Electrostatic phenomena Chapter 1 FREELY ADAPTED FROM PHYSICS Electromagnetism, Relativity and Quantum physics by Borracci and Carbone AND Electrostatic Phenomena – Foundations of Electricity, Lesson 38 by Julius Sumner Miller http://www.youtube.com/watch?v=o33VNRQdmAo CLIL Unit 1 A piece of history • • • • • V sec B.C. Thales of Miletus Pre-socratic phylosopher Mathematician Great physical discovery: If amber is rubbed with a piece of fur it acquires the property of attracting onto itself light bits of straw and dust. 1° experiment Nothing happens! → some iron dust rod →1 hard rubber electrostatically neutral = identical number of positive and negative electricity There is no interaction between the rod and the iron dust! Charges • Electric charge is a fundamental physical property of matter. • Charges come in two types arbitrarily named + positive charge, as the charge of the proton, - negative charge as the charge of the electron. • When on an object the net amount of positive charge is equal to the net amount of negative charge, we say that this object is neutral. Like charges repel, while unlike, or opposite charges attract. Static electricity caused by friction Rubbing the rod is a mechanical action that permits to separate the charges. The fur loses electrons becoming positively charged while the rod acquires electrons. The rod is charged . Only the electrons can move from an object to another one. 1° experiment bis … → a piece of fur • Rub the rod • Approach it to the iron dust • Iron dust is attracted by the rod! Rubber and glass: two different behaviors glass & glass rubber & rubber glass & rubber By convention we define • the charge on the rubber rod, rubbed with a fur, as negative • the charge on the glass rod with silk we will define the charge on the rod as positive. What happens with the glass rod and the silk? The glass loses electrons , becoming positively charged, while the silk acquires electrons becoming negatively charged. The law of conservation of charge silk silk fur glass silk fur glass Total charge =0 fur glass Total rubber charge =0 The net amount of charge that we have totally (considering the rod and the silk) is zero, as zero was the total amount of charge before I rubbed the glass rod with the silk. rubber rubber The net charge of an isolated system remains constant. Electric charge can neither be created nor destroyed The total amount of charge in the Universe is constant. The change in the amount of electric charge in any volume of space is exactly equal to the amount of charge flowing into the volume minus the amount of charge flowing out of the volume. 2° experiment Conductors and insulators • Insulators: the charge doesn’t flow. It remains in the same place where it was generated. pieces of paper not attracted pieces of paper attracted • Conductors: the charge flows freely. The electroscope metal knob metal rod light gold leaves 3° experiment The conduction • Rub the rubber rod on the fur • Touch to the metal knob with the rod • Some negative charge is deposited on the knob The electroscope is charged negatively by conduction Charging by conduction is typical of metals. • Touch to the metal knob with your finger • The electroscope is still neutral • Some negative charge has gone to the earth The electroscope has been grounded 4° experiment Determine the nature of an unknown charge Touch the electroscope with a charged sphere. Observe that the leaves diverge. Touch the electroscope Ground the electroscope. with another charged sphere and observe that the leaves diverge further. Now we can choose a unit of charge. The Coulomb Only within the past century has it become clear that an understanding of electricity originates from within the atom itself. This simplified model of an atom shows a small, dense, positively charged nucleus containing a mixture of protons and neutrons surrounded by negatively charged electrons. All protons and electrons have exactly the same magnitude of electric charge. Charge on the electron: e = −1.602× 10−19 C In the SI, the unit of electric charge is the Coulomb. Electric charge is quantised in units of electron charge. The total charge carried by any object is a whole multiple of the electron charge. A polar molecule is neutral overall, but its charge is not evenly distributed. water molecule 5° experiment The induction Approach the knob with a charged rod. The leaves diverge. The leaves collapse. The electroscope makes connection with the charged rod because of induction Metodi di elettrizzazione a confronto La polarizzazione
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