An Introduction to Electric Current

In this article, we will discuss about the basics of electric current.

We know that matter is made up of small particles. These particles are atoms or molecules. An atom contains positive and negative charges in it. Positive charges are protons (present in nucleus) while negative charges are electrons that revolve around the nucleus in specific orbits. The electrons of an atom are strongly bound to the nucleus as there is a strong force of attraction between protons and electrons.

        But the atoms of some elements, called metals, are not strongly bound to the nucleus. The electrons of outermost orbit can easily escape out due to lack of attraction by the nucleus. These outer most electrons are named as free electrons. Metals have enough number of free electrons. When these free electrons perform random motion within the element, the effect of their motion is cancelled out by each other. But if we tend to move these free electrons in a uniform way, the result may be interesting. To move these free electrons in a uniform way, we need a push that bound all the free electrons in the element to move in a single linear path. This can be done by applying an external potential difference across the ends of a metal substance with the help of a battery or voltage source. This uniform motion of free electrons in a metal substance is referred as electric current. So, we can define the term electric current as

“The rate of flow of charges (electrons) through a cross sectional area is called electric current.”

Mathematically

                                      I=Q/t

        Here, Q represents the amount of charge passing through a cross sectional area in t time.

With respect to the flow of electric current, elements have two categories: Conductors and Insulators

Conductors

The materials through which electric current can pass easily and have low resistance are called conductors. For example, iron and copper are conductors.

Insulators

The materials through which electric current cannot pass easily and have very high resistance are called insulators. For example, wood and rubber are insulators.

Unit of Electric Current

The unit of electric current is coulomb per second which is also called ampere (A) named after the French physicist A. M. Ampere (1775-1836).

If the rate of flow of electric charges through a cross sectional area is one coulomb per second then the electric current will be one ampere.

One ampere is a very high current. So, we often use small units for electric current micro amperes or milli amperes.

Nature

Electric current is a scalar quantity. It means that electric current does not possess any direction.

Electric current is divided into two kinds (electronic current and conventional current) with respect to the motion of negative or positive charges.

Electronic current

Current that flows from the negative terminal of the battery to its positive terminal due to the motion of negative charges.

Convention current

Current that flows from the positive terminal of the battery to its negative terminal due to the motion of positive charges.

Very initially, it was considered that the electric current in a material flows due the motion of negative charges. But after that, some scientists assumed the motion of positive charges as electric current. Now, it is an admitted fact that the electric current is actually the motion of free electrons (negative charges). But it is convenient to study or consider the motion of positive charges as electric current because it flows from high potential (positive terminal) to low potential (negative charges).

Sources of Electric Current

            Electric current can be obtained from

·        Cells (in which chemical energy is converted into electrical energy)

·        Solar Cells (in which light energy coming from sun is directly converted into electrical energy).

·        Electrical Generators (in which mechanical energy is converted into electrical energy)

·        Thermo-couples (in which heat energy is converted into electrical energy)

Ohm's law about electric current

A German physicist Georg Ohm (1789-1854) stated that if we do not change the physical conditions of a substance (temperature and pressure) then the electric current flowing through it will be directly proportional to the potential difference applied across its ends.

              I∝V

or

             V∝I           

             V=IR          

Here R is the resistance of the substance which is actually the opposition to the flow of electric charges. The unit of resistance is ohm (Ω) named after Georg Ohm.

            We can categorize all the conductors into two groups: Ohmic Conductors and Non-Ohmic Conductors

Ohmic Conductors

          Conductors that follow ohm’s law and have constant resistance are called ohmic conductors. For example, a piece of wire is considered to be ohmic.

Non-Ohmic Conductors

            Conductors that do not follow ohm’s law and have variable resistance depending on the temperature are called non-ohmic conductors. For example, filament and semi conductors are non-ohmic.

Joule’s Law

            The energy dissipated in an electric circuit by a load (resistor, bulb or electric fan etc) was explained by an English physicist James Prescott Joule (1818-1889). The statement given by him about the dissipation of energy is known as Joule’s Law. According to this

            “When electric current flows through a resistor during a specific time period, then the total electrical energy dissipated is equal to the product of square of electric current I, Resistance R and the time duration t.

                             Energy=I2Rt

          This energy is utilized in electric fan, iron and electric bulb etc. These devices convert the electrical energy into mechanical energy, heat energy and light energy respectively.