Electricity

1. Electricity

2. The importance of electrical power seems obvious in a modern industrial society. What is not so obvious is the role of electricity in magnetism, light, chemical change, and as the very basis for the structure of matter. All matter, in fact, is electrical in nature, as you will see.

3. Electric Charge

4. Electron Theory of Charge. • Electric Charge and Electrical Forces. • Electrons have a negative electrical charge. Joseph J. Thomson • Protons have a positive electrical charge • These charges interact to create an electrical force. • Like charges produce repulsive forces. • Unlike charges produce attractive forces.

5. A very highly simplified model of an atom has most of the mass in a small, dense center called the nucleus. The nucleus has positively charged protons and neutral neurons. Negatively charged electrons move around the nucleus a much greater distance than is suggested by this simplified model. Ordinary atoms are neutral because there is a balance between the number of positively charged protons and negatively charged electrons

6. Electrostatic Charge. • Electrons move from atom to atom to create ions. • positively charges ions result from the loss of electrons and are called cations • Negatively charge ions result from the gain of electrons and are called anions

7. (A) A neutral atom has no net charge because the numbers of electrons and protons are balanced. (B) Removing an electron produces a net positive charge; the charged atom is called a positive ion. (C) The addition of an electron produces a net negative charge and a negative ion.

8. Arbitrary numbers of protons (+) and electrons (-) on a comb and in hair (A) before and (B) after combing. Combing transfers electrons from the hair to the comb by friction, resulting in a negative charge on the comb and a positive charge on the hair

9. The charge on an ion is called an electrostatic charge. • An object becomes electrostatically charged by • Friction ,which transfers electrons between two objects in contact • Contact with a charged body which results in the transfer of electrons • Induction which produces a charge redistribution of electrons in a material • Charge is transferred in all three cases, it is not created or destroyed.

10. Charging by induction. The comb has become charged by friction, acquiring an excess of electrons. The paper (A) normally has a random distribution of (+) and (-) charges. (B) When the charged comb is held close to the paper, there is a reorientation of charges because of the repulsion of the charges. This leaves a net positive charge on the side close to the comb, and since unlike charges attract, the paper is attracted to the comb

11. Electrical Conductors and Insulators. • Electrical conductors are materials that can move electrons easily • Good conductors include metals. • Electrical nonconductors are materials that do not move electrons easily • These are also known as insulators • Semiconductors are materials that vary in their conduction and nonconduction, sometimes conducting sometimes not conducting.

12. Measuring Electrical Charges. • The magnitude of an electrical charge is dependent upon how many electrons have been moved to it or away from it. • Electrical charge is measured in coulombs. • A coulomb is the charge resulting from the transfer of 6.24 X 1018 of the charge carried by an electron • A very large amount of charge • A lightning discharge may transfer 200 coulombs of charge • A charged comb is less than 1 microcoulomb

13. The fundamental charge is the electrical charge on an electron and has a magnitude of 1.6021892 X 10-19 C • To determine the quantity of an electrical charge you simply multiple the number of electrons by the fundamental charge on an electron or: • q=ne • Where q is the magnitude of the charge, n is the number of electrons, and e is the fundamental charge.

14. Coulomb constructed a torsion balance to test the relationships between a quantity of charge, the distance between the charges, and the electrical force produced. He found the inverse square law held accurately for various charges and distances

15. Measuring Electrical Forces. • Force is proportional to the product of the electrical charge and inversely proportional to the square of the distance. • Coulomb’s Law • F is the force • k is a constant and has the value of 9.00 X 109 newtonmeters2/coulomb2 (9.00 X 10 9 Nm2/C2) • q1 represents the electrical charge of object 1 and q2 represents the electrical charge of object 2 • d is the distance between the two objects.

16. Force Fields. • The condition of space around an object is changed by the presence of an electrical charge. • The electrical charge produces a force field, that is called an electrical field since it is produced by electrical charge • All electrical charges are surrounded by an electrical field just like all masses are surrounded by gravitational fields.

17. A map of the electrical field can be made by bringing a positive test charge into an electrical field. • When brought near a negative charge the test charge is attracted to the unlike charge and when brought near a positive charge the test charge is repelled. • You can draw vector arrows to indicate the direction of the electrical field • This is represented by drawing lines of force or electrical field lines • These lines are closer together when the field is stronger and farther apart when it is weaker.

18. A positive test charge is used by convention to identify the properties of an electric field. The vector arrow points in the direction of the force that the test charge would experience

19. Lines of force diagrams for (A) a negative charge and (B) a positive charge when the charges have the same magnitude as the test charge.

20. Electrical Potential. • An electrical charge has an electrical field that surrounds it. • In order to move a second charge through this field work must be done • Bringing a like charge particle into this field will require work since like charges repel each other and bringing an opposite charged particle into the field will require work to keep the charges separated. • In both of these cases the electrical potential is changed.

21. Electric potential results from moving a positive coulomb of charge into the electric field of a second positive coulomb of change. When 1.00 joule of work is done in moving 1.00 coulomb of charge, 1.00 volt of potential results. A volt is a joule/coulomb.

22. The potential difference (PD) that is created by doing 1.00 joule of work in moving 1.00 coulomb of charge is defined as 1.00 volt • A volt is a measure of the potential difference between two points • electric potential =work to create . potential charge moved • PD=W • Q • The voltage of an electrical charge is the energy transfer per coulomb. • The energy transfer can be measured by the work that is done to move the charge or by the work that the charge can do because of the position of the field.

23. Electric Current.

24. Introduction • Electric current means a flow of charge in the same way that a water current flows. • It is the charge that flows, and the current is defined as the flow of the charge, it would be redundant to speak of a “flow of current”.

25. The Electric Circuit. • In order to have an electric current there must be a separation of the charge maintaining the electrical field (a potential difference). • This potential difference can push a charge through a conductor. • An electrical current is maintained by pumping charges to a higher electrical potential and the then do work as they flow back to a lower potential

26. The falling water can do work in turning the water wheel only as long as the pump maintains the potential difference between the upper and lower reservoirs.

27. An electrical circuit contains some device that acts as a source of energy as it gives charges a higher potential against an electrical field. • The charges do work as they flow through the circuit to a lower potential • The charges flow through connecting wires to make a continuous path. • A switch is a means of interrupting or completing the circuit. • The source of the electrical potential is the voltage source. • The device where the charges do work is the voltage drop.

28. A simple electric circuit has a voltage source (such as a generator or battery) that maintains the electrical potential, some device (such as a lamp or motor ) where work is done by the potential, and continuous pathways for the current to follow.

29. Voltageis a measure of the potential difference between two places in a circuit. • Voltage is measured in joules/coloumb. • The rate at which an electrical current (I) flows is the quantity (q) that moves through a cross section of a conductor in a give unit of time (t) I=q/t • the units of current are coulombs/second. • A coulomb/second is an ampere (amp) • In an electrical circuit the rate of current is directly proportional to the difference in electrical potential between two parts of the circuit IPD.

30. A simple electric circuit carrying a current of 1.00 coulomb per second through a cross section of a conductor has a current of 1.00 amp.

31. The Nature of Current. • Conventional current describes current as positive charges that flow from the positive to the negative terminal of a battery. • The electron current description is the opposite of the conventional current. • The electron current describes current as a drift of negative charges that flow from the negative to the positive terminal of a battery. • It is actually the electron current that moves charges. • Actually it does not matter which description is used, since positive charges and negative charges are mathematically equal

32. A conventional current describes positive charges moving from the positive terminal (+) to the negative terminal (-). An electron current describes negative charges (-) moving from the negative terminal (-) to the positive terminal (+)

33. (A) A metal conductor without a current has immovable positive ions surrounded by a swarm of chaotically moving electrons. (B) An electric field causes the electrons to shift positions, creating a separation charge as the electrons move with a zigzag motion from collisions with stationary positive ions and other electrons.

34. Electrons move very slowly in a direct current circuit. With a drift velocity of 0.01 cm/s, more than 5 hr would be required for an electron to travel 200 cm from a car battery to the brake light. It is the electric field, not the electrons, that moves at near the speed of light in an electric circuit.

35. The current that occurs when there is a voltage depends on: • The number of free electrons per unit volume of the conducting material. • The fundamental charge on each electron. • The drift velocity which depends on the electronic structure of the conducting material and the temperature. • The cross-sectional area of the conducting wire.

36. It is the electron field, and not the electrons, which does the work. • It is the electric field that accelerates electrons that are already in the conducting material. • It is important to understand that: • An electric potential difference establishes, at nearly the speed of light, an electric field throughout a circuit. • The field causes a net motion that constitutes a flow of charge. • The average velocity of the electrons moving as a current is very slow, even thought he electric field that moves them travels with a speed close to the speed of light.

37. What is the nature of the electric current carried by these conducting lines? It is an electric field that moves at near the speed of light. The field causes a net motion of electrons that constitutes a flow of charge, an alternating current. As opposed to DC.

38. Electrical Resistance. • Electrical resistance is the resistance to movement of electrons being accelerated with an energy loss. • Materials having the property of reducing a current and this is electrical resistance (R). • Resistance is a ratio between the potential difference (PD)between two points and the resulting current (I). • R=PD/I • The ratio of volts/amp is called an ohm ()

39. The relationship between voltage, current, an resistance is • V=IR • Ohms Law • The magnitude of the electrical resistance of a conductor depends on four variables. • The length of the conductor. • The cross-sectional area of the conductor. • The material the conductor is made of. • The temperature of the conductor.

40. The four factors that influence the resistance of an electrical conductor are the length of the conductor, the cross-sectional area of the conductor, the material the conductor is made of, and the temperature of the conductor

41. Electrical Power and Electrical Work. • All electrical circuits have three parts in common. • A voltage source. • An electrical device • Conducting wires.

42. The work done by a voltage source is equal to the work done by the electrical field in an electrical device. • W=PDq • The electrical potential is measured in joules/coulomb and a quantity of charge is measured in coulombs, so the electrical work is measure in joules. • A joule/second is a unit of power called the watt. • power = current (in amps) X potential (in volts) • P=IV

43. What do you suppose it would cost to run each of these appliances for one hour? (A) This light bulb is designed to operate on a potential difference of 120 volts and will do work at the rate of 100 W. (B) The finishing sander does work at the rate of 1.6 amp x 120 volts or 192 W. (C) The garden shredder does work at the rate of 8 amps x 120 volts, or 960 W.

44. This meter measures the amount of electric work done in the circuits, usually over a time period of a month. The work is measured in kWhr

45. Magnetism.

46. Magnetic Poles. • A North seeking pole is called the North Pole • A South seeking pole is called the South Pole • Like magnetic poles repel and unlike magnetic poles attract.

47. Every magnet has ends, or poles, about which the magnetic properties seem to be concentrated. As this photo shows, more iron filings are attracted to the poles, revealing their location.

48. Magnetic Fields. • A magnet that is moved in space near a second magnet experiences a magnetic field. • A magnetic field can be represented by field lines. • The strength of the magnetic field is greater where the lines are closer together and weaker where they are farther apart.

49. These lines are a map of the magnetic field around a bar magnet. The needle of a magnetic compass will follow the lines, with the north end showing the direction of the field.

50. The earth's magnetic field. Note that the magnetic north pole and the geographic North Pole are not in the same place. Note also that the magnetic north pole acts as if the south pole of a huge bar magnet were inside the earth. You know that it must be a magnetic south pole since the north end of a magnetic compass is attracted to it and opposite poles attract