BASIC PRINCIPLES IN ELECTRICITY AND MAGNETISM

3. All of us agree the importance of electricity in our daily lives. But what is electricity?

6. 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.

7. Charging by Induction 0

12. Coulomb’s Law: | F | = k | Q q o | / r 2 Rearranged: | F | = | q o [k Q/r 2 ] | Gives us: F = q o E where the electric field E is: | E | = | k Q / r 2 |

14. 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.

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

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20. 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 flow.

22. 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, a current .

23. (A) A metal conductor without a current has immovable positive ions surrounded by a swarm of randomly 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.

30. 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 .

33. 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.

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37. 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 .

40. 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.

41. A bar magnet cut into halves always makes new, complete magnets with both a north and a south pole. The poles always come in pairs. You can not separate a pair into single poles.

43. Oersted discovered that a compass needle below a wire (A) pointed north when there was not a current , (B) moved at right angles when a current flowed one way, and (C) moved at right angles in the opposite direction when the current was reversed .

44. (A) In a piece of iron, the magnetic domains have random arrangement that cancels any overall magnetic effect (not magnetic). (B) When an external magnetic field is applied to the iron, the magnetic domains are realigned, and those parallel to the field grow in size at the expense of the other domains, and the iron becomes magnetized .

45. A magnetic compass shows the presence and direction of the magnetic field around a straight length of current-carrying wire.

46. When a current is run through a cylindrical coil of wire, a solenoid , it produces a magnetic field like the magnetic field of a bar magnet. The solenoid is known as electromagnet .

48. A galvanometer measures the direction and relative strength of an electric current from the magnetic field it produces . A coil of wire wrapped around an iron core becomes an electromagnet that rotates in the field of a permanent magnet. The rotation moves pointer on a scale.

53. Electric currents create magnetic fields. 0

54. Charges moving in a magnetic field experience an electromagnetic force. 0

55. A changing magnetic field creates an electric field. 0

56. Induced electric currents act so as to oppose the motion that caused them. 0

57. FAITH ACADEMY, NEW DELHI-5. YEAR: 2008-09