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Electricity and Magnetism

Electricity and Magnetism. The Sciences chapter 5. Electricity. If you walk across a rug and pick up a charge from the carpet, are you negatively or positively charged?. Electricity describes charged particles. Rub a balloon with wool, picks up electrons

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Electricity and Magnetism

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  1. Electricity and Magnetism The Sciences chapter 5

  2. Electricity

  3. If you walk across a rug and pick up a charge from the carpet, are you negatively or positively charged?

  4. Electricity describes charged particles • Rub a balloon with wool, picks up electrons • Rub glass with silk, loses electrons • Objects attract or repel when “electrically charged” • “static electricity”

  5. Figure 5-1 The two kinds of electrical charges. Opposite charges attract, while like charges repel.

  6. Electricity describes charged particles • These charged particles can be at rest (“static electricity”) • or they may be moving (“current electricity”)

  7. Ben Franklin & Electric Charge • 1746, 1st to use “negative” and “positive” in e’statics • 1752 famous kite experiment • Invented lightning rod

  8. Lightning • Result of charges that become separated in thunder clouds • Negative charge on cloud induces positive charge on ground • Discharge causes lights and heats air

  9. The movement of electrons • What were Franklin’s “negative fluids”? • Need to know atomic structure to understand electricity

  10. Draw an atom. Label its parts. What do the different parts do? • Protons • Neutrons • Electrons

  11. Every atom has a positively charged nucleus surrounded by negatively charged electrons.

  12. The electrons of all atoms are identical. Each has the same quantity of negative charge and the same mass.

  13. Protons and neutrons compose the nucleus. Protons are 1800X more massive than electrons, but carry an amount of positive charge equal to the negative charge of electrons

  14. Neutrons have slightly more mass than protons and have no net charge.

  15. Static Electricity

  16. Coulomb’s Law • The electric force between two charged particles varies directly as the product of their charges and inversely as the square of the separation distances. • force (newtons) = k x 1st charge x 2nd charge / distance2

  17. Coulomb’s Law • …observed that if 2 electrically charged objects are moved farther away from each other, the force between them gets smaller, just like gravity. • If the distance between the 2 objects is doubled, the force decreases by a factor of 4. • (inverse square relationship)

  18. Electrical Field • A kind of “aura” or “force field” around every electric charge • Extends radially away from the proton and in opposite direction about the electron.

  19. Conductors Material that electrons are able to pass through Metals, ionic solutions Insulators Material that electrons do not through easily Glass, wood, rubber, plastics …distinguish betweenconductors and insulators

  20. Electrical potential and electric current • Movement of electric charge creates electric current • Charges move as current only when energy is supplied to them • Circuit • Switch • Voltage • Current

  21. If we use the water analogy… • Voltage = the pressure • Current = the rate of flow

  22. Voltage • The push that makes electrons move. • 1.5 V “D” cell or 6 V lantern battery • Higher voltage = greater push on electrons • Water analogy • Voltage causes current.

  23. Electric Current • Voltage creates current • Current is the amount of charge passing a point in a circuit in a second • Metric unit = Ampere (A) • Measures by an ammeter • Different devices often carry different amounts of current

  24. Direct current Current moves in one direction From dry cells or batteries Alternating current Pumped to us by Cobb EMC Oscillates back and forth at 60 Hz wall sockets …distinguish betweenDC and AC

  25. Ohm’s Law • How is current related to voltage? • Direct relation between the two led to discovery of “resistance” • Voltage / Current = Resistance (V / I = R)

  26. Resistance • measures how hard it is for current to move through a conductor (unit = Ohm). • Easier for electrons to move through thick wires than thin wires • Light bulb filament, thin, high resistance, heats up and glows

  27. Only one path for current flow Same amount of current thru entire circuit Cheap string of decorative lights Alternate paths for current flow Current divides up among the paths Wiring system for lights and elec outlets in homes & buildings …distinguish betweenSeries Circuits & Parallel Circuits

  28. Electrical Safety • Fuses • Circuit breakers • Ground-fault interrupter • “Atoms Family” ElecSafety link

  29. Electric Power • Power = energy used / time • Also calculated as product of current and voltage • Watts = amp’s x volts • Ex: 60W bulb draws .5 A on a 120V line 120W lamp draws 1A on a 120V line • If a 120V line to a socket is limited (by a fuse) to 15A, will it operate a 1200W dryer?

  30. Solution… • If a 120V line to a socket is limited (by a fuse) to 15A, will it operate a 1200W dryer? • 1200W = A x 120V A = 10, yes

  31. Review Questions from chap 5: Electricity and Magnetism • Try discussion questions 1-11, p. 114. • Problems 2,3,5,6,7, p. 114. • Answers are posted in study guide.

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