1Electric Charge and Coulomb’s Law Electric charge The positive charge and negative charge:Matter is made of atoms. Inside an atom, there is the nucleus that is surrounded by electrons.Inside the nucleus, there are two particles called proton and neutron. The smallest nucleus contains only one proton. This is the nucleus inside a hydrogen atom. Proton and electron attract each other. Proton and proton, electron and electron repel each other. This is a property of these matters (proton and electron) and we call it charge.Electric charge is a property of matter that can cause attraction and repulsion. We call the charge carried by electrons “negative (-e) ” and the charge carried by protons “positive (+e)”. Charge is a value, or a scalar, not a vector. It is fully described by a number.2Electric Charge and Coulomb’s Law Electric chargeElectric charge is a property of matter that can cause attraction and repulsion. We call the charge carried by electrons “negative (-e) ” and the charge carried by protons “positive (+e)”. Charge is a value, or a scalar, not a vector. It is fully described by a number. The SI unit for charge is the coulomb. An electron or a proton has a charge of magnitude e = 1.602 18×10−19C (coulombs). Some scientists, chemists in particular, use another unit, the esu or electrostatic unit. One esu equals 3.335 64×10−10C. To provide you with an idea of the magnitude of a coulomb, approximately 0.8 C of charge flows through a 100 watt light bulb every second. Or about 5 million trillion electrons every second. The rate of charges flowing through a conductor is called a current. We will get to this a few chapters later. No one has ever seen the charge, but we sure all see its effect in everyday life: electrostatic discharge in dry winter days to all the appliances (lights to motors to cell phones) that are powered by electricity.How much more do we know about the charge?3Electric Charge and Coulomb’s Law Electric charge When the numbers of electrons and protons in an object are the same, we say that this object is (electrically) nuetral. When they are not, we call it charged. There are many ways to charge up an object. A demo here:In this demo, we rub away or rub in electrons to make anobject positively or negatively charged.4Electric Charge and Coulomb’s Law Electric charge Electric charge is conserved. Charge can move between objects in the system, but the net charge of the system remains unchanged. Charges cannot be created or destroyed in the system, because charge is just a property of electrons and protons. They both are matter and matter conserves. A remark on anti-matter: matter and anti-matter annihilate into energy. So what’s more fundamental is the conservation of energy, but that’s beyond this class. The interactions between or among charges? the Coulomb’s Law5Electric Charge and Coulomb’s Law Electrostatics Electrostatics studies of interactions between stationary electric charges. Introduction of a few concepts: A conductor: An object or material in which charge can flow relatively freely. An insulator: An object or material in which charge does not flow freely. To ground: Charge flows from a charged object to the ground, leaving the object neutral. The ground: A neutral object that can accept or supply an essentially unlimited number of charges. The Earth functions as an electric ground. Application based on these physics concepts: the lightning rod on tall buildings.6Electric Charge and Coulomb’s Law Electrostatics Forces between charges and the induction.Unlike charges attract;Like charges repel.This provides a second way to charge an object: This process is call induction.To quantitively study the forces between charges, we introduce the law of this chapter:The Coulomb’s Law of forces between two point charges.Show
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