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Unit01_1_Web_Lecture_Notes

# Unit01_1_Web_Lecture_Notes - For two stationary point...

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Notes from lecture: 21 Jan 2008. There are two types of electric charges. We call them positive ( + ) and negative ( ). Opposite charges ( + and ) attract, while like charges ( + & + , or & ) repel. In the demonstration, the PVC rod became ( ) while the clear Lucite became ( + ). Electrons are negatively charged particles that are transferred from one object to another. A deficit of electrons results in a ( + ) charge. A surplus of electrons results in a ( ) charge. Electric charge is conserved. The total charge in an isolated system does not change. Electric charge is quantized. That is, it comes in discrete amounts. The smallest magnitude of charge is called the elementary charge e . The charge of an electron is – e . The charge of a proton is e . The charge of a neutron is 0. In the international system (called SI for the French name: Système Internationale) of units, the unit of charge is the Coulomb (C). In SI units, the elementary charge is 19 10 602 . 1 × = e C. The electrostatic force between charged particles is described by Coulomb’s law.

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Unformatted text preview: For two stationary point particles, with charges 1 q and 2 q , separated by a distance r , the magnitude of the electrostatic force exerted by one particle onto the other is 2 2 1 1 2 2 1 r q q k F F = = ← ← . The constant k in Coulomb’s law is called the electrostatic constant. The units of k are = ] [ k Nm 2 /C 2 , and 9 10 99 . 8 × = k Nm 2 /C 2 . We will often use the approximation 9 10 . 9 × ≈ k Nm 2 /C 2 . Often Coulomb’s law is written in terms of the permittivity constant, 12 10 85 . 8 − × = ε C 2 /(Nm 2 ), rather than in term of k . These two constants are related by 4 1 πε = k . The electrostatic force obeys the principle of superposition. For n stationary charged point particles: n F F F F ← ← ← ← + + + = 1 3 1 2 1 all 1 L . Note that this is a vector sum. Force directions: 1 > q 2 > q 1 > q 2 < q 1 < q 2 < q 2 1 ← F r 2 1 ← F r 2 1 ← F r 1 2 ← F r 1 2 ← F r 1 2 ← F r...
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Unit01_1_Web_Lecture_Notes - For two stationary point...

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