & fast; with ac the motion is more jiggle back & forth- but the essential thing is as many electrons return as leave-ie it is not the electrons providing power but the em energy from the source.

Let's think of an AC voltage that is generated by a coil (rotating in a clockwise direction)inside an AC generator. When the generator coil is in line with the magnetic field, the coil will see zero magnetic flux linkage and the magnitude of generated voltage is nil. As the coil starts to rotate upright, the magnetic flux linkage and the induced voltage will rise from zero Volt to a maximum 1.414Vrms when the coil is 90 degree perpendicular to the magnetic field. As the coil rotates pass 90 degree, the voltage starts to fall and become zero when the coil is flat in line with the magnetic field again. The coil has rotated by 180 degree then. You can arbitrarily call this half cycle as positive - you will get a positive current when you connect a passive element (R, L or C) across the output of the coil.

Now when the coil starts to rotate pass 180 degree, the voltage you get from the coil will be 'negative' because the magnetic flux linkage now seeing the back side of the coil. This is describe by the sine wave where it goes into the negative half cycle. Again, it will reach the maximum 1.414Vrms as the coil reaches the 270 degree point. So between 180 - 360 degrees, the voltage seen by a passive element (R, L or C) across the output of the coil is 'negative' and the resulting current is negative.

If you wish to visualise it using electrons using a piece of electric wire, let's think of a large group of hot steel ball bearings (electrons) stored in a big container (energy source) connected to a steel tube with the right hand end an empty container (the same energy source as the left hand container), when the voltage is positive, they flow (move) from left to right in small but increasing/decreasing quantities proportional to the half-cycle sine wave, then all these heated steel ball bearings are at the right hand container. During the negative cycle, these ball bearings will flow in increasing/decreasing numbers back to the left container. In a 60Hz system, this alternating flow will occur 60 times a second. If you now touch the steel tube (i.e., the electric wire), you will be able to feel the energy loss by the electric current as heat.

Hope the above make sense.

It is just a change of direction of the current flow. Imagine a white dot painted on your car tire. When you drive the car, the dot will go up and down. When the dot goes up that is positive; when it goes down, that is negative (by my definition).