Learn Basic Physics - Understanding Electric Voltage


Do you want to learn the basics about electric voltage, a term used by all of us in connection with the electricity being used in our houses? This article provides in details what electric voltage is, from where it originates, and what for it is used.

Introduction

The invention of electricity was one of the greatest milestones in scientific research and development and later it helped the scientists and researchers to devise so many machines and gadgets running on it. The houses were lit up with electricity and electric bulbs were invented with different illuminations to be used at different places. With the advent of time, new gadgets and appliances were designed and invented which worked on this hidden but extremely useful source of power. The advances in electronics and electrical industry made revolutionary contributions in our lives and today we cannot think of life without electricity as many of the things we use are running on it. We require it continuously in an uninterrupted way.

We often use the word voltage when we talk about the electricity that comes to our house through the transmission line and our elders often tell us that there is a voltage in the line and one must be careful and cautious not to touch any open-wire or electrical point with naked hands otherwise one can get an electrical shock which may sometimes be fatal for the person. In this article, we will like to learn the basic concepts as what this voltage is and how it is generated and what it does when it is used in various ways in our house or to that matter in the shops and industries around. Once the fundamental concepts about this basic entity are understood we can then move to understand other concepts in the physics about the field of electricity.

Understanding voltage from the basic electric cell or battery

To understand the entity voltage we will first try to understand the basic principle behind an electric cell or electric battery that we use at so many places in our household. We use cells in a torch for glowing the light in the dark, we use them in the wall clocks, we use cells in toys and other electronic accessories, we use a battery in the car which starts our car, we use a battery in our mobile for its working etc. Cells or batteries are sources of electrical power for our appliances and gadgets. One thing that is common with all these electrical sources is that they provide some electrical voltage to us which is used to operate the various gadgets.

A basic electric cell has two metal electrodes and in between them, some electrolytic material is placed. The chemical reactions between these electrodes and electrolyte produce some charged ions and these charged ions are the source of the electrical power. To understand the things more clearly one has to understand that the atoms of the elements in an electrolytic material as well as the material of the electrode have negatively charged electrons some of which become free of the atoms during the chemical reactions and can move through the electrolyte. So the net result is that there is some electrical charge accumulation between the two electrodes and if externally we connect a metal wire between these two electrodes the charges would start flowing through this wire and then we say that some electric current is flowing in the wire. So, there is some electrical power between the two electrodes and in scientific terms, we denote it with the word voltage. This word is coined to give honour to the great scientist Alessandro Volta who invented the first electrical cell.

So, each cell has two terminals corresponding to these electrodes and we call them positive and negative terminal. If you look at the simple torch cell that comes in various sizes you will find a capped metal tip which is the positive terminal and at the other end of the cell, you might see a metal flat plate which is actually a part of the container for the electrolyte and also working as the negative terminal. A basic torch cell is generally of 1.5 -volt rating which means that it has an electrical voltage equal to 1.5- volts between its two terminals. If we connect a torch bulb with the help of two conducting metal wires to the terminals of this cell, an electric current will start flowing through the circuit and bulb will start glowing giving us light.

Cell and battery

We use these two terms interchangeably but strictly speaking, a battery is a composite of many cells while a cell is a single unit. If you observe a car battery then generally you would find that it is made of 6 cells each having a voltage of 2 volts and because we have connected them in a series the total voltage across the battery is 12 volts (6 x 2 volts). So, the battery is nothing but many cells connected in a particular fashion.

The current capacity of a cell or battery

As we have seen that there is an electrical voltage present between the two terminals of a cell, now we have to find what good for this voltage is? What is its use? To understand this, first, we will learn what are the materials which conduct electricity and what are those materials which do not. For example, a copper wire is a very good conductor of electricity and we use it for electrical wiring in our houses. Aluminium wire is also a good conductor but not as good as the copper, so this is the second choice for the house wiring. Now take a wooden stick which is a non-conductor of electricity and connect it to cell terminals through some copper wires. What happens? Nothing happens. No current flows through the wood. It does not respond to the voltage of the cell. It means that if we have a voltage source like a torch cell then we can use it to send current only in the conductors. To understand it more clearly let us take the example of a torch bulb (it could be an old filament type bulb or modern LED bulb) and it would glow when connected to a cell using copper or any other conducting metallic wires. The bulb is glowing now because the electrical current is flowing through it. If we disconnect the connecting wires from the terminals of the cell the circuit will be broken and no current will be able to flow. So, remember, current will only flow when the circuit is complete. How do we off the lights in our house? By putting the switch off which breaks the circuit.

Once we have understood that current flows through certain materials when they are connected to a voltage source, the next question which comes in the mind is how much current would flow? The amount of the current depends on the electrical resistance of the material through which it is flowing. If the electrical resistance is more, less current will flow and if the electrical resistance is less, more current will flow. This is a very important concept in electricity.

This is about the current which flows through a material. Now, what about the power or strength of the voltage source? How much current it can deliver? Can it deliver unlimited current? The answer to this question is very simple that every cell has a capacity of delivering current up to a limit only and if we try to draw more from it then it will fail and would not be able to provide it or even may get damaged in the process. So this current delivering capacity is an important thing and we should not draw current beyond the stipulated limit from a cell or any other type of voltage source.

What is a short circuit ?

At this point, it will be relevant to understand what is a short circuit which often happens in our houses and the main switch or circuit breaker automatically goes to off position. In layman language, if the current in a circuit increases beyond a permitted limit then it is a condition of short circuit and the fuse or cut out provided in the electric line goes to off position and the circuit is broken and current ceases to flow in the circuit. This is a safety measure otherwise high current flowing through the circuit will damage the circuit itself and the connecting wires might get burned and start a fire. That is the importance of circuit breakers or fuses which are mounted at the electricity main panel in our houses.

How powerful a battery is and how long it will work

A button cell (generally used in children toys or wristwatches) can deliver small currents, a torch cell (the common cell used in various household purposes like the TV remote, wall clock etc) can deliver better currents, while a car battery can provide large currents. So, they all have different powers or capacities. Technically, it is denoted with a unit known as Ahr (Ampere hour). The small unit is mAhr (milliampere-hour where 1 Ahr = 1000 mAhr).

So one thing is clear now that there is a limit of current that we can draw from a cell. Another thing is the time for which we can draw this limited current from it. The cell will provide limited current for the period as determined by its Ahr capacity and after that, it will be discharged fully. If it is an ordinary cell like the torch cell then we have to throw it and put a new cell in its place. But if it is a rechargeable cell like the battery of a mobile phone then we have to charge it to regain its electrical power and then we can use it by charging every time it is fully utilised and discharged. We can use it for so many times but once the battery has gone bad or old then we have to go for a new battery even for mobile.

To understand the above concept in a practical way a 500 mAhr mobile battery can provide a current of 250 mA (milliampere) for 2 hours only while a 5000 mAhr mobile battery can provide the same current of 250 mA for 20 hours. So the first mobile will require charging much earlier than the second one.

Series and parallel combination of cells

Sometimes we want to increase the voltage or increase the current capacity of a combination of cells by connecting them in a particular fashion which leads to making of a battery bank. For doing this there are two rules. If we connect the cells in series the voltage will increase. If we connect the cells in parallel voltage will remain the same but it will increase the current capacity. Let us take a simple example of connecting some car batteries each of 12- volt rating to make a required battery bank which will give 48 volts for some specific purpose. So what we do is we just connect 4 of them in series by connecting the positive terminal of the first battery to the negative terminal of the second and the positive terminal of the second battery to the negative of the third battery and so on and after this, we will have 48 volts between the negative terminal of the first battery and the positive terminal of the fourth battery. This is called a series combination. Suppose we feel that this combination might not provide the sufficient current for our purpose for which we are using them, then what we would do is we will make another same set with other 4 batteries in series and these two sets we will connect in parallel so that the voltage of this combination remains at 48 volts only but the current capacity is almost doubled. So these are the techniques of increasing the voltage as well as current capacities of the combinations in a battery bank.

Direct Current (DC) and Alternating Current (AC)

So far we have learnt about the voltage from the examples of cell or battery. Let us now think of the main voltage that is coming to our house for illumination and running our gadgets. What is it? It is also an electric voltage but it is not like the voltage that we are getting from a cell or a battery. The main difference is that in a cell the electric current in the circuit or the torch bulb or a toy flows in one direction only. This is known as Direct Current (DC). The convention adopted in Physics is that current is shown as flowing from positive terminal to negative terminal outside the cell. One can observe it while seeing some electrical or electronics diagram where an arrow is shown to depict the current flowing. But in the mains voltage coming to our house, it is not so as it changes its direction very fast about 50 times in a second and that is why it is known as Alternating Current (AC).

Historically, when electricity was invented, in the beginning, DC voltage was only there in the cities to light the houses. But it was found that the electrical power loss in the transmission lines with DC was quite substantial and later when AC voltage was produced it was found that the line losses were minimal and then AC was adopted in a big way and today we have AC only flowing through the electrical transmission lines to our houses.

Generation of electric voltage

The electricity which comes to our house (generally known as 220 volts mains voltage) is generated in a power generation unit commonly known as a powerhouse. It could be a generated by falling water on the turbines of a hydroelectric generation unit or it could be by burning of coal in a thermal power plant or converting the nuclear energy to electricity in Nuclear energy plant, the result is electricity only. This electric voltage is sent to our houses through a transmission line made of conducting metallic wires. The nature of this voltage is alternating (AC) and has a frequency of 50 c/s (Hz). All our gadgets in our house, work on this main source of electrical energy. Only thing is we have to use it appropriately. For example, our mobile device generally requires an electric source of 4.5 to 5 volt (DC source) to get its battery charged every time it is discharged. So for that, we require a charger inside which we have an electronic circuit which converts the mains 220 volt AC into 4.5 / 5 volt DC and then only we can use it. Please note that electricity is a great enabler but it has to be used in the right way and the prescribed manner. Everything is having a rating. We have to use the correct source of electric voltage for our purposes.

Current changes from gadget to gadget

This is one important point to note that though the voltage source is constantly giving us the same voltage the current in different gadgets will be different depending upon the design and electrical power utilisation of the gadget. We will just see some examples and it will be more clear. The unit of current is ampere and remember the conversion 1 ampere is equal to 1000 milliampere (1 A = 1000 mA).

An old filament type electric bulb of 100 watts used with 220- volt mains voltage in our house draws a current of about 450 mA (milliampere). A modern LED bulb of 10 watt draws only 50 mA (though it can give comparable glow as compared to an old 100- watt bulb). That is why electric consumption in the LED bulb is much less. Consumption depends on current. An electric iron of 800 - watt rating draws a high current of about 4 A (4 Amperes). A microwave or OTG having a rating of 1500- watt draws a current of about 8 A (8 Amperes). These gadgets consume high electrical power inflating our electric bills.

Conclusion

Electric voltage is the measure of the electric potential in the source of the electrical energy and we use it for various purposes. The electric current is a dependent factor and depends upon the material or gadget which we connect to this power source. The consumption of electricity in any gadget or circuit or electric device depends upon how much current is flowing through it.


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