EMF/Voltage/Potential Difference & Current

EMF/Voltage/Potential Difference:

Absolute potential of a point is the work done in moving a unit positive test charge from infinity to the point. The potential difference between two points is the work done in moving a unit positive test charge from one point to the other.

If we have two points A and B and it requires work to move a unit positive charge from B to A, then A is said to be at higher potential with respect to B or the potential difference between points. A and B is positive when we move from point B to point A, so experience a rise in potential. Conversely from point A to B there is a fall of potential.

The SI unit of potential of potential difference is volt (symbol V).It is defined as the potential difference across a resistance.

Since the potential difference between two points A and B may be positive or negative, it is more appropriate to write as it as, say V_AB which means potential of point A with respect to that of point B. If A is at higher potential than B, V_AB is positive. Then V_AB (i.e. potential of point B with respect to that of A) is negative. Thus V_BA= -V_AB. The terms potential difference and voltage are synonymous.

 The term emf (electromotive force) is also used instead of voltage. Strictly speaking emf is the total voltage of a source (e.g. a battery or a generator). There would always be some voltage drop in the source itself and the voltage at the terminals of the source would be a bit less than the source emf. The voltage at the terminals is known as output voltage or terminal voltage.

Electric Current:

An atom consists of a nucleus and electrons which are in different orbits around the nucleus. The electrons in the outermost orbit are known as valence electrons. This valence electrons are rather loosely bound to the parent atom. When the different atoms of a conducting material are packed to form a solid, the valence electrons are free to move from one atom to another. 

The motion of valence electrons is in random directions. When a field is applied to the conductor, the motion of electrons gets channelized. More electrons flow in on correction that in the reverse direction. Thus there is occurs the flow of electrons. The rate of flow of electrons is electric current. 

It so happens that the reference direction of flow of positive current is reverse of the direction of flow of electrons. The direction of current was assumed much before the concept of electron was discovered.

Later on it was established that electric current is essentially the flow of electrons. However, the original convention about the direction of positive current is still being used.

When a current is flowing in a conductor having a non-uniform cross sectional area, the current is the same for all cross sections of the conductor. This is due to the principle conservation of charge.

The SI unit of current is ampere (symbol A). 1A is defined as the constant current which, when following in each of the two straight infinitely long parallel conductors, situated in vacuum and

spaced one meter apart, produces a force equal to 2×10^-7 newton per meter length between them. 

1A also means flow of 1C (i.e. 6.24×10⁸ electrons) charge past ant point in a conductor in on second. However, this statement doesn’t indicate the speed at which electrons are moving. This speed known as drift velocity is very small about 1 mm per second. 

Nevertheless, electric signals travel along the conductor at a very high speed. The phenomenon is similar to that in fluid mechanics. The speed of water particles is very slow. However when a pressure flows through at one end of a long pipe full of water, a pressure wave travels rapidly along the pipe.

In solids, the current is solely due to electron. In liquids, the current is due to the motion to positive ions and negative ions. In gasses, the current is due to electrons, positive ions and negative ions. In vacuum tubes, the current is due to electrons emitted from the cathode. In semiconductors, the current is due to electrons and holes.

A current may be direct (dc) or alternating current (ac). A direct current flows continuously in the same direction. However, an alternating current flows first in one direction for a brief instant and then in the reverse direction for another equally brief instant. Thus the direction of an alternating current keeps on changing continuously. The supply of electric power to consumers now-a-days is through alternating current.

The current density is the current per unit area

The units for current density is  A/m²