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PHYSICS

PHYSICS. Static Electricity. A. Charge. 1. Charge - basis for electricity a. two types - “+” or “-” based on the excess or deficiency of electrons. Named by Ben Franklin b. can be created chemically, mech., etc c. Like charges repel, unlike charges attract. 2. Microscopic view of Charge.

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PHYSICS

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  1. PHYSICS Static Electricity

  2. A. Charge 1. Charge - basis for electricity a. two types - “+” or “-” based on the excess or deficiency of electrons. Named by Ben Franklin b. can be created chemically, mech., etc c. Like charges repel, unlike charges attract

  3. 2. Microscopic view of Charge e- a. Atom is neutral – electrons (e-) balanced by same # of protons (p+). b. With addition of energy, electrons removed leaving a positive ion. Freed electrons become attached to other material and have a negative ion. • neutral objects charged by rubbing • conservation of charge is critical • protons are stationary, lattice structure p+

  4. 3. Conductors and Insulators • Insulators - materials through which charges do not move easily. Charges stuck in place i. Air an insulator, but sparks can occur. b. Conductors - materials such as metals that allow charges to move • Charge is carried by the movement of electrons. • Charged electrons repel each other so move as far apart as possible. • electrons act as though they belong to the entire whole, sea of electrons

  5. 4. Concentration of Charge (conductor) a. charged body distributes charge over the surface b. distributed to bring about equilibrium c. pointed surfaces have a higher concentration of charge d. Grounding - same charge per unit area, removal of excess charge.

  6. B. Electric Force 1. more powerful than gravity and both attractive and repulsive 2. Charging by Conduction - charging a neutral body by touching it with a charged body. Positive or negative 3. Charged by induction - charge an object without touching

  7. 4. Coulomb’s Law a. 1785 - Charles Coulomb devises an experiment to measure the electric force. b. “The magnitude of the force that a tiny sphere with charge q exerts on a second sphere with charge q’, separated by a distance d, is given by the equation:

  8. i. Forces that q exerts on q’ and that q’ exerts on q are equal [p1&p2] c. Distance, d, is between the center of charges. d. Unit of charge: Coulomb (C) = A s i. 1 C = charge on 6.25 x 1018 electrons ii. lightning bolt: 7 -10 C iii. elementary charge on one electron, e- = -1.60 x 10-19 C e. For charge in Coulombs, distance in meters, and force in Newtons k = 9 x 109 N m2 C-2

  9. f. Coulomb’s Force - a vector quantity, must show direction. i. repulsive forces - positive signs, force on object is away from second object. ii. attractive forces - negative signs, force on object is toward second object. • A charged object is always attracted by a neutral object (and, by 3rd Law, a neutral object is attracted to a charged object) > like charges move  away

  10. 4. Problems • a. Two point charges of magnitude 3 x 10-9 C and 6 x 10-9 C are separated by 30 cm. Find the force of repulsion. F = (9x109 N m2 C-2 ) (3x10-9 C)(6x10-9 C) (0.3 m)2 = 1.8 x10-6 N

  11. b. A 4.5 x 10 -9 C charge is located at 3.2 m from a - 2.8 x 10 -9 C charge. Find the electrostatic forces. c. An electron and a proton of a hydrogen atom are separated by a distance of 5.3 x 10-11 m. An electron has a charge of - 1.60 x 10-19 C and a proton has a charge of 1.60 x 10-19 C . Find the electrostatic forces between the two.

  12. 5. Principle of Superposition a. When two or more charges affect a different charge, the resultant force acting on the charge equals the vector sum of the various individual charges present. • 2.2 x 10 -9 C charge is located at x = -1.5m, 5.4 x 10 -9 C charge is located at x = 2 m, & 3.5 x 10 -9 C charge is located at the origin. Find the net force acting on the charge at the origin.

  13. F3-2 F3-1 + + + q1 1.5 q3 2 m q2 • F 3-2 = 3.08 x 10 -8 N F 3-1 = 4.25 x 10 -8 N Fnet = F 3-1 - F 3-2 = 1.17 x 10 -8 N towards q2

  14. c. Determine the resultant force acting on charge q 3 in the following situation. q 1 = 6 x 10 -9 C q 2 = - 2 x 10 -9 C q 3 = 5 x 10 -9 C 3 m 4m q 2 q 3 q 1

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