![]() Creating the force law How does the force depend on the charge?įirst, let's ask ourselves how the force between two charges must depend on the magnitude of the charges. This is just the reciprocal of the number of charges in a Coulomb:Į = the magnitude of the charge on a proton or electron = 1.6 x 10 -19 Coulombs. Since we are going to use "Coulombs" instead of counting protons and electrons, it's useful to introduce the number that is the value of the amount of charge on an electron or proton in our Coulomb unit. It will turn out to be quite convenient for checking and proposing new electrical equations using dimensional analysis. The result is the standard unit for charge is taken to be the Coulomb:ġ Coulomb = the charge on 6.24 x 10 18 protons.Īctually, it seems rather reasonable to do this, since counting up to 10 18 protons might take us a very long time! But since we have a new arbitrary measurement scale, we can assign a new dimension to go with M, L, and T (mass, length, and time). ![]() The understanding of matter as made up of atoms didn't come till almost 100 years later and the understanding of atoms as made up of electrons and protons not till 50 years after that.Īs a result, the scale for measuring charge was based on other things. Electrical forces were analyzed and the theory worked out in the last half of the 18th century. One way to assign a number to the charge on an object is just to count.Ī plausible way to quantify charge would be to assign the number: Q = (number of protons) - (number of electrons). (We are temporarily assuming all these charges are pretty much in the same place so we are ignoring polarization effects for now, since they are much smaller than the electrical effects from unbalanced charges - though polarization does turn out often to be important!) The total effect of neutral matter would be zero and observable effects would come from having an excess of one or another. ![]() Since they seem to cancel and be equal, it seems that a natural way to assign a number to charge is to count the number of protons and subtract the number of electrons. This is why atoms with the same number of electrons and protons don't seem to have obvious electric effects. ![]() The electrical effects of an electron and a proton on other charged particles are opposite to each other and equal in strength. One basic fact that is easily demonstrated is: "Charge" is what we are calling that property of the basic particles of matter - electrons and protons - that create and feel the electric force. We first have to answer some questions about charge: What is it? How does it behave? And what mathematical model is appropriate for describing it? Quantifying charge In this webpage, we will elaborate these four ideas to create a model of electric force that both encompasses these qualitative ideas and allows us to quantify electric forces so that we can use them in Newton's 2nd law. The forces two charges exert on each other should be equal and opposite (satisfy Newton's 3rd law).The direction of the force between charges is along the line from one to the other.įrom our work with the Newtonian theoretical framework, we also expect.The forces between charges get weaker as the charges get farther apart.Charges of the same kind repel each other, charges of different kinds attract.When you bring them close to another pair of charged tape strips, they either repel or attract.) Then you will see that the dangling ends of the tape strips attract each other. ( You can see this for yourself in a simple experiment with Scotch Magic Tape TM,: Simply pull two strips of tape off each other - both strips become charged. In the SI system of units, the value of the elementary charge is exactly defined as e īy combining the best measured value of the antiproton charge (below) with the low limit placed on antihydrogen's net charge by the ALPHA Collaboration at CERN.There are two kinds of charge and there are three qualitative properties of the electrical force between charged objects. The elementary charge, usually denoted by e, is a fundamental physical constant, defined as the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 e. Charge carried by one proton or electron Elementary charge
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