Net Charge
Net Charge
In our everyday experience we find that most objects have a nearly equal number of protons and electrons and are thus considered electrically neutral, or are said to contain no net charge. In some situations like when you go down a plastic slide at a playground or when you pull clothes out of a dryer, objects may acquire a noticeable net charge. In such situation there is an imbalance between the positive protons and the negative electrons on those charged objects. While the effects of this imbalance are often easily seen, the imbalance is still very slight.
Positively charged objects have missing electrons and negative objects have surplus electrons. We speak of the electrons rather than the protons since electrons are more easily transferred upon charging since protons are stuck in the nuclei of atoms.
We will soon see that if the number of unbalanced electrons became something approaching even 1% of the object's electrons, it would have devastating effects. Before addressing that problem, let's discuss how objects tend to acquire a net charge in nature. There are multiple ways for it to happen - more even than we'll discuss at this time. We will stick to the most common ways for now.
Charge Calculating
If we wished to find the mass of a macroscopic object, we would simply add up the masses of all the atoms making it up. Besides minute effects of the binding energies which we'll discuss toward the end of the book, the parts make up the mass of the whole. This works because all the little masses are positive. There are no negative masses in nature. So adding more little parts to a whole only ever serves to increase the total or net mass of the object.
On the other hand, when dealing with charge, the process is different since there are both positive protons and negative electrons. In most cases the electrons are roughly equal in number to the protons in the nuclei so that the total or net charge is zero. Let's take a moment to do this math more carefully.
Total Number of Protons
Given a piece of bulk material like iron, we can find the total number of protons by finding the number of atoms of iron and then recognizing that there are 26 protons in each iron nucleus. The total number of atoms can be found as is done in chemistry by using the known molar mass of iron which is where 1 mole is Avogadro's number of atoms, or If we know the mass of the bulk iron to be M, then the number of atoms N may be found by using:
This means that a 10kg plate at the gym contains N iron atoms. Plugging into this equation using M=10kg gives Given the fact that each iron nucleus (and atom) contains 26 protons, we have . To find the total positive charge contained in that 10kg plate, we multiply the number of protons times the fundamental charge and get So that plate contains 4.48 million coulombs of positive charge! That single plate would have a much bigger impact on your world than earth's gravity if it were not balanced by a roughly equal number of electrons in the 10kg plate. To give you a sense of how large a coulomb of charge is, a lightning strike only delivers around 10 to 30 coulombs of charge! So if all the charge in a block of iron could be liberated, we'd have serious problems.
Continuing our discussion, we should expect that there are just as many electrons as protons in the block of iron, and therefore the total negative charge is equal to Adding the positive and negative charge together, the plate has no net charge. That's not to say that a plate of metal can't have a net charge (it can), but unless we do something to it to charge it, it will be nearly zero left to its own.
If even one in a million of the protons did not have a corresponding electron, the object would have 4.48C of charge, which is nearly equivalent to a lightning bolt worth of charge! That's not going to happen. So you can expect that a very small fraction of the atoms will have extra or missing electrons in charged objects. Maybe one in a trillion or less.
It turns out that excess electrons will not be willing to stay on an object beyond some limit since they will repel one another. Too few electrons will also present problems beyond a certain point since the object lacking electrons will be very positively charged and will tend to "steal" electrons from the surrounding air molecules.
Electric welding torches operate on this principle of transferring electrons between materials (the torch and the workpiece), and the operator can actually choose whether the torch has excess electrons in which case it shoots them at the metal, or lacks electrons in which case it takes them from the metal being welded. The torch produces a sustained spark - like a mini lightning bolt. The choice of polarity (positive or negative torch) actually makes a notable difference to the operator since the choice will affect things like penetration depth and heat deposit of the weld. Can you guess which way makes the metal hotter (and torch less so)? Hint: Electrons are being transferred and they contain kinetic energy and momentum when they travel. If you guessed the torch being negative (and delivering electrons), that's correct. On the other hand, changing the polarity such that the welder is positive (and receives electrons) makes the majority of the energy heat the tip of the welder and less of it heats the workpiece.
Our Terminology
We now know about the great quantity of both positive and negative charge in macroscopic objects. Yet, as we discussed, in most cases objects have a nearly perfect balance between the protons and electrons, leaving the object neutral or uncharged. In our future discussions when we speak of a charged object, we will understand this to mean the net charge rather than the positive proton charge or the negative electron charge taken on its own.