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Notes 29 - Topics 5 & 6 - Electricity *

Notes 29 - Topics 5 & 6 - Electricity * - --------------------------------------------------------------------------------- 5 .1.7 - 5.1.8 Coulomb’s Law l

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Notes 29 - Topics 5 & 6 - Electricity *

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  1. Notes 29 - Topics 5 & 6 - Electricity* ---------------------------------------------------------------------------------- 5.1.7 - 5.1.8 Coulomb’s Law l • Coulomb’s Law, Form 1 (Charles-Augustin de Coulomb) - the force between two charged objects is directly related to the product of the two charges in C’s and indirectly related to the square of their separation; Coulomb Very, Very Cool!! http://www.colorado.edu/physics/2000/waves_particles/wavpart2.html Fe = k q1q2r -2; ...where q is the charge in coulombs, r is the separation in meters, and k is Coulomb’s constant; (k = 9.0 x 109 N m2C-2) Sample Problem 1: What is the electric force between the proton and electron in a hydrogen atom? (show solution in NB) Given: q1 = +1.6 x 10-19 C; q2 = -1.6 x 10-19 C; r = 5.3 x 10-11m; Unknown: Fe = ? Equation: Fe = k q1q2 / r -2 Fe = (9 x 109) (+1.6 x 10-19 ) ( -1.6 x 10-19) / (5.3 x 10-11)2 Fe = - 8.2 x 10-7 N Sample Problem 2: At what minimum speed must the electron in a hydrogen atom be moving as it orbits the proton? (hint: UCM; show solution in NB) Given: Fc = 8.2 x 10-7 N; r = 5.3 x 10-11m; me = 9.11 x 10-31 kg; Unknown: v = ? Equation: Fc = mv2 / r; v = √[Fc (r) / m] v = [(8.2 x 10-7 N) (5.3 x 10-11m) / (9.11 x 10-31 kg)].5 v = 2.2 x 107 ms-1

  2. • Coulomb’s Law, Form 2: - the constant (k) in Coulomb’s Law can be expressed as a function of another constant, the Permittivity of Free Space (eo); - the Permittivity of a material is a natural property and measures how easily an electric field (see 5.1.9) can pass through a material (ie., how easily the material “permits” the passage of an electric field); - Permittivity of Free Space... eo =8.9 x 10-12 Nm2C-2 Fe = q1q2 (4 πeo r 2)-1 ...where q is the charge in coulombs, r is the separation in meters, and eo is the Permittivity of Free Space; k = (4 πeo )-1

  3. 5.1.9-11 Electric Field (E)... ...as a function of the force exerted on a positive test charge (q1) by another charged object (q2); - radiates away from + charged objects and inward toward - charged objects; E = Fe / q1 ...where E is strength of electric field in N C -1, Fe is electrostatic force in N, and q1is charge of the test charge in C; ...as a function of the charge (q2) that establishes the field (the Field Charge) and the distance from it; - Substitute Fe from Coulomb’s Law in the the equation above to get the electric field caused by a single point charge: E = Fe / q1 = [k q1q2 / r2] / q1 = k q2 r -2; ...where r is the distance from the charged object in meters, and q2is the Field Charge in coulombsthat establishes E; - Electric Fields around/between charged objects... A. Electric field lines are drawn as arrows from positive to negative with the point of the arrows toward the negative charge; B. The more field lines per unit area, the stronger the field; C. The number of field lines starting or ending at a charged object is proportional to the charge; D. When field lines curve, the field at any point is tangent to a field line; E. E lines never cross. Electric Field 1 http://www.gel.ulaval.ca/~mbusque/elec/main_e.html Electric Field 2 http://www.nep.chubu.ac.jp/~kamikawa/electricfield/elefi_e.htm

  4. Diagram 1: E around +point charge Diagram 2: E around -point charge Diagram 3: E around 2 point charges...+ and + Diagram 4: E around 2 point charges...+ and -

  5. Diagram 5: E around 2 point charges...- and - Diagram 6: E around+point charge and +plate Diagram 7: E around +point charge and -plate Diagram 8: E around/between + plate and + plate

  6. Diagram 9: E around/between -plate and -plate Diagram 10: E around/between +plate and -plate E2 http://lectureonline.cl.msu.edu/~mmp/kap18/RR447app.htm E3 What is really wrong about the simulation? http://www.colorado.edu/physics/PhysicsInitiative/Physics2000.05.98/waves_particles/wavpart3.html

  7. Two charged smoke particles exert a force of 4.2 x 10-2 N on each other. What will be the force if they are moved so they are only 1/8 as far apart? ...3 times as far apart? What will be the force if the charge of one doubles? ...if the charge on both double? Equations:

  8. What is the net charge of a person who picks up -70. mC in electrons? By how much does the person’s mass increase? Equation 1: Equation 2:

  9. What is the total charge of all the electrons in 10. g of hydrogen gas? Equations:

  10. A proton is released in an uniform electric field and it experiences a force of 3.2 x 10-14 N toward the south. What are the magnitude and direction of the electric field? Equations:

  11. What is the magnitude and direction of the electric field midway between -10. mC charge and a +20. mC charge that are 10. cm apart? Equations:

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