# 1 ELECTRICITY I Statics 2 If you rub a piece of amber With the fur of a rabbit It will attract bits of stuff (paper, leaves, etc) Electrostatics if the.

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1 ELECTRICITY I Statics

2 If you rub a piece of amber With the fur of a rabbit It will attract bits of stuff (paper, leaves, etc) Electrostatics if the study of electrical charges that can be contained in one place

3 Charges are measured in units called Coulombs. 1 C = 6.25 X 10 18 electrons1 C = 6.25 X 10 18 electrons The charge of one electron is 1.602 x 10 -19 C. This magnitude is called e -.The charge of one electron is 1.602 x 10 -19 C. This magnitude is called e -. e also can be used to represent a proton charge.e also can be used to represent a proton charge. Charges are often expressed in units of e. (Ex: 2e, 3e - )Charges are often expressed in units of e. (Ex: 2e, 3e - )

4 Charge and Mass of Atomic Particles electron-1.60x10 -19 9.109 x 10 -31 proton+1.60x10 -19 1.673 x 10 -27 neutron01.675 x 10 -27 Particle Charge (C) Mass (kg)

Elementary Charges What is the charge of 4 electrons?What is the charge of 4 electrons? What is the charge of a helium nucleus?What is the charge of a helium nucleus? How many electrons in 1.92 E-19C?How many electrons in 1.92 E-19C? Is a charge of 2E-19 possible?Is a charge of 2E-19 possible? 5

6 The Law of Conservation of Electric Charge… Charge can neither be created nor destroyed, only transferred.Charge can neither be created nor destroyed, only transferred. Transferring charge creates ions…which are charged particles.Transferring charge creates ions…which are charged particles. Removing electrons creates a positive charge.Removing electrons creates a positive charge. Adding electrons creates a negative charge.Adding electrons creates a negative charge.

7 Electric Charges Like charges repel Unlike charges attract

8 Polarization Bringing a charged object near (but not touching) a neutral object polarizes (temporarily separates) the charge of the neutral object. Like charges in the neutral object are repelled by the charged object. Unlike charges in the neutral object are attracted by the neutral object. The neutral object returns to normal when the charged object is removedBringing a charged object near (but not touching) a neutral object polarizes (temporarily separates) the charge of the neutral object. Like charges in the neutral object are repelled by the charged object. Unlike charges in the neutral object are attracted by the neutral object. The neutral object returns to normal when the charged object is removed

9 Electric Dipoles An object that is electrically neutral overall, but permanently polarized, is called an electric dipole. Example: H 2 0 moleculeAn object that is electrically neutral overall, but permanently polarized, is called an electric dipole. Example: H 2 0 molecule 123 physics.com123 physics.com

10 Charging by Contact If a charged object is brought in contact with a neutral object, charges will be repelled from (or attracted to) the charged object. The neutral object will gain a charge of the same sign as the charged object.If a charged object is brought in contact with a neutral object, charges will be repelled from (or attracted to) the charged object. The neutral object will gain a charge of the same sign as the charged object.

11 Charging by Induction Bring a charged object near (but not touching) a neutral object. Ground the neutral object. Remove the ground. Remove the charged object The neutral object now has a charge opposite to the charged object.Bring a charged object near (but not touching) a neutral object. Ground the neutral object. Remove the ground. Remove the charged object The neutral object now has a charge opposite to the charged object.

12 Grounding Providing a path from a charged object to the Earth is called grounding it. Charges will be attracted from (or repelled to) the Earth by the charged object. Since the Earth is so large, both the charged object and the Earth are neutralized.Providing a path from a charged object to the Earth is called grounding it. Charges will be attracted from (or repelled to) the Earth by the charged object. Since the Earth is so large, both the charged object and the Earth are neutralized.

13 Electrons in Solids When atoms are arranged in a solid they “share” electronsWhen atoms are arranged in a solid they “share” electrons –High electron mobility = conductor –Low electron mobility = insulator In order to produce a net flow of electrons, they must increase their energyIn order to produce a net flow of electrons, they must increase their energy

14 Superconductors Superconductors are materials that lose all resistance to charge movement at temperatures near absolute zero (0 K or about -273 o C). Recently, “high temperature” (above 100 K) superconductors have been discovered.Superconductors are materials that lose all resistance to charge movement at temperatures near absolute zero (0 K or about -273 o C). Recently, “high temperature” (above 100 K) superconductors have been discovered.

15 The Periodic Table

16 Static charges in Nature A typical thunder cloud has both + and – charges A typical thunder cloud has both + and – charges Lightning is static electricity Lightning is static electricity –Bolt travels negative to positive

17 LIGHTNING Though air is not a conductor, the buildup of charges during a storm forces the electricity to flow through the air anyway, ripping apart gas molecules along the way. This “energized gas” state is called plasma - which is a good conductor.

18 Question 1 Explain from an atomic standpoint why charge is usually transferred by electrons.Explain from an atomic standpoint why charge is usually transferred by electrons. Protons are relatively fixed in the nucleus of an atom, while electrons can be transferred from one atom to another.Protons are relatively fixed in the nucleus of an atom, while electrons can be transferred from one atom to another.

19 Question 2 Calculate the net charge on a substance consisting of a combination of 7.0 x 10 13 protons and 4.0 x 10 13 electrons.Calculate the net charge on a substance consisting of a combination of 7.0 x 10 13 protons and 4.0 x 10 13 electrons. 4.8 x 10 -6 C4.8 x 10 -6 C

20 Question 3 A negatively charged balloon has 3.5  C of charge. How many excess electrons are on this balloon? A negatively charged balloon has 3.5  C of charge. How many excess electrons are on this balloon? 2.2 x 10 13 electrons2.2 x 10 13 electrons

21 ELECTRICITY I Coulomb’s Law

22 Coulomb experimented with charged spheres. He found that:He found that: –electrostatic force varied inversely with square of the distance between the spheres. –electrostatic force varied directly with magnitude of the charge. He combined these discoveries to make his law.He combined these discoveries to make his law. F e = kqq’/d 2

23 Electrical Forces The electrical force between 2 charges depends on: The size of each charge More charge means more force. The distance between the charges More distance means less force.The electrical force between 2 charges depends on: The size of each charge More charge means more force. The distance between the charges More distance means less force.

24 Comparing gravitational force to electric force GravityGravity –Is a weak force –Acts over long distances –Is attractive only ElectricityElectricity –Is a strong force –Acts over short distances –Is attractive and repulsive

25 What are q,d and k? q is the magnitude of the charge. q is the magnitude of the charge. –q’ can indicate a test charge d is the distance between the charged particles. d is the distance between the charged particles. k is Coulomb’s constant of proportionality. k is Coulomb’s constant of proportionality. – k = 8.99 x 10 9 N·m 2 /C 2 (Compare this to G which is 6.67 x 10 -11 N·m 2 / kg 2 !)

26 Sample Problem: The electron and proton of a hydrogen atom are separated, on average, by a distance of about 5.3x10 -11 m.The electron and proton of a hydrogen atom are separated, on average, by a distance of about 5.3x10 -11 m. Find the magnitudes of the electric force and the gravitational force that each particle exerts on the other.Find the magnitudes of the electric force and the gravitational force that each particle exerts on the other.

27 For the electric force, we use Coulomb’s Law: F e = kqq’/d 2 Fe =Fe =Fe =Fe = (8.99 x 10 9 N·m 2 /C 2 ) * (1.6 x 10 -19 C) * (-1.6 x 10 -19 C) (5.3 x 10 -11 ) 2 (5.3 x 10 -11 ) 2 = -8.2 x 10 -8 N

28 For the gravitational force, we use Newton’s Law of Gravitation: F = Gm 1 m 2 /d 2 Fg =Fg =Fg =Fg = (6.67 x 10 -11 Nm 2 /kg2) * (9.11 x 10 -31 kg) * (1.67 x 10 -27 kg) (5.3 x 10 -11 m) 2 (5.3 x 10 -11 m) 2 = 3.6 x 10 -47 N

29 Question 4 How does the electric force between two charges change when the distance between them is doubled?How does the electric force between two charges change when the distance between them is doubled? The force is quarteredThe force is quartered

30 Question 5 What is the mathematical representation of Coulomb’s Law?What is the mathematical representation of Coulomb’s Law? F = kqq’/d 2F = kqq’/d 2

31 Question 6 What is the numerical value and unit of Coulomb’s constant?What is the numerical value and unit of Coulomb’s constant? 8.99 x 10 9 N m 2 / C 28.99 x 10 9 N m 2 / C 2

32 ELECTRICITY I Electric Fields

33 The Electric Field An electric field caused by a point charge exerts a force on any other charge in its environment.An electric field caused by a point charge exerts a force on any other charge in its environment. A collection of all the forces makes up an electric field.A collection of all the forces makes up an electric field. Faraday (1791-1867) defined the electric field as the region of space around a charged object.Faraday (1791-1867) defined the electric field as the region of space around a charged object. When another charged object enters the field, electrical forces arise.When another charged object enters the field, electrical forces arise.

34 Electric Field lines for a positive point charge Electric Field lines for a negative point charge

35

36 Electric Dipoles Field lines for two point charges of equal magnitudes, but opposite signs. This charge configuration is called an electric dipole.

37 Electric Field A positive test charge experiences a force directed away from the central charge + + As the charge is moved closer to the central charge the force increases in magnitude + +

38 Effects of Fields No effect on neutrals.No effect on neutrals. Electric fields can accelerate charged particles or repel them.Electric fields can accelerate charged particles or repel them. Magnetic fields can turn a charged particle in a circleMagnetic fields can turn a charged particle in a circle

39 Electric Field Intensity is the ratio of the force on a test charge to its charge. The units are Newtons per Coulomb (N/C) E is the electric field intensityE is the electric field intensity F is the forceF is the force q’ is the test chargeq’ is the test charge

40 ELECTRICITY I Electric Potential

41 Electric Potential It takes a force to raise an object in the Earth’s gravitational field. When we do raise an object in the field we change its gravitational potential energy. (mgh)It takes a force to raise an object in the Earth’s gravitational field. When we do raise an object in the field we change its gravitational potential energy. (mgh) When a charge is moved against an electric field, its potential energy also changesWhen a charge is moved against an electric field, its potential energy also changes

42 Electric Potential A positive charge experiences a downward force in the direction of the E-field It takes work to move the charge against the electric field increasing its PE

43 Electrical Potential (V) The electrical potential (V) is defined as the electrical potential energy per unit charge.The electrical potential (V) is defined as the electrical potential energy per unit charge. Potential (V)= Potential (V)= electrical potential energy charge electrical potential energy charge Electric potential is a scalarElectric potential is a scalar

44 Electrical Potential and Current A common analogy is to compare potential to water pressure.A common analogy is to compare potential to water pressure. The higher the potential, the higher the pressure to transfer charge.The higher the potential, the higher the pressure to transfer charge. A battery maintains a continuous potential difference.A battery maintains a continuous potential difference.

45 Electrical Potential (Voltage) The unit for electrical potential is the Volt, named after Alessandro Volta, an Italian scientist who developed the batteryThe unit for electrical potential is the Volt, named after Alessandro Volta, an Italian scientist who developed the battery 1 Volt = 1 Joule / 1 Coulomb1 Volt = 1 Joule / 1 Coulomb

46 Potential Difference (Voltage) The voltage difference between 2 points is equal to the work done against the field to move a positive charge from A to B with no acceleration

47 Uniform Electric Field Two parallel conducting plates a distance, d, apart constitute a uniform electric field.Two parallel conducting plates a distance, d, apart constitute a uniform electric field. The uniform field emanates from the positive plate to the negative plate.The uniform field emanates from the positive plate to the negative plate.

48 Voltage in a Uniform E-field Voltage equals work per unit charge.Voltage equals work per unit charge. That is V = Nm/CThat is V = Nm/C N/C is E (electric field intensity).N/C is E (electric field intensity). m is distance between the plates. m is distance between the plates. Therefore…Therefore…

49 Electron Volt (eV) When a particle with a charge equal to that of an electron moves through 1 volt in an E-field, it changes energy by 1 eV.  PE = q  V 1 eV = 1.6 x 10 -19 J

50 ELECTRICITY I Capacitance

51 Capacitance Capacitance is the ability to store charge.Capacitance is the ability to store charge. A charge storing device is called a capacitor.A charge storing device is called a capacitor.

52 Capacitance At a given potential (V), the amount of charge (Q) that can be stored by a body depends on its physical characteristics.At a given potential (V), the amount of charge (Q) that can be stored by a body depends on its physical characteristics. These physical characteristics are lumped together under the term capacitance (C).These physical characteristics are lumped together under the term capacitance (C).

53 Capacitance (C) C = Q/VC = Q/V The unit of capacitance is the Farad (F), named to honor Michael FaradayThe unit of capacitance is the Farad (F), named to honor Michael Faraday 1 Farad = 1 Coulomb / 1 Volt1 Farad = 1 Coulomb / 1 Volt

54 Capacitance 1 Farad is a large amount of capacitance.1 Farad is a large amount of capacitance. Usually a capacitor found in a piece of electronic equipment will be rate in microfarads (  F) or picofarads (pF)Usually a capacitor found in a piece of electronic equipment will be rate in microfarads (  F) or picofarads (pF)

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