Conventional current and electric current When an external field is applied ,then electrons move in the direction opposite to that field, I want to know that does conventional current moves in the direction of field? A confusion is there in my mind that the direction of electric current is opposite to the direction of electron flow so can the direction of conventional current be from negative to positive terminal (,opposite to proton flow)?
 A: 
does conventional current moves in the direction of field?

Yes. Conventional current assumes positive charge-carriers and field directions are likewise assumed as the direction a positive charge would move. 

the direction of electric current is opposite to the direction of electron flow 

Not understood. What is the difference between "electric current" and "electron flow"? 

can the direction of conventional current be from negative to positive terminal (,opposite to proton flow)

No. Negative and positive terminals are as well, per convention, defined as seen from a positive charge. A positive charge will always want to move towards lower potential, so towards a negative terminal. Since conventional current assumes positive charge-carriers, conventional current will always want to flow towards a negative terminal.

In general, remember that the term "conventional current" is just a way for us to talk about currents without having to take into account the actual charge-carrier. It may seem odd, but the reason is that it doesn't matter. There are several different options of charge-carriers in different types of circuits:


*

*Usual metallic circuits (all wiring and most usual components such as resistors, capacitors and inductors) contain electrons as majority charge-carriers, so negative.

*In semiconductors (transistors as used in enormous amounts in computers, solar panels, thermoelectrics, catalysts etc.) the charge-carrier is either electrons (negative) or so-called holes (basically a "missing electron") (positive). 

*In ionic parts of circuits (electrolytes in batteries, fuel cells, membranes, dielectrics in capacitors, liquids inside the body etc.) we have atoms split into their positive and negative ions, that both will move when a potential is applied. So both charge-carriers types are present here.

*In plasmas (active materials in fusion reactors, lightning strikes etc.) you may have actual "free" protons, electrons and other particles, and thus again a mixture of charge-carrier types.

*Etc.


All such charge-carriers may be a part of a full circuit. If we by "current" only meant "electrons moving", then it would work when talking about the metallic parts of the circuit, but would suddenly leave out big contributions in ionic parts and would not make sense at all in semiconductors etc. If we on the other hand by "current" only meant "protons", then it would very rarely make sense (only partly for pure plasmas).
So, we've got to find another meaning of the term "current". What we notice in all types of the above circuit parts is that positive and negative charge-carriers always behave exactly opposite. So, a negative charge-carrier (electron, negative ion...) moving one way is equivalent to a positive charge moving the opposite way (there is now less negative charge where they came from, corresponding to a positive charge arriving there). In the same way, a positive charge-carrier (proton, positive ion, hole...) moving one way is equivalent to a negative charge moving the other way.
This realization is important. It means that if we forget about the type of carrier and only consider their effect as a current, then we can consider, for instance, all negative charge-carriers as if they were positive charges moving the opposite way and thus as if they were a positive current. Then that equivalent positive current can be added to the actual positive charge-carrier current. And then we have included all contributions and have the full current.
Therefor it has been decided per convention in international science that every time we say the term "current", without specifying further, we mean "current of (equivalent) positive charges". This is called conventional current. In different situations you must then keep in mind that the actual flow of charge might be the exact opposite way - it doesn't matter at the macro-scale, you should just be aware of it if you ever look deeper.
This convention is used throughout in electronics. Also in field directions (always defined as the direction a positive charge would be pushed), potential energies, potentials and voltages (a point of low potential is where a positive charge wants to move) etc.
A: Electrons are what carry electric current. Protons are not used to carry electricity, although they do have a positive charge. In most of the things we think of as conductors, the protons are kept in place by the electrical forces acting on the rest of the atom. That leaves the electrons.
When electricity was first investigated, positive and negative poles were arbitrarily assigned to the terminals of a battery, and current was assumed to flow from the positive to the negative pole. 
Then, when electrons were discovered to be the carriers of electricity, it was found that their charge is negative in the old convention. As a result, the "real" current, the one carried by electrons, goes from the negative pole to the positive. However, as many people find it easier to think of current going from + to -, the convention is still used when thinking about (DC) circuits. 
The problem arises when you start thinking about the internals of circuit elements where you have to remember that the electrons go the other way. Especially with semiconductors this is very important, as their behaviour depends on the behaviour of electrons (and of holes, i.e. "missing" electrons).
