The flow of charges from one point to another constitutes a current and the direction of flow of this charge is governed by the potential difference between the two points.

1. ELECTRIC POTENTIAL

Electric potential of a point is defined as the work done in moving a unit positive charge from infinity to that point. Mathematically electric potential is given by;

The potential at infinity is zero. For convenience, the earth’s potential is taken to be zero and can be considered as infinity. Therefore electric potential of a point can be defined relative to the earth.

Electric potential is a scalar quantity. Work is measured in joules (J) while quantity of charge is measured in coulombs (C). Therefore electric potential is measured in J/C or the volt (V).

Therefore a point is at a steady potential of 1 V if 1 J of work must be done to move a 1C of positive charge from the earth to the point.

Meanwhile a point is at a steady potential of 1000 V if 1000 J of work must be done to move 1 C of positive charge from the earth to the point.

The electric potential of a neutral body or earth is zero.

Positive and negative potential

A point is at a positive potential if work must be done (energy used) to move a unit positive charge from the earth to the point. Conversely if a point is at a negative potential, energy is instead released when a unit positive charge is moved from the earth to the point.

This is illustrated in the figure below;

Electric potential is comparable to gravitational potential. For example, the higher you go the greater the gravitational potential. To raise a body to a certain height, energy is used, but when a body falls to a lower altitude, energy is released.

Potential difference and electron flow

If two points are at different potentials, then a potential difference (p.d.) exists between them. Work must be done to move a unit positive charge from the smaller to the higher potential.

By definition potential deference (p.d.) or voltage between two points is the work done in moving a unit positive charge.

If two points at different potentials are joined by a conducting wire, electrons will flow from the lower to the higher potential until both points are at the same potential. That is, electrons will always flow towards the more positive potential and this constitutes an electric current (figure above). In doing so, energy is always released.

Cells and batteries have two terminals. Chemical reactions inside the cell or battery create a potential difference between the two terminals, one being positive (higher potential) and the other negative (lower potential). If the two terminals are connected with a conducting wire, electrons will flow from the negative to the positive terminal. In so doing, the potential energy of the electrons is released in the form of electrical energy which can be used to light bulbs, play radios, etc.

Potential difference between two points is measured in volts using an instrument called the voltmeter.

If the potentials of the two points are know, then the potential difference is given by;

                Potential difference = higher potential - lower potential

1. ELECTRIC CURRENT

Electric current is the rate of flow of charge in a wire. Mathematically:

Electric current = charge movedtime taken or in symbols,  I=Qt →Q=It

Electric current can also be calculated if the number of charges moving is known.

Quantity of charge, Q = ne →I=net and n= Ite 

The SI unit of current is the ampere (A), defined as the current in a circuit when 1C of charge moves through a section of the circuit in s. Therefore 1A = 1 C/s.

The measuring instrument for current is the ammeter

How current flows in a conductor

All conductors have free mobile electrons which move about randomly in all directions.

The number of electrons moving in one direction at any instant is balanced by the number moving in the opposite direction.

But when a battery is connected across the ends of the conductor, the electrons (which are free to move about) accelerate towards the positive terminal of the battery this constitutes an electric current.

Non-metals do not conduct electricity because they do not have free mobile electrons as all the electrons are used in covalent bonding.

On the other hand, graphite conducts electricity because in each atom, only three of the four valence electrons are used in covalent bonding while one is free to move about.