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Electrostatic Forces. “Static” Electricity When you comb your hair and… … bring your comb over a pile of paper bits

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Presentation on theme: "Electrostatic Forces. “Static” Electricity When you comb your hair and… … bring your comb over a pile of paper bits"— Presentation transcript:

1 Electrostatic Forces

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

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

4 “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. http://nsdl.org

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

6 “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. http://nsdl.org

7 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. http://nsdl.org

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

9 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. http://nsdl.org

10 Electric charge in cloud

11 Possible lightning paths

12 Charge and image

13 The “Leader”

14 Main Stroke

15 Lightning striking tree

16 Lightning striking Empire State Building

17 Shoes of man struck by lightning Man not hurt!!

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

19 Pt II: Making a Magic Tape Electroscope First Step: Bend back one end on each of two tapes to make a handle. Second Step: Lay each piece of magic tape on top of each other on a smooth surface.

20 Third Step: Pick up the tape “handles” and pull the tapes apart. Making a Magic Tape Electroscope http://nsdl.org

21 Making a Magic Tape Electroscope The tapes separate as shown. What do you think causes this to occur? 1. 2. 3.

22 Making a Magic Tape Electroscope Charges on the surface of the tapes shown are alike Do we know whether these charges are positive or negative? How could we tell? Discuss these questions with your group and record answers in your notebook http://nsdl.org

23 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

24 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 same charge 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.

25 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)

26 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).

27 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

28 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 negative neutral induction 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. remember the ball is a conductor!

29 Concept Checks – Conductors (2) Two neutral conductors are connected by a wire and a charged rod is brought near, but does not touch. The wire is taken away, and then the charged rod is removed. What are the charges on the conductors? 1.00 2.+– 3.–+ 4.++ 5.– – 0 0 ? ?

30 Concept Checks – Conductors (2) Two neutral conductors are connected by a wire and a charged rod is brought near, but does not touch. The wire is taken away, and then the charged rod is removed. What are the charges on the conductors? positive charge will flow from the blue to the green ball due to polarization charges will remain on the separate conductors While the conductors are connected, positive charge will flow from the blue to the green ball due to polarization. Once disconnected, the charges will remain on the separate conductors even when the rod is removed. 1.00 2.+– 3.–+ 4.++ 5.– – 0 0 ? ?

31 Concept Check – Charging by Induction A positively charged object is placed close to a conducting object attached to an insulating glass pedestal (a). After the opposite side of the conductor is grounded for a short time interval (b), the conductor becomes negatively charged (c). Based on this information, we can conclude that within the conductor 1. both positive and negative charges move freely. 2. only negative charges move freely. 3. only positive charges move freely. 4. We can’t really conclude anything.

32 Concept Check – Charging by Induction A positively charged object is placed close to a conducting object attached to an insulating glass pedestal (a). After the opposite side of the conductor is grounded for a short time interval (b), the conductor becomes negatively charged (c). Based on this information, we can conclude that within the conductor 1. both positive and negative charges move freely. 2. only negative charges move freely. 3. only positive charges move freely. 4. We can’t really conclude anything.

33 Electromagnetic Charge

34 Positive and Negative Charge Positive (+) Air Human Hands Asbestos Rabbit's Fur Glass Human Hair Mica Nylon Wool Lead Cat's Fur Silk Aluminum Paper Cotton Steel Wood Lucite Sealing wax Amber Polystyrene Polyethylene Rubber balloon Sulphur Hard rubber Nickel, Copper Brass, Silver Gold, Platinum Sulfur Acetate, Rayon Polyester Celluloid Polyurethane Polyethylene Polypropylene Vinyl Silicon Teflon Saran Wrap Negative (-)

35 Insulators and Conductors + + + + + + + + + + + + + + + + + Conductor + + + + + + + + + + + + + + + + + Nonconductor

36 Insulators and Conductors + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Q + + + + + + + + + + + + + + + + + + Q/2

37 Concept Check – Coulomb’s Law What is the magnitude of the force F 2 ? 1.1.0 N 2.1.5 N 3.2.0 N 4.3.0 N 5.6.0 N Q Q F 1 = 3N F 2 = ?

38 Concept Check – Coulomb’s Law What is the magnitude of the force F 2 ? 1.1.0 N 2.1.5 N 3.2.0 N 4.3.0 N 5.6.0 N same magnitude force of one on the other of a pair is the same as the reverseNote that this sounds suspiciously like Newton’s 3rd Law!! The force F 2 must have the same magnitude as F 1. 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!! Q Q F 1 = 3N F 2 = ?

39 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:

40 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:

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

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

43 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 QF QFr Q ? Q ? 3r3r3r3r

44 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 Originally we had: Now we have: 1/9 as big as which is 1/9 as big as before. QFQFr Q F/9 Q F/9 3r3r3r3r

45 Electric Charge Model Su mmary of things we know: There is a property of matter called electric charge. (In the metric system its units are Coulombs.) Charges can be negative (like electrons) or positive (like protons). In matter, the positive charges are stuck in place in the nucleus. Matter is negatively charged when extra electrons are added, and positively charged when electrons are removed. Like charges repel, unlike charges attract. Charges travel in conductors, not in insulators Force of attraction or repulsion F=Kqq / r 2 p158

46 TWO kinds of electric energy Static Electricity Current Electricity

47 A few thousand volts

48 Van de Graaff Born 1901 Invented static electricity generator in 1929 This is the generator we will use today

49 Here is a bigger Van de Graaff generator

50 An even bigger one!

51 A giant Van de Graaff generator

52 The biggest--25 Million Volts Oak Ridge National Lab in Tennessee

53

54 Electrons jump onto the belt at the bottom Electrons jump off the belt at the top

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

56

57 Why does your hair do this??

58 Because all the (-) charges (electrons) repel! Finish : Lab handout Give me a charge(van de graf) CDP 32-1 32-2


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