Electric Charge and Coulomb’s LawSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Superposition Principle, ExampleZero Resultant Force, ExampleElectrical Force with Other Forces, ExampleElectrical Force with Other Forces, Example cont.1Electric Charge and Coulomb’s LawElectric chargeThe 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 LawThe unit of electric chargeThe SI1 unit for charge is the coulomb. An electron or a proton has a charge of magnitude e = 1.602 18×10−19 C (coulombs). Some scientists, chemists in particular, use another unit, the esu or electrostatic unit. One esu equals 3.335 64×10−10 C. 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?1. The International system of Units, more reading: http://physics.nist.gov/cuu/Units/index.html.3Electric Charge and Coulomb’s LawCharge and charge upWhen the numbers of electrons and protons in an object are the same, we say that this object is (electrically) neutral. 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 LawConservation of electric chargesElectric 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.8 = 10 + (-2), so O-21 decays to Ne-21 plus two electrons.5Electric Charge and Coulomb’s LawMovement of electric charges in matterA conductor: An object or material in which charge can flow relatively freely. Example: metal, carbon, …An insulator: An object or material in which charge does not flow freely. Example: plastic, glass, …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 LawElectrostatics – forces between chargesUnlike charges attract; Like charges repel.7Electric Charge and Coulomb’s LawForces between charges – charge inductionThe force between charges provides a second way to charge up an object: This process is call induction.Charging an inductor:Charge rearrangement in insulators:8Electric Charge and Coulomb’s LawTo quantitatively study the forces between charges, we introduce the law of this chapter:The Coulomb’s Law of forces between two point charges in vector form:121212rrF221022141rqqrqqkePLAYACTIVE FIGURE221200022902112mNC 1085428space. free ofty permittivi theis 41CmN 1098768constant. Coulomb thecalled is to from directedr unit vecto theis . of because experience force theis Here/./.kk.qqqqe1212rF9Electric Charge and Coulomb’s LawDiscussions about Coulomb’s LawThe term point charge refers to a particle of zero size that carries an electric chargeThe force is inversely proportional to the square of the separation r between the charges and directed along the line joining themThe force is proportional to the product of the charges, q1 and q2, on the two particlesThe force is attractive if the charges are of opposite signThe force is repulsive if the charges are of like signElectrical forces obey Newton’s Third LawThe force on q1 is equal in magnitude and opposite in direction to the force on q2With like signs for the charges, the product q1q2 is positive and the force is repulsiveWith unlike signs for the charges, the product q1q2 is negative and the force is attractive2112FF10Electric Charge and Coulomb’s LawThe Superposition PrincipleThe resultant force on any one charge equals the vector sum of the forces exerted by the other individual charges that are presentIf there are four charges from q1 to q4, the resultant force on q1 is the vector sum of all the forces exerted on it by other charges: Remember to add forces as vectors: Problem solving template1 21 31 41= + +F F F Fr r r rTemplateStep 1, formulas or related concepts.Step 2, known quantities.Step 3, direct application of the formulas/concept or the condition to form an equation.Step 4, vector involved?Step 5, unit in the final answer correct? Answered all were asked?11Electric Charge and Coulomb’s LawExample 1X axisStep 1, formulaStep 2, known quantities:x Fr F221x221 :axis-X theAlong , Law sCoulomb'rqqkrqqkeeC. 10601electron an of charge theand constant Them 3m 14 m, 6m )5(1 distances536192312321.ekrr,eq,eq,eqe12Electric Charge and Coulomb’s LawStep 3, direct application of the formula twice:Step 4, vector involved? Yes, and the answer
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