17-31summary - Chapter 17 Electric charge and Coulombs law...

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Chapter 17 - Electric charge and Coulomb’s law - there are two types of electric charge, positive and negative. Protons have a positive charge +e, and electrons carry a negative charge –e. Charge is measured in coulombs (C), with e = 1.60x10 -19 C - the electric force between two point charges q 1 and q 2 is given by Coulomb’s law. If the charges are separated by a distance r, this force has a magnitude F=k(q 1 q 2 )/r 2 and is directed along the line joining the two charges - the electric force is repulsive for like charges (charges with the same sign) and attractive for unlike charges (those of opposite sign). Here, k is a constant of nature with value k=8.99x10 9 Nm 2 /C 2 and ε 0 is a universal constant called the permittivity of free space, with the value 8.85x10 -12 C 2 /Nm 2 - the electric field E can be measured using a test charge q and is proportional to the electric force, with F=qE. The electric field due to a point charge q can be determined from Coulomb’s law. Its magnitude is E=kq/r 2 - Electric flux and Gauss’s law - when an electric field of magnitude E passes through a surface of area A while making an angle θ with the normal to the surface, the electric flux through the surface is Φ E =EAcos θ - Gauss’s law relates to the electric flux Φ E through a closed surface to the amount of electric charge inside the surface: Φ E =q/ ε 0 - Gauss’s law gives a powerful way to calculate the electric field in cases that are very symmetric. Coulomb’s law can be derived from Gauss’s law. - Conservation of charge - a fundamental conservation law of nature is that electric charge is conserved. Charge is also quantized; it comes in discrete amounts, such as electrons and protons, which have charges –e (an electron) and +e (a proton) - principle of superposition - Coulomb’s law gives the electric field due to a single point charge. For a collection of point charges, the principle of superposition states that the total electric field is the vector sum of the fields produced by the individual charges - Conductors and insulators - in a metal, some of the electrons are free to move, whereas in an insulator, there are no mobile charges, For a metal in static equilibrium, the electric field inside is zero and all excess charges reside on the metal’s surface. The electric field inside an insulator is zero - DNA fingerprinting - the technique of electrophoresis uses the electric force to separate ions according to their size and charge. This is the basis of DNA fingerprinting Chapter 18 - electric potential energy - the electric force is a conservative force, so there is an electric potential energy PE elec . The potential energy of two point charges separated by a distance r is PE elec = kq 1 q 2 /r - electric potential - the electric potential V (also sometimes called just the potential) is proportional to the electric potential energy. If the electric potential energy of a charge at a particular location is PE elec , the electric potential at that point is V= PE

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17-31summary - Chapter 17 Electric charge and Coulombs law...

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