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Electricity

Electricity. Electricity. Coulomb ’ s Law. Charge. Ancient Greeks discovered that amber that is rubbed will attract small objects. If you run a comb through your hair .. It will attract small bits of lint. Why? This is due to an electrical force. Important Vocabulary.

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Electricity

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  1. Electricity

  2. Electricity Coulomb’s Law

  3. Charge • Ancient Greeks discovered that amber that is rubbed will attract small objects. • If you run a comb through your hair .. It will attract small bits of lint. • Why? • This is due to an electrical force

  4. Important Vocabulary • Conductors- materials that allow electrons to flow • Insulators- materials that impede electron flow • If a charge is negative this is due to an excess of electrons • If a charge is positive this is due to a lack of electrons • You can build a charge on insulators because… • Electrons cannot easily flow, on conductors the electrons would spread out, but insulators do not allow electrons to do this

  5. Law of Charges • Like charges repel • Opposite charges attract • Neutral charges are attracted to charged (+ or -) objects

  6. Induction • Why is it that a piece of amber that is rubbed can attract objects if it is not charged? Or combs? • If an object has a buildup of excess electrons, that charge can induce a charge in a neutral object • A neutral object has positive and negatively charged particles, and electrons can move freely

  7. More Examples of Electrostatic Charges • Rubbing your feet on the carpet then touching a doorknob • Jumping on a trampoline then stepping on the metal rail • When you pull your clothes out of the dryer, and a sock is sticking to the sweater

  8. Electric Force • Coulomb determined that the force between two objects is proportional to their charge and inversely proportional to the square of their distances • FE=Kq1q2 r2 • q1= charge of first particle (in coulombs, C) • q2=charge of second particle (C) • r=distance between charges (m) • k=Coulomb’s constant 9.0 x 109 N m2/C2

  9. Gravity Versus Electric Charge • FE=Kq1q2 r2 • What does this look like? • Fg=Gm1m2 • r2 • Compare the constants G and k • What do you notice? • k is MUCH larger • So…… • Electric Force is much greater • Also another important difference • Gravity ALWAYS attracts, electric force can repel also

  10. When solving for Electrostatic Force • We will not use +/- signs of charges even though force is a vector • We will determine the direction of force based on whether it is an attraction of a repulsion

  11. Example • Two 85 kg students are 1.0 m apart. If these two students each have a charge of 2.0 x 10-3 C what is the electrostatic force between them? • r=1.0 m • q1=2.0 x 10-3 C • q2=2.0 x 10-3 C • k=9.0 x 109 N m2/C2 • FE=? • FE=Kq1q2 r2

  12. Example • Two point charges of 1.8 x 10-6 C and 2.4 x 10-6 C produce a force of 2.2 x 10-3 N on each other. How far apart are these two charges? • q1=1.8 x 10-6 C • q2=2.4 x 10-6 C • Fe=2.2 x 10-3 N • k=9.0 x 109 N m2/C2 • r=? • FE=Kq1q2 r2

  13. Example Honors • A charge of 1.7 x 10-6 C is placed 2.0 x 10-2 meters from a charge of 2.5 x 10-6 C and 3.5 x 10-2 m from a charge of -2.0 x 10-6. What is the net electric force on the 1.7 x 10-6 C charge?

  14. Electricity Electric Fields

  15. Electric Fields • Charged objects are surrounded by electric fields, just like any mass is surrounded by a gravitational field • Are dependent on the size of and distance to the charge (just like gravitational fields) • Defined as the force per unit charge • E=FE/q • E=electric field (N/C) • FE=electrostatic force (N) • q=test charge (C) • Or….. E=kq/r2

  16. Electric Fields are Vectors • Are force fields • So they are vectors • Draw fields with arrows • Define direction of the electric fields as the direction a positive charge would move in that field • Strength is indicated by density of arrows

  17. Example of Isolated Charges

  18. Example of Positive and Negative Charge

  19. Example of Positive Charges

  20. Example of Negative Charges

  21. Example • What is the electric field strength at a point where a 2.00 μC charge experiences an electric force of 5.30 x 10-4 N? • q=2.00 μC • q= 2.00 x 10-6 C • FE=5.30 x 10-4 N • E=? • E=FE/q • E=5.30 x 10-4 /2.00 x 10-6 • E=265 N/C

  22. Example • At a distance of 7.50 x 10-1 m from a small charged object the electric field strength is 2.10 x 104 N/C. What is the charge of this particle? At what distance from this same object would the electric field strength be 4.0 x 10 4 N/C? • r=7.50 x 10-1 m • E=2.10 x 104 N/C • k=9.0 x 109 N m2/C2 • q=? • E=kq/r2 • Q=1.312 x 10 -6 C • E=4.0 x 10 4 N/C • r=?

  23. Electricity Electric Fields from Multiple Charges

  24. Electric Fields are Vectors • Force fields are vectors • When multiple charges overlap we add them up or subtract as vectors • Like charges will repel, so subtract • Opposites attract, so add

  25. Example • What is the strength of a electric field midway between a 2.00 μC charge and a -4.00 μC that are 0.60 m apart? • q1=2.00 μC • q1=2.0 x 10-6 C • q2=-4.00 μC • q2=-4.00 x 10-6 C • r=0.60 m • Electric field strength will depend on the distance away from the 2 charges • Midway… so really r=0.30 m • k=9.00 x 109 Nm2/C2 • Find the Strength for both charges first • E=kq/r2

  26. Example • Two 5.25 μC charges are 0.40 m apart. What is the strength of the electric field between them at a point 0.10 m away from the first charge and 0.30 m from the second? • q1=5.25 x 10-6 C • q2=5.25 x 10-6 C • Field strength depends on the distance away from the charge • r1=0.1 m • r2=0.3 m • k=9.0 x 109 Nm2/C2 • E=kq/r2

  27. Example Honors • Find the magnitude of the electric field the point p due to the charges as shown. 12.0 μC 6.0 m -8.0 μC 4.5 m

  28. Electricity Electric Potential

  29. Electric Potential Energy • If a charged object is in an electric field it has electric potential energy, that is it has the potential to move in that field. Note that the potential energy it has could be used to attract or repel

  30. Electric Potential • Electric Potential (V)- is the amount of electric potential energy that a charge would have when located at a specific point. • V=kq/r • V=electric potential measured in volts • A volt=J/C • k=9.0 x 109 Nm2/C2 • q=charge • R=distance (m)

  31. Electric Potential • Defined in terms of the moving of a positive charge • + charges moves towards low potential • - charges moves towards high potential • We will use +/- signs for these problems

  32. Example 3.0 μC -2.0 μC 5.0 cm 5.0 cm P • Calculate the potential at point P as shown in the diagram • q1=3.0 x 10-6 C • r=.05 m • q2=-2.0 x 10-6 C • r=.05 m • V=? • V=kq/r • V1=(9.0 x 109)(3.0 x 10-6)/(.05) • V1=514 000 V • V2=(9.0 x 109)(-2.0 x 10-6)/(.05) • V2=-360000 V

  33. Potential Difference • The electric potential between two points due to a charge • With electric potential you are looking for the potential at one specific point, potential difference you are looking at 2 specific points • VAB=VB-VA

  34. Example A 1.00m 0.50m • What is the potential difference between points A and B due to the charge shown? • q=8.00 x 10-6 C • r1=1.00 m • r2=0.50 m • VAB=? • Find V at each difference first B 8.00 μC

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