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Electricity and Magnetism Electric Charge Coulomb’s Law Capacitors Electric Charges and Forces

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Objectives 1. Describe and calculate the forces between like and unlike electric charges. 2. Identify the parts of the atom that carry electric charge. 3. Apply the concept of an electric field to describe how charges exert force on other charges. 4. Sketch the electric field around a positive or negative point charge. 5. Describe how a conductor shields electric fields from its interior. 6. Describe the voltage and current in a circuit with a battery, switch, resistor, and capacitor. 7. Calculate the charge stored in a capacitor.

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Vocabulary Terms: homework charge electrically neutral static electricity positive charge negative charge electric forces charge by friction electroscope protons neutrons electrons gravitational field charged induction Coulomb’s law capacitor parallel plate capacitor microfarad coulomb electric field capacitance charge polarization shielding test charge farad field inverse square law discharged field lines

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Electrostatics In Physics

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Electric Charge Key Questions: 1. Define electrically neutral 2. Define net charge or excess charge 3. What causes static electricity

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Electric Charge All ordinary matter contains both positive and negative charge. You do not usually notice the charge because most matter contains the exact same number of positive and negative charges. An object is electrically neutral when it has equal amounts of both types of charge.

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Electric Charge Objects can lose or gain electric charges. The net charge is also sometimes called excess charge because a charged object has an excess of either positive or negative charges. A tiny imbalance in either positive or negative charge on an object is the cause of static electricity.

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Electric charge Answer the following: 4. What is the unit of electrical charge? 5. What is the mass of an electron, proton, and neutron? 6. What are the charges in coulombs of a protron, neutron, and electron? 7. Describe the attracting and repelling characteristics of protons and electrons

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Electric Charge Electric charge is a property of tiny particles in atoms. The unit of electric charge is the coulomb (C). A quantity of charge should always be identified with a positive or a negative sign.

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Electric forces Electric forces are created between all electric charges. Because there are two kinds of charge (positive and negative) the electrical force between charges can attract or repel.

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8. Draw a picture of an electroscope with a charge (positive or negative) 9. Describe how the electroscope becomes positively charged through conduction 10. Describe how the electroscope becomes negatively charged through conduction

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Electric forces The forces between the two kinds of charge can be observed with an electroscope.

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Electric forces Charge can be transferred by conduction.

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11. What moving particles create a current in metals? 12. Define current 13. Write the formula for current 14. What is the unit for current?

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In conductive liquids (salt water) both positive and negative charges carry current. In solid metal conductors, only the electrons can move, so current is carried by the flow of negative electrons. The direction of current was historically defined as the direction that positive charges move. Both positive and negative charges can carry current.

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Electric current Current is the movement of electric charge through a substance. Current (amps) Charge that flows (coulombs) Time (sec) I = q t

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Calculate current 15. Two coulombs of charge pass through a wire in five seconds. Calculate the current in the wire.

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16. Compare the movement of an electron in a conductor and insulator 17. Describe a semi-conductor 18. Describe charging by friction 19. What causes a negatively charged balloon to stick to a wall?

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Conductors and insulators All materials contain electrons. The electrons are what carry the current in a conductor. The electrons in insulators are not free to move—they are tightly bound inside atoms.

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Conductors and insulators A semiconductor has a few free electrons and atoms with bound electrons that act as insulators.

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Conductors and insulators When two neutral objects are rubbed together, charge is transferred from one to the other and the objects become oppositely charged. This is called charging by friction. Objects charged by this method will attract each other.

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20. Define coulomb’s law and write the formula 21. What is coulomb’s constant? 22. Describe the force between two charges if the distance between them is tripled 23. Describe the force between two charges if both charges are increased by a factor of two.

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Coulomb's Law Coulomb’s law relates the force between two single charges separated by a distance. Force (N) Constant 9 x10 9 N. m 2 /C 2 Distance (m) F = K q 1 q 2 r 2 Charges (C)

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Coulomb's Law The force between two charges gets stronger as the charges move closer together. The force also gets stronger if the amount of charge becomes larger.

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Coulomb's Law The force between two charges is directed along the line connecting their centers. Electric forces always occur in pairs according to Newton’s third law, like all forces.

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Coulomb's Law The force between charges is directly proportional to the magnitude, or amount, of each charge. Doubling one charge doubles the force. Doubling both charges quadruples the force.

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Coulomb's Law The force between charges is inversely proportional to the square of the distance between them. Doubling the distance reduces the force by a factor of 2 2 = (4), decreasing the force to one-fourth its original value (1/4). This relationship is called an inverse square law because force and distance follow an inverse square relationship. Now answer questions #22, 23

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Calculating force 24. Two balls are each given a static electric charge of one ten- thousandth (0.0001) of a coulomb. Calculate the force between the charges when they are separated by one-tenth (0.1) of a meter.

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25. Describe an electric field 26. What type of field does mass create? 27. Compare the gravitational force and electrical force between two charges. 28. Draw an electrical field between: a. A proton and electron b. A proton and proton c. An electron and electron 29. Draw the electric field coming from a single proton and a single electron

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Fields and forces The concept of a field is used to describe any quantity that has a value for all points in space. You can think of the field as the way forces are transmitted between objects. Charge creates an electric field that creates forces on other charges.

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Fields and forces Mass creates a gravitational field that exerts forces on other masses.

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Fields and forces Gravitational forces are far weaker than electric forces.

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Drawing the electric field

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30. Describe the units and equations for both a gravity field and electric field 31. Describe how electric fields accelerate particles causing changes in voltage 32. Why is shielding necessary around electrical wires inside of sensitive electronic equipment?

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Electric fields and electric force On the Earth’s surface, the gravitational field creates 9.8 N of force on each kilogram of mass. With gravity, the strength of the field is in newtons per kilogram (N/kg) because the field describes the amount of force per kilogram of mass.

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Electric fields and electric force With the electric field, the strength is in newtons per coulomb (N/C). The electric field describes the amount of force per coulomb of charge.

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Accelerators An electric field can be produced by maintaining a voltage difference across any insulating space, such as air or a vacuum. Electric fields are used to create beams of high-speed electrons by accelerating them. Electron beams are used in x-ray machines, televisions, computer displays, and many other technologies.

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Electric shielding Electric fields are created all around us by electric appliances, lightning, and even static electricity. These stray electric fields can interfere with the operation of computers and other sensitive electronics. Many electrical devices and wires that connect them are enclosed in conducting metal shells to take advantage of the shielding effect.

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Practice problems: 33. A particle in a microwave has a charge of -9 x 10 -15 C and is deflected into popcorn with a force of 4.5 x 10 -2 N. What is the strength of the electric field. USE F = Eq q = -9 x10 -15 C F = 4.5 x 10 -2 N E = F/q = 4.5 x 10 -2 N/- 9 x10 -15 C = 5 x 10 12 N/C

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34. A positive and negative particle, each with a 5 x 10 -8 charge, are separated by a distance of 6 mm. What is the Force acting on them? Use coulomb’s law F = k q 1 q 2 /r 2 F = (9x10 9 ) (-) (5 x 10 -8 ) (5 x 10 -8 ) / (6 x 10 -3 ) 2 = -6.25x10 -1 N

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35. What would the force be in number 34 if the distance were tripled F = k q 1 q 2 /r 2 If r is tripled the r 2 = 3 2 = 9 so the force would be 1/9 as strong = -6.25x10 -1 N/9

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36. Find the size of the electrical field at the location of one charge due to the other charge? USE F = Eq, Use information from question #35 E = F/q = -6.25x10 -1 N/ (5 x 10 -8 )

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37. Find the potential energy of the charged pair. Use U E = k e q 1 q 2 /r = (9x10 9 ) (-) (5 x 10 -8 ) (5 x 10 -8 ) / (6 x 10 -3 )

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38. How many excess electrons does the microwave particle in #33 have? Q = Ne where Q = the total charge N = the number of electrons e = electronic charge 1.6 x 10 -19 C N = Q/e = 9 x10 -15 C/ 1.6 x 10 -19 C = 5.625 x 10 4

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Capacitors 39. What is a capacitor? 40. What is the symbol for a capacitor in a circuit diagram? 41. How is a capacitor charged? 42. How is a capacitor discharged? 43. What determines flow in and out of a capacitor? 44. Describe a parallel plate capacitor 45. Describe how the parallel plate capacitor works

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Capacitors A capacitor is a storage device for electric charge. Capacitors can be connected in series or parallel in circuits, just like resistors.

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Capacitors A capacitor can be charged by connecting it to a battery or any other source of current. A capacitor can be discharged by connecting it to any closed circuit that allows current to flow.

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Capacitors The current flowing into or out of a particular capacitor depends on four things: 1. The amount of charge already in the capacitor. 2. The voltage applied to the capacitor by the circuit. 3. Any circuit resistance that limits the current flowing in the circuit. 4. The capacitance of the capacitor.

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How a capacitor works inside The simplest type of capacitor is called a parallel plate capacitor. It is made of two conductive metal plates that are close together, with an insulating plate in between to keep the charges from coming together. Wires conduct charges coming in and out of the capacitor.

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46. What determines how much charge a capacitor can hold? 47. Define capacitance 48. What is the formula for capacitance 49. What is the unit for capacitance 50. What is a microfarad?

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How a capacitor works inside The amount of charge a capacitor can store depends on several factors: 1. The voltage applied to the capacitor. 2. The insulating ability of the material between the positive and negative plates. 3. The area of the two plates (larger areas can hold more charge). 4. The separation distance between the plates.

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Capacitance The ability of a capacitor to store charge is called capacitance (C). Charge (C) Capacitance (coulombs/volt) q = C V Voltage (volts) Cameras use capacitors to supply quick bursts of energy to flash bulbs.

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Capacitance Capacitance is measured in farads (F). A one-farad capacitor can store one coulomb of charge when the voltage across its plates is one volt. One farad is a large amount of capacitance, so the microfarad (μF) is frequently used in place of the farad.

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Calculate capacitance 51. A capacitor holds 0.02 coulombs of charge when fully charged by a 12-volt battery. Calculate its capacitance and the voltage that would be required for it to hold one coulomb of charge. C = q/V

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Capacitors: electric field = V/d 52. 2000 volts moves across a parallel plate capacitor with a separation of 5 mm. What is the electric field across the capacitor? E = V/d = 2000V/5x10 -3 m = 4 x 10 N/C

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Application: How a Television Works

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