Understanding Conventional Current & EMF: Answers to Your Questions

[Student_93]

so here is my problem. . my physics book is trying to explain me about conventional current and i am not getting it !
urghh. .so at last i decided to copy paste what my book says so some 1 else can explain me better . .
so here it is...
(text from book)
Now let us see how current flows in a conductor.Consider a conductor in the form of a copper wire . It has a large number of free electrons which are in random motion just like the molecules of a gas confined in a container.Their movement does not obey any symmetry but they move in all directions .In the absence of an electric field the rate at which the free the rate at which the free electrons cross any section of the wire from right to left is equal to the rate at which they cross from left to right with the result that the net rate is zero.So in spite of the fact that electrons are in motion, no current flows through any section of the conductor .Now if one end of the copper wire is connected with the positive terminal of the battery and the other with its negative terminal, an electric field E is established at each point of the wire. Now the free electrons ,because of there negative charge,experience a force in a direction opposite to the direction of the electric field E. Because of this force a net directed flow of free electron take place from the negative terminal of the battery towards its positive terminal and an electric current begins to flow through the wire from the positive terminal towards the negative terminal of the battery .Note the current flow due to negative charges has been charged with conventional current.This current flows in the wire from positive to negative terminal of the battery i.e, current flows from a point of higher potential to a point of lower potential.

So the Questions are:
If the electrons are flowing towards positive terminal of battery from the negative terminal,then what is the need of saying that the current is flowing from + to -?
it's confusing me a lot . . . please help me ! !
and also explain me emf( electromotive force ). .
waiting . .i have a test :(

 

[cupid.callin]

Current means transfer of something in unit time. Water current means water flow from one place to other. If water flows out form a tap into a bucket the amount of water in the bucket increases with time.

The same with the charge. The charge should increase at that place to where the current flows that means that the current must be the flow of positive charges.
Of course, the charge would increase by the same amount if equivalent negative charges flow AWAY. But we consider the current positive if it increases the charge at the place to where it flows.

It would be more logical to take direction of current would be the same as the direction of electron flow. Why not then? Because of the tradition.
******
"Charge was discovered by the Ancient Greeks who found that rubbing fur on various substances, such as amber, would build up an electric charge imbalance. The Greeks noted that the charged amber buttons could attract light objects such as hair. ... The word electricity derives from ηλεκτρον, the Greek word for amber.

By the 18th century, the study of electricity had become popular. One of the foremost experts was a man named Benjamin Franklin. Franklin imagined electricity as being a type of invisible fluid present in all matter. He posited that rubbing insulating surfaces together caused this fluid to change location, and that a flow of this fluid constitutes an electric current. He also posited that when matter contained too little of the fluid it was "negatively" charged, and when it had an excess it was "positively" charged. Arbitrarily (or for a reason that was not recorded) he identified the term "positive" with the type of charge acquired by a glass rod rubbed with silk, and "negative" with that acquired by an amber rod rubbed with fur. So you see that current from + to - is just a convectional , as it is named
People thought that something positive flowing makes so they accepted it that way. Later we did found that its negative charge that makes up current but by then there were many theories made upon this convention that instead of improving them we just took current to be opposite and called it conventional current!

 

[Student_93]

so if the current really flows from - to + , then why my book is insisting me to believe that it flows from + to -? ?


and the electrons(-) will surely be attracted towards the (+) terminal of the battery, and if there are protons(+) they would love to go towards the (-) terminal of the battery. .

Now the point is:
Is the Battery providing us the Positive charges ? and they are gathering at one place and kicking the negative charges out? and hence the current is flowing ?

Can someone please explain me the role of the Battery in this situation, and what kind of charges it supply to the conductor?

Thanks

 

[gneill]

"Conventional" current is just that, a convention. It is based upon an arbitrary assumption that what was flowing in the wire was positive charges moving from the positive terminal, through the circuit, and back to the negative terminal of the battery. This choice was made before the discovery of the electron and its role in electric current, and was based upon an arbitrary designation of positive and negative to the charges created in particular ways in static electricity experiments.

As it turns out, this ultimately forced the assignment of "negative" to the charge on the electron when it was discovered.

If the arbitrary naming choice had gone the other way, then the + and - terminals of the battery would have been reversed, the electron would have been a 'positive' charge carrier, and "conventional" current would then match the direction of flow of electrons in a circuit. But the choice was made as it was, and we're stuck with the convention.

What you need to realize is that the naming choice doesn't affect the analysis of circuits. A current of imaginary positive charges moving in one direction is analytically the same as a current of negative charges flowing in the opposite direction. Both represent the same net movement of charge from one place to another, and the same amount of work is done in doing so.

The historical accident that produced our current naming convention (Ha! a pun!) isn't problematical unless one needs to understand the underlying physics of particular situations or devices -- such as why the plate electrode of a vacuum tube diode has to be positive with respect to the cathode in order for current to flow, implying that the charge carriers must be negatively charged particles.

 

[cupid.callin]

And also note that in real (the general current we use) there is no positive charge (like protons) flowing! Only electrons flow through the circuit.

If you have studied the structures of solids, you would know that nucleus(with electrons) is crucial in making the structure of solid and therefore cannot move to produce current (an of course protons cannot also move as its impossible for electrostatic force from battery to overcome nuclear forces of nucleus). If this was not true you would see copper wires in your home disappear during current flow and you would have to pay each year for new wires!

And the battery also do not provide any any positive charge. Of course also in battery there exists atoms who would not give protons because of what i just explained. Battery's positive terminal is having less electrons and on connecting the two ends, the excess electrons from negative terminal flows through wire to nullify the positive charge!

Imagine battery as a dam and electrons as water. The side filled with water is the negative terminal having excess water(electrons). The other side of dam (with less water) is the positive terminal. And you won't expect water to flow int the Dam, would you? Water(electrons) flow from Higher volume (Negative terminal) to lower volume (positive terminal). Water flows due to potential energy from Earth and in same way electrons flow due to electrostatic field from battery.

 

[Student_93]

Thanks :)