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Electricity

Electric Charges. electricity comes from the Greek word elektron, which means amber. Amber is petrified tree resin and the Greeks noticed that if you rubbed an amber rod with a cloth it would attract small bits of leaves Today rubbing a balloon, a rubber rod, a glass rod or any number of substances causes attraction of other objects (Charging an object).

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Electricity

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    1. Electricity Electrostatics and Fields

    2. Electric Charges electricity comes from the Greek word elektron, which means amber. Amber is petrified tree resin and the Greeks noticed that if you rubbed an amber rod with a cloth it would attract small bits of leaves Today rubbing a balloon, a rubber rod, a glass rod or any number of substances causes attraction of other objects (Charging an object)

    3. There are only two possible types of charge. Positive charge – rubbing glass rod with silk Negative charge – rubbing rubber rod with fur Unlike charges attract and like charges repel Charged objects also tend to attract neutral objects

    4. Repulsion/Attraction of Charges

    5. What Happens when a object becomes charged? Atoms have three subatomic particles: protons (+), electrons (-) and neutrons In neutral atoms the # of protons = The # of electrons The electrons, with addition of some energy, can be removed and transferred to other objects Rod + fur – electrons move from fur to rod

    6. Charging Objects

    7. Electrons move, but protons do not! Positively charged objects usually end up that way because they have lost electrons leaving them with more protons that electrons Remember – we are just moving electrical charge from place to place

    8. Conduction and Induction There are other ways to give an object an electrical charge Insulators vrs conductors

    9. Conduction Bring a neutral object in contact with a previously charged object. Both objects will end up with the same charge although it will be smaller than the initial charge Electroscope – used to illustrate the behaviour of electrical charges

    10. Conduction

    11. Induction A charged object is just brought near a neutral object. The charged object never actually touches the neutral object.

    12. Induction The electrons flow downward away from the neg. rod and they repel each other

    13. Using Induction to Determine Charge

    14. Grounding The Earth because of its immense mass can gain or lose many negative (or positive) charges and still remain neutral. As a result when you make contact between a charged object and the Earth it will immediately lose its charge and become neutral. This process is known as grounding. The third wire on an electrical outlet is grounded so as to avoid any dangerous charge buildup

    15. Grounding

    16. Electrical Forces We have seen how electrically charged objects can repel or attract other charged objects. Is there a way to mathematically describe the attraction/repulsion? This question was answered by Charles Coulomb. the greater the quantity of charge involved the greater the force of attraction or repulsion. Also, The force of attraction/repulsion actually depends on the square of the distance.

    17. F = force of attraction/repulsion (Newtons or N) q1 and q2 = quantity of charge on each object (coulombs or C) d = distance between the two charged objects (m) K = 9.00x109 N m2/C2

    18. Conversions Typically charges are usually measured in micro-coulombs (µC) ( 1C is a large quantity of charge) Is there a limit to how small a charge can be? As it turns out there appears to be. No charge smaller than the charge found on an electron (or proton) seems to exist. This quantity of charge is called the elementary charge and has a value of: e= 1.60 x 10-19 C

    19. Examples Two positive charges each of quantity10 µC are separated by a distance of 5.00x10-8 m. What force will each charge experience?

    20. Three examples A -30 µC charge is placed 40 cm from a second unknown charge. If the -30 µC charge experiences a net attractive force of 25.3 N, what is value of the second charge? A 40 µC and -20 µC charge exert a force of magnitude 1000 N on each other. How far apart are the two charges? How many protons are needed to have 1.0 C of charge?

    21. Example Three charges are arranged at the corners of a right triangle as shown below. Calculate the net force exerted on charge B.

    22. Both charges A and C exert a force on B. The net force on B will therefore be the resultant of the force that A and C exert on it. Even though A and C exert a force on each other these forces have no effect on B so we can ignore them

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