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Basic Physics lesson-14 : Voltage and Current

Umesh is a freelance writer contributing his creative writings on varied subjects in various knowledge and educational sites in internet.

basic-physics-lesson-14-voltage-and-current

Introduction

In the earlier Physics lessons in this series we learned the basic concepts of linear motion of bodies and laws governing them, then we learned about gravity. We also learned a good deal about force, work and energy. Going ahead, we learned about frictional forces. Further, we discussed and learned various other types of motions like (1) circular motion, (2) rotational motion, and (3) simple harmonic motion.

Now, it is time to move to other learnings in Physics by understanding the basic Physics principles behind them. In this particular lesson we are taking up the field of electricity and will try to understand about two basic entities that are voltage and current. When we deal with electricity, we often hear these words. They are very commonly used by even the laymen. So, let us learn some basics about them.

Recollect

Electricity

Electricity has revolutionised the human life on this planet in great ways and the invention of electricity was not only an amazing thing but was also one of the greatest leap in modernising the whole world and today it is the most used essential commodity in our households. When we talk of electricity from a layman's perspective then it simply means that some voltage is coming to my house through the electric line and some current is flowing in that line. So, in this lesson we would try to understand what that voltage and current actually mean and what are the basic scientific concepts in this regard.

Understanding voltage from an electric cell or an electric battery

To understand voltage let us go back to olden days when the manufacturing of electric cells (dry cells) or batteries (car batteries) was started for commercial use like lighting a torch bulb or sending a current through a wire or running an electric circuit. So let us take an example of the electric dry cell that we use in our torch or wall clock or toys. These cells come in various sizes and used in various gadgets. Nowadays, low electrical power button cells have also become very common as devices working with lower electrical power have been invented. Anyway, for understanding purpose we would restrict to the common dry cell available in the market. Generally, they have a rating written on their label as 1.5 Volt. In scientific language we call this voltage as the electromotive force (EMF). This denotes that the cell has a packaged electrical power and it can send a current through a material when it is connected to the two terminals of this cell. The term battery is also used in place of cell by laymen but actually battery means a bank of cells. For example in a car a 12 volt battery is used which is generally comprised of 6 cells, each having a voltage capacity of 2 volts. As they are added together in a particular pattern called 'in series', the total pack becomes 12 volt. We will come to know shortly how they are added when we come to the ways of using multiple cells sometimes for increasing voltage and interestingly sometimes for increasing current also when it is so required.

Voltage can also be perceived as the electrical pressure from an electrical power source that pushes charged electrons (the tiny atomic level negatively charged particles that are present in the atoms of the elements) that can move through a conductor (like a metal wire) and that flow of electrons is actually the flow of electrical current in scientific language.

The word voltage is coined to honour the scientist Alessandro Volta (1745-1827), inventor of the voltaic cell, the primitive form of present household dry cell.

The easiest way to understand voltage is to understand about electric charges which are present on the terminals (electrodes) of a cell. Such a cell consists of two electrodes in between which an electrolyte (a chemical) is placed and the reaction between these electrodes and electrolyte produce some charged ions (positive and negatively charged) which are essentially the source of electric power that we are able to see at the two terminals of a cell. These cells will give electric power for some time and when the electrolytic material is fully used up the cell ceases to generate electricity and we throw it and replace with a new one. Later on, some improved and different cells were invented which could be recharged from mains electric power in our houses using a special charger. These cells are known as rechargeable cells and they can be used repeatedly for quite a number of cycles of recharging.

Understanding current capacities of electric cells and batteries

These small cells or small batteries are having limited power. When we say limited power then we mean that their current providing capacities over a time period are low. To understand it better just go through the label on your mobile battery where its voltage rating and current providing capacity are mentioned. Generally, voltage mentioned would be around 4 to 5 volts but current capacities will vary tremendously and that actually determines as how long the battery will work before it is required to be recharged for next cycle of use. Ordinary phone batteries would have a current capacity of say 500 mAh (milli ampere hours) while good phones can have even 5000 mAh or more. What does it mean? It simply means that the above ordinary mobile phone can deliver a current of say 100 mA for 5 hours while the good and advanced phone as mentioned above can deliver same current for a good 50 hours. That is the major difference between them. Their voltage is same but their current providing capacities are amazingly different.

Let us compare this with a car battery which has generally a voltage rating of 12 volt and a current providing capacity of say 50 Ah (ampere hours). That is actually equal to 50000 mAh, which practically means that it can give 100 mA current for a whopping 500 hours!

Relationship between voltage and current

As we discussed above that voltage is the electromotive force available between the two terminals of a cell then what is the meaning of electric current and how it is related to the voltage?

Relationship between voltage and current is very straightforward and in simple terms it depends on the nature and characteristics of the material across which this voltage is applied. In scientific terms we call this characteristic as the electrical resistance of the material. For example if we connect a metal wire across a cell a very high current will flow through it and the wire will heat up and the cell will get discharged soon. We have to avoid such mishaps as they are called 'short circuiting' in electrical field and it means that we have shorted the electrical source that is cell in this case. It can even damage the cell due to that high current. Why did a high current flow through the metallic wire? The answer is that metals have a low electrical resistance and if we apply a voltage to them, a high current would flow. Now in place of a metal wire if we take a wooden stick and connect it to the cell then what happens? Nothing happens as wood is a high resistance material for electricity and no current flows through it. So if we have a source of electrical energy like a cell or battery then it is not necessary that it will be able to force an electrical current in every material. It would depend on the material to which this electrical energy is being fed. This is a very important point and we should understand that current is a totally dependent factor.

For conducting materials like metals the relationship between voltage and current is governed by the famous Ohm's law -

Voltage = Current x Resistance

or in scientific notation:

V = I R

A simple exercise:

A filament type electrical miniature bulb is connected to a 1.5 Volt cell and starts glowing. The current flowing through the bulb is 150 mA. Find out the resistance of the bulb.

Let us use Ohm's law to solve the above -

Given V = 1.5 Volt, I = 150 mA = 0.15 A (converting mA to A)

So R = 1.5 / 0.15 = 10 Ohm (Ohm is the unit of resistance)

How to increase voltage or current capacity using more cells

This is a very important concept and once understood it helps in understanding many phenomenons in the field of electricity.

For increasing the voltage the cells are to be connected in series one after another. Every cell has two terminals one is positive and other is negative. Connecting in series means that the negative terminal of first cell will be connected to the positive terminal of the second cell. Now these two cells are in series and the total voltage between the positive terminal of the first cell and negative terminal of the second cell is now two times of than that of one cell. It means if we connect two 1.5 volt cells in that fashion it would become 3 volt and in fact it is used in a torch in the same fashion where this is used to glow an electrical bulb which requires 3 volt to glow. One can similarly use more cells in series and voltage will go on increasing.

The other way to connect is 'in parallel'. In this scheme we connect the negative terminal of the first cell to the negative terminal of the second cell and positive terminal of the first cell to the positive terminal of the second cell and the common negative and common positive points become the two terminals of this cell package but its voltage remains only 1.5 volt. One can put as many cells in that fashion but the voltage will remain only 1.5 volt. So, what is the gain to us? We have gained in the current providing capacity as the package is now able to give more current as compared to a single cell. Its capacity has increased, almost two times.

Let me explain it by an example of a battery bank where someone having a number of 12 volt 100 Ah (ampere hour) batteries with him, requires to make a high current capacity battery bank of 36 volt having 400 Ah (ampere hour) capacity for use in some equipment. How can he make it? So, what he will do is that first he will connect 3 batteries in series making it 36 volt. The current capacity remains at 100 Ah only. Like this he will make 4 sets. After making this he will connect these 4 sets in parallel and he will have a battery bank of 36 volt, 400 Ah.

Direct current (DC) and Alternating current (AC)

So far, we have not talked about the direction of the electric current because what we learned till now is that the current was flowing from the cell or battery into the torch bulb or some circuit in our car or some toy and it was actually direct current as it was flowing in one direction only. As per the scientific convention and nomenclature the direction of current is taken opposite to the direction of flow of negatively charged tiny atomic particles that is electrons. If we connect a torch bulb through metal wires to a cell then these negatively charged electrons will get attracted to and flow from negative terminal of cell to the positive terminal through these wires and the bulb. So, as the convention is, the current direction is taken from positive terminal to negative terminal in the simple circuit mentioned. You might have seen an arrow mark frequently in the electrical or electronic circuits and that is this current direction.

Let us go back in the history when electricity was invented. At that time it was invented as a direct current. For quite some period the cities were lighted with DC current only. There were some inherent deficiencies in direct current transmission like line losses of electrical energy and some safely considerations, which were a big hindrance in sending it to far off place from the generation site. So, subsequently it was replaced with alternating current.

The major difference between a DC and AC is in the ways of the direction of current flowing. As we saw that in DC, the current direction is in one direction only but in AC the current changes its flow from one direction to the opposite direction so fast that we can not perceive it but it flows and glows a filament bulb or any other AC light with much ease and gives the illumination just like a DC current passing through a bulb. Further, It flows through the transmission lines also with much ease as compared to the DC. For those who are not from science background it will be surprising or amazing to note that in AC the current changes direction in 50 or 60 times (in western countries 60 and in India it is 50) in a second! This is known as the frequency of AC and is denoted as cycles per second or Hertz (Hz). This is the frequency with which it is travelling to far off places without much significant line losses which was an issue and big problem in DC transmission over long distances.

So, today the electricity which we are getting in our household, which we also commonly call as mains voltage, is AC (in some countries it is 110 volt, 60 Hz while in some others it is 220 volt, 50 Hz). If someone wants to use it to charge ones mobile which works on 4.5 volt DC then one has to use the mobile battery charger which generally comes with a new mobile or alternatively can be bought from the local market. A charger is a converter to change from mains AC voltage to 4.5 volt DC which we connect in the mobile to charge the rechargeable mobile battery.

Voltage and Current

Some practical observations

  • The electrical resistance of thin metal wires is more than thick wires of the same metal. That is the reason why we use thick wires for the electrical line (commonly known as power line) in our houses where high current is expected to flow and use thinner wires where less current is expected to flow.
  • When we use a simple screwdriver style line tester then we insert the metal tip in the mains electrical phase point and touch the metallic cap with our finger. A feeble current flows and a small bulb glows inside the tester telling us that line is live. We do not get any shock because due to presence of a high resistance inside the tester which comes in between the line and our finger a very small current flows through us to the ground through our feet and the circuit gets completed but we do not feel it. On the other hand let us visualise as what happens when we touch the mains point directly with our finger. We get a terrible shock because now there is no resistance in between and our body is exposed to the high voltage coming there (about 110 volts in US and about 220 volts in India), which sends a high current through our body giving us a electric shock.
  • If a 60 watt old filament type bulb is connected to mains 220 volts then current of about 0.28 Amperes (280 mA) flows through it. This is more and less the same current which flows through a torch bulb when connected to two dry cells of 1.5 volt connected in series (making it practically as 3 volt). This gives an interesting thing to observe that if we connect a torch bulb and 60 watt mains bulb in series and connect it to the mains voltage line then both will glow. As the resistance of torch bulb is very less in comparison to the mains bulb, it does not affect the current in the circuit. One can see it oneself by experimenting.
  • If you have small miniature bulb or LEDs each of which glows with 1.5 volt and the mains voltage in your area is about 110 volt then if you make a garland of these miniature bulbs by connecting them in series, then using 110/1.5 = approximately 74 of them, the garland will glow nicely. To get a slightly less glow you can add a few more LEDs in the garland. In my country, India, I will have to use 220/1.5 = approximately 147 such LEDs for making a garland to glow with the mains voltage (220 volt) available here.

Conclusion

Voltage and current are interconnected and depending on the voltage and the medium on which it is applied, the current would flow. Voltage is like an electrical pressure or energy available in a battery or mains AC line but we feel it only when we apply or connect it to a gadget and visualise the current flowing through that. Understanding voltage and current is fundamental in acquiring the learnings in the field of electricity.

References

1. https://www.electronics-tutorials.ws/dccircuits/dcp_2.html

2. https://www.fluke.com/en-in/learn/best-practices/measurement-basics/electricity/what-is-voltage

3. https://www.electronics-tutorials.ws/dccircuits/electrical-energy.html

4. https://byjus.com/physics/ohms-law/

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Umesh Chandra Bhatt

Comments

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on November 09, 2020:

Lakshmi, thanks a lot for your visit and comment.

Lakshmi from Chennai on November 09, 2020:

Hi, great article with an elaborate description.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on November 07, 2020:

Liz, thanks a lot for your visit and acknowledging my little effort. Highly appreciate.

Liz Westwood from UK on November 07, 2020:

I have a vague recollection of studying this. It is a useful topic to refresh in our memories.