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### Understanding Electrostatic Forces: A Study in Physics ###

Explore the phenomena of lightning, shocks, and static cling to uncover the physics behind electrostatic forces. Investigate and record your findings as you complete the steps outlined in the study. Discover how charges interact, attracting or repelling each other based on their polarity. ###

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### Understanding Electrostatic Forces: A Study in Physics ###

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  1. Electrostatic Forces

  2. What do you see in the cartoon? Turn and tell your partner 2 things Share out Record in your notebook What do you think caused the shock? Record answer in your notebook Investigation #1 (p792-794) p147

  3. P147 Investigation #1 (p792-794) • The study of lightning, shocks, and static cling can reveal important physics • Complete steps 1-9 • Get your notebook stamped when you finish today

  4. http://nsdl.org “Static” Electricity When you comb your hair and… … bring your comb over a pile of paper bits

  5. http://nsdl.org “Static” Electricity What will happen? A. B. Why did this happen?

  6. http://nsdl.org “Static” Electricity • The force felt by the paper bits is due to a difference in charge on the comb compared to the paper. • This “force of attraction” was first observed by the Greeks who found that piece of amber (“elektron”) attracted other objects when rubbed.

  7. C. A. B. http://nsdl.org Match the diagram below with its correct description. 1. 2. 3. Excess of Electrons Net Charge Negative Deficiency of Electrons Net Charge Positive Balanced atom Net Charge Zero

  8. http://nsdl.org “Static” Electricity • Usually charges balance each other out, and nothing happens. But when two objects with like charges (all positive or all negative) come together, the charges repel and the objects move away from each other. • Objects with opposite charges attract each other because the different charges want to balance each other.  • Objects can get a negative charge by picking up electrons from other objects.

  9. http://nsdl.org Common Misconceptions about “Static” Electricity Actually, the thing we call static electricity is an imbalance in the amounts of positive and negative charges found on the surface of an object.

  10. http://nsdl.org Common Misconceptions about “Static” Electricity Lightning is like static electricity, except on a much bigger scale. Both lightning and static electricity happen because of the attraction between the opposite charges.

  11. Electric charge in cloud

  12. Possible lightning paths

  13. Charge and image

  14. The “Leader”

  15. Main Stroke

  16. Lightning striking tree

  17. Lightning striking Empire State Building

  18. Shoes of man struck by lightning Man not hurt!!

  19. http://nsdl.org Self Check? Charges flow from less negative to more negative areas. Like charges repel.

  20. Concept Check - Electrostatics • Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges? • 1. one is positive, the other is negative • 2. both are positive • 3. both are negative • 4. both are positive or both are negative

  21. Concept Check - Electrostatics • Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges? • 1. one is positive, the other is negative • 2. both are positive • 3. both are negative • 4. both are positive or both are negative The fact that the balls repel each other only can tell you that they have the same charge, but you do not know the sign. So they can be either both positive or both negative.

  22. Concept Check - Electrostatics • From the picture, what can you conclude about the charges? • 1. have opposite charges • 2. have the same charge • 3. all have the same charge • 4. one ball must be neutral (no charge)

  23. Concept Check - Electrostatics • From the picture, what can you conclude about the charges? • 1. have opposite charges • 2. have the same charge • 3. all have the same charge • 4. one ball must be neutral (no charge) The PERIWINKLE and BLACK balls must have the same charge, since they repel each other. The RED ball also repels the PERIWINKLE , so it must also have the same charge as the PERIWINKLE (and the BLACK).

  24. Concept Checks – Conductors • A metal ball hangs from the ceiling by an insulating thread. The ball is attracted to a positive-charged rod held near the ball. The charge of the ball must be: • 1. positive • 2. negative • 3. neutral • 4. positive or neutral • 5. negative or neutral

  25. remember the ball is a conductor! Concept Checks – Conductors • A metal ball hangs from the ceiling by an insulating thread. The ball is attracted to a positive-charged rod held near the ball. The charge of the ball must be: • 1. positive • 2. negative • 3. neutral • 4. positive or neutral • 5. negative or neutral Clearly, the ball will be attracted if its charge is negative. However, even if the ball is neutral, the charges in the ball can be separated by induction (polarization), leading to a net attraction.

  26. Electromagnetic Charge

  27. Positive and Negative Charge

  28. Insulators and Conductors Conductors are materials that electric charges move easily and spread out evenly + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Nonconductor Conductor Insulators are materials that electric charges can not move through

  29. + + + + + + + + + + + + + + + + + Conductors (Q2 charged by direct contact) Q + + + + + + + + + + + + + + + + + + + + + + + + + + + + Q/2 Q/2 + + + + + + + +

  30. + + + + + + + + + + + + + + + + + Insulators (not charged by induction Q + + + + + + + + + + + + + + Q/2 Q/2 + + + +

  31. + + + + + + + + + + + + + + + + + Conductors (Q2 charged by induction) + + + + + + + Q + + + + + + + + + + + + + + + + + + + + + + + + + + + + Q/2 Q/2 + + + + + + + +

  32. http://nsdl.org Methods of Charging Induction Direct contact Usually results in a temporary rearrangement of charges Results in a transfer of charges

  33. F2 = ? F1 = 3N Q Q Concept Check – Coulomb’s Law • What is the magnitude of the force F2? • 1. 1.0 N • 2. 1.5 N • 3. 2.0 N • 4. 3.0 N • 5. 6.0 N

  34. F2 = ? F1 = 3N Q Q Concept Check – Coulomb’s Law • What is the magnitude of the force F2? • 1. 1.0 N • 2. 1.5 N • 3. 2.0 N • 4. 3.0 N • 5. 6.0 N The force F2 must have the same magnitude as F1. This is due to the fact that the form of Coulomb’s Law is totally symmetric with respect to the two charges involved. The force of one on the other of a pair is the same as the reverse. Note that this sounds suspiciously like Newton’s 3rd Law!!

  35. Concept Check – Electric Force • Two uniformly charged spheres are firmly fastened to and electrically insulated from frictionless pucks on an air table. The charge on sphere 2 is three times the charge on sphere 1. Which force diagram correctly shows the magnitude and direction of the electrostatic forces:

  36. Concept Check – Electric Force • Two uniformly charged spheres are firmly fastened to and electrically insulated from frictionless pucks on an air table. The charge on sphere 2 is three times the charge on sphere 1. Which force diagram correctly shows the magnitude and direction of the electrostatic forces:

  37. F2 = ? F1 = 3N Q Q F2 = ? F1 = ? 4Q Q Concept Check – Coulomb’s Law (2) • If we increase one charge to 4Q, what is the magnitude of F1? • 1. 3/4 N • 2. 3.0 N • 3. 12 N • 4. 16 N • 5. 48 N

  38. F2 = ? F1 = 3N Q Q F2 = ? F1 = ? 4Q Q Concept Check – Coulomb’s Law (2) • If we increase one charge to 4Q, what is the magnitude of F1? • 1. 3/4 N • 2. 3.0 N • 3. 12 N • 4. 16 N • 5. 48 N • Coulomb’s Law: • Now we have: • which is 4 times bigger than before.

  39. F F Q Q ? ? Q Q r 3r Concept Check – Coulomb’s Law (3) • The force between two charges separated by a distance r is F. If the charges are pulled apart to a distance 3r, what is the force on each charge? • 1. 9 F • 2. 3 F • 3. F • 4. 1/3 F • 5. 1/9 F

  40. F F Q Q F/9 F/9 Q Q r 3r Concept Check – Coulomb’s Law (3) • The force between two charges separated by a distance r is F. If the charges are pulled apart to a distance 3r, what is the force on each charge? • 1. 9 F • 2. 3 F • 3. F • 4. 1/3 F • 5. 1/9 F • Coulomb’s Law: • Now we have: • which is 1/9 as big as before.

  41. TWO kinds of electric energy Static Electricity Current Electricity

  42. A few thousand volts

  43. Van de Graaff Born 1901 Invented static electricity generator in 1929 This is the generator we will use today About 100,000 or 150,000 volts

  44. Here is a biggerVan de Graaff generator

  45. An even bigger one!

  46. A giant Van de Graaff generator

  47. The biggest--25 Million VoltsOak Ridge National Lab in Tennessee

  48. So, how does our Van de Graff Generator work?

  49. p150 Electrons jump off the belt at the top Electrons jump onto the belt at the bottom

  50. One of the properties of matter Where the electrons go when two things are rubbed together

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