# Electric charge

Learning objectives

Display equations ?

# Electric charge

From the moment we wake up in the morning, we barely stop to notice the sponginess of the pillow pressing against our face, the hardness of the floor beneath our feet, the wetness of the water splashing on our hands or the softness of the clothes brushing against our skin. The routineness of these feelings has in some sense made us numb to them.

But if we choose to pause and pay attention to the details and even ask questions to find out why things are the way they are, we’d find ourselves living in a more beautiful and stunning world. If only one morning as we wake up we wonder what makes the pillow cushiony and the floor hard, why is water wet and clothes soft, and spend at least a few minutes seeking answers to these.

Let us make a start now, let us ask what makes hard things hard and soft things soft? What gives shape and size to different objects? At first, such questions might sound silly and we might be tempted to think that hard things are just hard, that’s the way it is. But science has taught us that everything has a reason and often a profound one. So, we need to be a little more patient, have faith that there is an answer waiting to be found by us, and continue thinking.

So, what gives things their shape and hardness? For example, if we press hard on a pebble with our hands, we feel its hardness but do the same with a piece of sponge and we feel its softness. The sponge gives in essentially because it has a large number of pores in it that allow for the compression. You’ll know that sponge has pores if you dip it in water.

At first we might be tempted to say, as was the practice of the ancients, that it is \textit{in the nature of the material} of the objects to be hard or soft. But we would like to find out why that nature is so. And it turns out that there is a simple but profound answer to this. Electric forces give matter shape, size, texture and hardness.

To get started, let us put on some rubber shoes, walk on a rug for a while dragging our feet a little, and then touch with our bare hands a metal door-knob. What we’ll experience is a mild electric shock. This gets us wondering, what exactly caused us to feel the shock? Let’s repeat the same experiment, except this time let’s walk barefoot on plane floor and then touch the same metal knob. No shock this time. It should have been the rubbing between the soles of the rubber shoes and the rug that lead ultimately to the shock. If we understand what exactly happened during the rubbing, we’d have glimpsed something deep about nature.

Since we’ve figured out it was the rubbing between the rubber shoes and the rug that lead to the shock, let’s rub together as many things as we can get our hands on and wait for a surprise. We try a variety of materials from plastic, wood, rubber and glass, to different fabrics like silk, cotton and fur. During one such specific trial, when we rubbed a glass rod with a silk cloth, we observed something peculiar – every time after the rubbing was done and the glass rod and the silk cloth were again brought close to each other, the cloth clung to the rod (like how opposite poles of two weak magnets do). But if we separate the two and bring them close again, the cloth no longer clings to the glass rod (unlike how it is with magnets that continue to attract).

We again suspect that something happened during the rubbing of the glass rod and the silk cloth that then caused the cloth to stick to the glass. Incidentally, we came across one more of such phenomenon, when we rubbed a rubber rod against cat fur. The fur would momentarily stick to the rubber every time we brought them close after rubbing them together but the mysterious attraction would still vanish if we separated them and brought them close again.

The best we can make of these observations at first is to think that something happens when different materials like glass and silk or rubber and fur are rubbed against each other – more specifically, these materials on rubbing, gain a strange ability to attract one another, but lose it as soon as they touch.

To make the concept of charge more intuitive, we might even imagine an invisible fluid flowing from one object to another during the rubbing. After the rubbing, one of the objects has excess of this mysterious fluid and the other is deficient of it. It is in order to restore the lost balance in the fluid concentration that the attraction occurs. Once brought together again, the two objects exchange the fluid, restoring the balance.

We further note something interesting: when the rubber rod after rubbing with fur is brought close to the glass rod after rubbing it with the silk cloth, the two tug at one another. We might now begin to think that rubbing always results in attraction – not just between things that are rubbed against one another but in general between rubbed objects. We hence must expect two rubber rods rubbed separately (with two different pieces of fur) must attract one another, too. To our surprise, we see that two such rubbed rubber rods do not attract but repel one another. Similarly, two different glass rods that have been rubbed (with two silk cloths) separately, also repel one another.

It may seem a bit confusing at this point. But let us try the fluid-exchange description we proposed earlier and see if we can go any further with it. When a rubber rod is rubbed with fur, let us imagine the invisible fluid flows from the fur and deposits in excess on the rod. If we bring another such rubber rod that in itself contains an excess of the fluid, it is natural for them to repel since there is no room for any exchange (neither is deficient of the fluid). But when the glass rod is rubbed against a silk cloth, say the reverse happens – the fluid flows from the glass onto the cloth, so that the glass rod is deficeint of the fluid. Would it not be natural for this glass rod to be attracted towards a rubbed rubber rod? The rubber has excess of the invisible fluid and the glass is deficient of it. When they touch one another, fluid exchange happens and the natural level of the fluid is attained, causing the attraction to vanish.

Therefore it all comes down to the losing of an excess of the fluid and the gaining of it if there is a deficiency – ultimately attaining a balanced amount of the fluid in every object. Using this description, you could even explain what happened when you walked with your rubber shoes on a rug and then touched the metal knob. When you dragged your feet along on the rug, there was a flow of this fluid from the rug onto your shoes and hence you. This excess fluid discharged into the metal knob the moment you touched it. The discharge giving you a sensation of electric shock. It still isn’t fully clear as for why the fluid would want to flow into the knob, but we’ve come a long way in finding a meaningful answer to the initially puzzling happenings with rubbed objects.

There, however, is a serious problem with this explanation. If the fluid is invisible for all practical purposes, how can we be so sure that it even exists? Could we as well not do with an invisible fire that flows from one object to another on rubbing? Or even an invisible ghost? The point is, all we suspect is that there is an exchange of some entity when objects are rubbed. We don’t know what this entity is exactly. We know that it is likely that it flows. Hence we imagine it as a fluid.

In order to avoid imagining hypothetical, invisible fluids or fires, let us name this entity that transfers from one object to another on rubbing, about which we are yet to learn more, \textbf{charge}. We can therefore say that when say glass is rubbed with silk, charge flows from the silk cloth onto the glass rod. This imbalance in charge causes them to attract one another. So that when they are brought together again, the glass loses the excess charge and returns it to the silk cloth. Similarly, when rubber is rubbed against fur, charge flows from rubber to fur, leaving the rubber rod with a deficiency of charge and the fur with an excess of it.

To keep track of which object has excess and which a lack of, the charge, let us call an excess of charge, positive charge and a lack of it, negative charge. Note that we aren’t introduing two types of charge here, rather just two forms of it. And all of this solely for our convenience. It now becomes easier to say that when rubber is rubbed against fur, the rubber loses some charge and hence becomes negative while the fur gains the same charge and becomes positive. Similarly, when glass is rubbed against silk, the glass gains the charge and becomes positive while the silk cloth becomes negative.

Is it not now convenient to say that the negatively charged rubber rod attracts the positively charged fur. Likewise, the positively charged glass attracts negatively charged silk. And more interestingly, positively charged glass attracts negatively charged rubber, but when allowed to touch, the positive and negative charges flow back, leaving the objects in their original uncharged state. The same is true when we speak of two negatively charged rubber rods or two positively charged glass rods repelling one another.

A further generalisation is that charging by rubbing is not unique to rubber, fur, glass and silk. A variety of other materials such as plastic, mica, nylon, wool, aluminium, paper, steel, wood, copper, gold, plastic, etc

All we need to remember is when two objects are rubbed against one another, charge flows from one object to another. The one that loses the charge is said to become negatively charged and the other that gains the charge is said to become positively charged. Thereon, until the two objects are touched, positively charged objects will attract negatively charged ones but repel other positively charged objects. Positive-negative attract, positive-positive repel and negative-negative repel. But once the objects are touched, the charges flow back, and all attractions and repulsions vanish.

So far, it seems as though all we have done is logically refined the language used to describe what’s happening when two different objects rub against one another and begin attracting or repelling one another. And such logic is built with experimental observations in mind. It however is not fully refined. We haven’t answered questions such as: why does charge flow on rubbing? Why does it flow one way and not the other? Is charge real or is it just convenient vocabulary?

Answering these questions will take us deeper into the mysteries of nature and will eventually lead us to an understanding of the question we raised earlier – what gives things their shape, size and hardness – what makes pillows soft and floors hard.

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