Introduction The invention of electricity was a big turning point leading to further progress and development is scientific areas. Lighting an electro bulb by just pressing an electrical switch was one of the most surprising and amazing events in the scientific history and then there was no looking back and today everything in our life and around us is happening by the flow of currents in the electrical and electronic circuits in our houses or in the electrical and electronic gadgets. Electricity has become a main source of energy in our household and from lighting a room to heating and cooking everywhere we are using it in a variety of ways. It has also got a big role in industries where a variety of machines and equipment run on this invisible power.
In another related article we learnt a good deal about the electric voltage that is either produced in a battery or cell (like a car battery or a torch cell) or in a power station (like a hydroelectric power plant or a thermal electric plant) and then used for lighting our houses or running electric motors or appliances like fan, heater, fridge, washing machine, geyser etc. Now in this lesson, we will try to understand the fundamentals of the electric current and learn about it in details. We would be learning in details about this crucial entity known as the electric current which flows in a medium or gadget when it is connected to the electric voltage source. We must distinguish between these two entities known as voltage and current as one is the source of electric power and other depends on the medium or material which is connected to the voltage source and then depending upon the electrical nature of that material, the current would flow through it. We would come to know more about it as we progress forward in our learning about electric current.
Electrical properties of materials There are various types of materials but they have different electrical properties. In some materials good electrical current flows but in some no current flows. There are some through which a limited small current flows. Let us try to understand why such a difference is there. The materials are categorised in different categories and types like metal, wood, plastic, water, milk, soil, cotton, salt, chemicals, wax, human body and many others. Now we have a source of electricity and let us say that we have a car battery. Let us connect the two terminals of the battery to a metal wire. What happens? The wire starts heating up because of the current flowing through it as metal is a very good conductor of electricity. Due to the flow of high current through this metal wire after a few seconds wire might burn due to the excessive heat generated by the flow of electric current and wire may eventually break in pieces. What happens now? The metal wire is burnt and broken, so current cannot flow in the open air as the air is a bad conductor of electricity.
As per the scientific understanding, the metals are having low resistance to the passage of electrical energy and that is why electric current flows through them so easily. Let us now take a car headlight bulb and connect it to a car battery using two electric wire pieces. What do we see? The bulb glows continuously. This time the wires do not burn. Why? Because the electric bulb is having a designed electrical resistance in it which controls the electrical current flowing through the simple circuit that we have just assembled ourselves.
In the first case when we had connected a metal wire directly to the battery terminals we literally shorted it. We have taken this as an example only for understanding purposes and in real life situation, we should never do it. It is unsafe to do so, as the wire would burn due to this shorting and the surrounding area might catch fire. It is advised not to attempt such a thing even in curiosity.
Let us now take a different material. Let us take a wooden stick and connect it to the battery using two electric wires. What happens? Nothing happens because the wood is a non-conductor of electricity and no current flows through it. A non-conductor is known as an insulator also. It will not allow electrical energy to propagate through it. In our house, many electrical fittings are having conspicuous insulators at places to avoid having direct contact with the naked electric wires as that might give a shock to us if we are accidentally exposed to the high voltage (220 volts) coming in our houses and is very much available on those wires.
At this juncture, someone might ask that when we touch a car battery terminal why we do not get any electrical shock while the household electricity does that to us if we accidentally make contact with a live naked wire. So, the answer lies in the value of the voltage which is a measure of the electromotive force applied on the medium or material through which it passes. A battery gives only 12 volts while the mains voltage is 220 volts. The human body can only tolerate voltages up to something like 60-70 volts (this may also change slightly from individual to individual) and after that everything is a shock to us and would have a devastating effect. Never touch a live wire with hands and always use gloves or other protective tools while manipulating them. The reason why the electrical wires are covered with an insulating cover made of plastic or rubber is very obvious now that is to protect ourselves from electric shock. Plastic and rubber are non-conductors and commonly used to cover the metal wires.
In our houses, the electrical wiring is done using the copper or aluminium wires from one point to another as these are the conductors of electricity and these wires take it to every gadget or machine fitted in our house. Metal wires are good carriers of electricity from one place to another.
Relation between Voltage and Current Many people confuse between these two terms (voltage and current) and use them interchangeably. It is not so. Voltage is the source of electrical power like a car battery (generally 12 volts) or the mains supply (generally 220 volts) coming to our houses. This source is fixed in its nature. But the current which flows in a medium or a gadget due to this electrical energy is not fixed and would change a great way depending upon the material or gadget connected to the source of electricity. A water purifier in our houses may consume only one-fifth of the electrical current as compared to that of the fridge though both are being fed from the 220-volt mains voltage. So it is interesting to remember that the voltage source is fixed in its nature but current varies significantly from one gadget or appliance to others.
If we keep all the electrical appliances in our house in off position and switch off all the lights then the total current flowing in our house reduces to zero. In such a situation, there is no consumption of electricity in our house. We pay every month the electric bill for the electric consumption and if it is zero then the electric bill will also be zero. In such a case, we would be paying only the minimum electric meter rent or things like that which would be merely something around a hundred rupee or maybe less than that. So, next time you with your family members leave your house for a long trip then ascertain that all electrical switches are in off position except the ones like fridge etc which you want to function even in your absence.
Flow of current in conductors - Ohm's lawLet us now try to understand the relation between the applied voltage and the resulting current that it takes to flow through a conductor. Ohm's law explains it and says that the voltage applied on a conductor and current produced in it is having a relation of proportionality between them which means that if you increase the voltage then the current would also increase and decreasing voltage will decrease the current. If we denote the voltage with V and current with I then using Ohm's law, we say that V is proportional to I.
In mathematical terms, it is written as V = IR where R is the constant of proportionality and is known as the resistance of the conductor. The unit of V is in volt, I in ampere, and R in ohm. This electrical resistance of the materials varies from one material to others and is the main thing that determines as how much current will flow through it when it is subjected to some electric voltage.
Relation of current with the size and shape of the conductorThe flow of electric current through various mediums is a complex process but when we talk of the flow of current in conductors then it can be easily explained by the flow of free electrons in the conducting materials. Generally metals or metal alloys like Copper, Aluminium, Iron, Brass, Tin etc are good conductors of electricity and that is why they are used to take the electric voltage from the powerhouse (a place where electricity is generated) to our houses or the shops in the town or industries present in any area. When we say good conductors, it precisely means that their electrical resistance is lower and they allow the electric power to propagate through them easily and in that process, a small amount of the electric power is lost through them which is technically known as line losses. A good electric company will adopt measures to minimise these line losses because they will be eating up a part of their revenues.
Have you anytime observed that the main electric line going from one place to another uses thick metal wires while inside our house we use much smaller diameter wires and in some of our electrical gadgets we even use a very thin wire or chord inside which metal wires are encapsulated? Using a particular size of the wires depends on the current considerations. Wherever we have a small flow of current we use thin wires and wherever it is more we use thick wires. In analogy, we can compare it with the flow of water in the town water supply. Wherever we want more water to flow we keep bigger pipes but for individual houses, we use smaller pipes of around one-inch diameter.
The electrical resistance of a conductor like an electrical wire depends on its size in twofold ways. First is the length of the conductor. The resistance of the conductor is proportional to its length. If the length of the wire is more its resistance will be more. For example, if the resistance of a 10-meter long Aluminium wire of some thickness is, say, 0.2 ohms then the resistance of the 400-meter long same diameter wire will be 0.2 * 400 / 10 = 8 ohms. The second thing is the diameter of the wire. It is of utmost importance and has many consequences in the electric industry. The resistance is inversely proportional to the area of cross-section of the wire. The cross-section means the small circular area that we obtain when we cut the wire perpendicular to its length. In mathematical terms the cross-sectional area of a wire having a diameter of 'd' is given by (pi * d^2) / 4 where pi is having a value of about 3.14 and reflects here because of the properties of circles which tells us that area of a circle is pi * r^2 where r is the radius of the circle. We also know that radius is half of the diameter. Now, let us increase the thickness of this wire by increasing the diameter to say four times so that it becomes comparatively a thicker wire. So the new diameter is now '4d'. What would be the area of the cross-section for it? Using the same logic we get the cross-sectional area as 4 * pi * d^2.
Please note that by increasing the diameter of the wire four times the area of the cross-section has increased by 16 times. As the resistance is inversely proportional to the area of cross-section it implies that the thick wire would have considerably less resistance than that of the thin wire. This relationship is used for optimal designing of electrical and electronic circuits and associated wire connections everywhere in the industry and is the basic building stone in the learning of electrical sciences or electrical technologies.
ConclusionElectric voltage is fixed in its nature but the electric current depends upon the medium or material through which it propagates and in this article, we tried to understand its propagation through the conductors like metal wires. We also learned about the entity known as electrical resistance which is a characteristic property of a medium or substance and impedes or accepts the flow of electricity through the material.