Phys15

# Phys15 - Unit 15 Electrostatics and Gausss Law 15.1 15.2...

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1 Unit 15 Electrostatics and Gauss’s Law 15.1 Electric charge 15.2 Coulomb’s law 15.3 Shell theorems for electrostatics 15.4 Electric field 15.5 Electric field lines 15.6 Shielding and charging by induction 15.7 Electric flux 15.8 Gauss’s law 15.1 Electric charge There are two kinds of charges, namely, positive (+) charge and negative ( ) charge. Like charges repel Unlike charges attract Objects with zero net change are said to be electrically neutral. Electric charges are generated after rubbing between materials. Example After rubbing plastic rod (or amber rod) with fur, the plastic rod (or amber rod) becomes negatively charged and the fur is positively charged. + + +

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2 After rubbing glass rod with silk, the glass rod becomes positively charged and the silk is negatively charged. A familiar example of an electrically neutral object is the atom. Atoms have a small, dense nucleus with a positive charge surrounded by a cloud of negatively charged electrons. All electrons have exactly the same electric charge. This charge is very small, and is defined to have a magnitude, e = 1.60 × 10 19 C. S.I. unit of charge is coulomb, C. Clearly, the charge on an electron, which is negative, is e . This is one of the defining, or intrinsic, properties of the electron. Another intrinsic property of the electron is its mass, m e : m e = 9.11 × 10 31 kg In contrast, the charge on a proton – one of the main constituents of nuclei – is exactly +e. As a result, since atoms have equal numbers of electrons and protons, their net charge is precisely zero. The mass of the proton is m p = 1.673 × 10 27 kg. Note that this is about 2000 times larger than the mass of the electron. The other main constituent of the nucleus is the neutron, which, as its name implies, has zero charge. Its mass is slightly larger than that of the proton: m n = 1.675 × 10 27 kg. Example How is it that rubbing a piece of amber with fur gives the amber a charge? Answer: Rubbing the fur across the amber simply results in a transfer of charge from the fur to the amber – with the total amount of charge remaining unchanged. Before charging, the fur and the amber are both neutral. During the rubbing process some electrons are transferred from the fur to the amber, giving the amber a net negative charge, and leaving the fur with a net positive charge. At no time during this process is charge ever created or destroyed.
3 This, in fact, is an example of one of the fundamental conservation laws of physics: Conservation of electric charge . When charge is transferred from one object to another it is generally due to the movement of electrons. In a typical solid, the nuclei of the atoms are fixed in position. The outer electrons of these atoms, however, are often weakly bound and fairly easily separated. The atom that loses an electron is a positive ion, and the atom that receives an extra electron becomes a negative ions. This is charging by separation.

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Phys15 - Unit 15 Electrostatics and Gausss Law 15.1 15.2...

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