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Electricity

Electricity. Sections 12.1 and 12.2. Reminders. In-class Quiz #5 on today No lab this week Mallard Reading Quiz due Tuesday before class (Chapter 13: Magnetism & Electricity) Weekly Reflection #10 sent out after class today. Electrostatics. Examples of electrostatic discharges

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Electricity

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  1. Electricity Sections 12.1 and 12.2

  2. Reminders • In-class Quiz #5 on today • No lab this week • Mallard Reading Quiz due Tuesday before class (Chapter 13: Magnetism & Electricity) • Weekly Reflection #10 sent out after class today.

  3. Electrostatics • Examples of electrostatic discharges • Lightning – static cling in dryers • “foot dragging” – “shocking” cats • Electrical charges come in two types, + and – • Amber (Gk. electron) is associated with – • Glass is associated with + • Charging: • By induction • By conduction (conductors and insulators)

  4. Electrical Charges • Negative (electron) • Positive (proton) • Charge is measured in Coulombs, C • Quantities of charge • Electron = -1.6 x 10-19 C • Proton = +1.6 x 10-19 C • Opposite charges (+ and –) attract whereas like charges (+ and + or – and –) repel

  5. Electric Field Strength • E = Fe/q by definition (force per unit charge) • OrFe = Eq (where E = electric field strength) • Units of E are N/C • Parallels W = mg (where g = grav field strength) • Electric field lines: • Directed from positive to negative charge • Show the path that a positive unit test charge would take.

  6. Coulomb’s Law • First worked out using a torsion balance • Pendulum apparatus easier to understand • Fe = mg tan(θ) found proportional to q1 and q2 • Fe = mg tan(θ) found inversely proportional to r2 • Fe = keq1q2/r2 where ke = 9 x 109 Nm2/C2 • Parallels Newton’s universal law of gravitation • Fg = Gm1m2/r2 where G = 6.67 x 10-11 Nm2/kg2 • Note that Fe/Fg = 4.17 x1042; therefore Fe>>>> Fg

  7. Why Gravity Dominates in Universe • Because Fe/Fg = 4.17 x 1042and Fe>>>> Fg, then why does gravity dominate in the universe? • The universe is “neutral” in terms of overall charge; that is, the number of electrons is balanced by the number of protons. • Individual objects are most commonly neutral because they contain roughly equal numbers of positive and negative charges.

  8. A Couple of Examples • Fe = keq1q2/r2 where ke = 9 x 109 Nm2/C2 • Analogously Fg = Gm1m2/r2 • However, Fg is only attractive (unless we are talking the grand scale of the universe...) whereas Fe can be either attractive or repulsive • Fe = qE • Analogously W = mg • This is why g is sometimes called the gravitational field strength and is expressed in units of N/kg

  9. Extra Credit Problems (2pts total) • What is the electrical field strength 1 meter away from a 1C positive charge? (Hint: Use both formulas on last slide eliminating one “q”.) • What is the direction of the field 1 m away from the 1C charge, in or out of the 1C charge? • What force would a 1.4x10-5C negative charge experience at 1m distance from the 1C positive charge? • Would the force be directed toward or away from the 1C charge?

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