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Unformatted text preview: chapter 17 - two types of electric charge, positive and negative- protons have +e, electrons have –e- e = 1.60 x 10- 19 coulombs (C)- Coulomb’s law – electric force between two point charges- F = k q 1 q 2 /r 2 = q 1 q 2 /4 Πε r 2- electric force repulsive for like charges (same sign) and attractive for unlike charges (diff signs)- k = 8.88 x 10 9 Nxm 2 /C 2- ε = 8.85 x 10-12 C 2 /Nxm 2- electric field – charged particle creates electric field - density gives strength, # of lines proportional to Q, lines never cross- arrow gives direction, start on positive, end on negative- positive charge lines radiate outward; negative charge lines radiate inward- patterns: density of lines between charges indicates strong electric field- equal and unlike charges – two lones leave the +2q charge for each line that terminates on –q- equal and like charges – at a great distance looks the same as that of a single charge 2q- electric field E can be measured using a test charge q F=qE- electric field due to a point charge can be determine by Coulomb’s law: E = k q/r 2- electric flux through the surface when electric field E passes through surface area A while making angle with the normal to the surface: φ E = EAcos θ- closed surfaces: + when outgoing, - when ingoing- Gauss’s law: φ E = q/ ε 0 – way to calculate electric field in cases that are very symmetric- principle of superposition: for a collection of point charges, the total electric field is the vector sum of the fields produced by the individual charges- what is electric force due to multiple charged particles- calculate force on a test charge due to one of the charges then calculate the others (combine effect)- add like vectors when many charged particles are present- in metal (conductor) some electrons are free to move- all excess charges are on surface of metal, electric field inside is zero- in insulator no mobile charges – electric field inside does not need to be zero- electrophoresis: uses electric force to separate ions according to size and charge – DNA fingerprinting- triboelectric = friction; conduction=contact; induction=proximity/ground (bring charged rod near, polarizes conductors, positive charged rod)...
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