1 / 27

Basic Principles of Electricity and Magnetism

Learn about the existence of electric charge, properties of electric charges, insulators, conductors, semiconductors, charging by induction, and Coulomb's law.

sstoddard
Télécharger la présentation

Basic Principles of Electricity and Magnetism

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. PHY 112 Basic Principle of Physics II – Electricity and Magnetism

  2. Electric Charge(Existence of Electric Charge) • A comb attracts bits of paper after it has been run through hair on a dry day. • An inflated balloon that has been rubbed with a wool adheres to a wall, sometimes for hours • Materials that behave the way the COMB and the BALLOON behaved are said to electrically charged

  3. Properties of Electric charges • There are two kinds of electric charges (positive and negative) • Like charges repel one another unlike charges attract one another. Applications Some cosmetics take advantage of electric force by incorporating materials that are electrically attracted to the skin or hair, causing the pigment to stay once applied.

  4. Properties of Electric charges • Electric charge is always conserved Meaning When one object is rubbed against another charge is not created in the process. The electrified state is due to transfer from one object to the other. One object gain some amount of negative charge while the other object gains an equal amount of positive charge.

  5. Properties of Electric charges • Electric charge (q) is quantized. Meaning q is integer multiplied by a quantity with a constant value. The quantity was found to be electron charge i.e. q = Ne

  6. Insulators, Conductors and Semiconductors • Substances are classified based on whether electric charge can flow through them or not • Insulators: These are materials in which electric charges can not move freely. Examples are – glass, rubber and wood. Implication: When insulators are charged by rubbing, only the area rubbed becomes charged because the charge will not be able to flow to other regions of the material.

  7. Conductors • Conductors: These are materials in which electric charges move freely. Examples are – copper, aluminium and silver. Implication: When the materials are charged in one region, the charge readily redistributes itself over the entire surface of the material. • Note: A metal that is held in the hand when it is charged will not attract a piece of paper because all the charges would have moved through the hand into the Earth. • However if the metal is held with an insulator, the charge will be retained in it and will therefore attract paper. • The insulator will prevent the charge from reaching the hand.

  8. Semiconductors • Semiconductors: These are materials with electrical properties between those of insulators and semiconductors. Examples are – silicon and germanium. Implication: When the materials are charged in one region, the charges move in the materials but not as fast as in conductors.

  9. Grounding of a conductor • When a CONDUCTOR is connected to the EARTH by means of a conducting wire or pipe, it is said to be GROUNDED. • The EARTH is usually referred to as an infinite “SINK” to which electrical charge can migrate.

  10. Charging by Induction • Apart from rubbing, materials can also be charged by induction Steps • Step 1: Take a neutral (uncharged) conducting sphere insulated from the ground

  11. Charging by Induction • Step 2: Bring a negatively charged rubber rod near sphere, the region nearest the rod will end up having excess positive charges while the region farthest away will have equal excess of negative charges.

  12. Charging by Induction(without insulation from ground) • Step 1: Take a neutral (uncharged) conducting sphere connected to the ground with a conducting wire.

  13. Charging by Induction(without insulation from ground) • Step 2: Bring a negatively charged rubber rod near sphere, the region nearest the rod will end up having excess positive charges while the region farthest away will have equal excess of negative charges. • Some of the electrons in the conductor are so strongly repelled by the presence of the negative charge in the rod that they move out through the wire to the Earth.

  14. Charging by Induction(without insulation from ground) • Step 3: When the wire to the ground is removed the conducting sphere contains excess induced positive charge.

  15. Charging by Induction(without insulation from ground) • Step 4: Finally when the rubber rod is removed, the induced positive charges remain and are uniformly distributed over the surface of the sphere due to the repulsive forces among them.

  16. Coulomb’s Law • The law describes the electric force between two charged objects. • The law states that the electric force between two stationary charged particles is (1) inversely proportional to thee square of the separation r between the particles and is directed along the line joining them. r q1 q2

  17. Coulomb’s Law (2) proportional to the product of charges q1 and q2 on the two particles i.e. (3) attractive if the charges are of opposite sign and repulsive if they have the same sign.

  18. Coulomb’s Law Equation • From the three observations, the Coulomb law equation for calculating the magnitude of the electric force (Coulomb force) between two point charges is • k is the Coulomb constant and has a value 8.9875 x 109 N.m2/C2= • o, permittivity for free space, is 8.8542 x 10-12 C2/N.m2

  19. Coulomb’s Law Equation • Since force is vector quantity, the coulomb equation in the vector form is • This is the electric force exerted by charge q1 on charge q2. • The force obeys Newton’s third law, the electric force exerted by q2 on charge q1 is

  20. Coulomb’s Law Equation • If q1 and q2 have the same sign, the force is positive and it is therefore repulsive. • If q1 and q2are of opposite signs, the force is negative and it is therefore attractive. • Hence product q1q2 determines the direction of the force. • When more than two charges are present, e.g. charges q1, q2, q3 and q4, then the resultant force exerted on q1 by q2, q3 and q4 is

  21. Questions • Question 1 • Solution to Question 1 • First calculate the number of atoms in 10.0g of silver using the relation

  22. Question • Where N is the number of atoms • atoms • (a) The number of electrons in the pin = number of atoms multiply by number of electrons per atom =5.58 x 1022 x 47 = 2.62 x 1024 electrons.

  23. Questions • The total number of electrons addede to make a charge 1 mC = this charge value divided charge per electron = 1mC/1.6x10-19C = 1x10-3C/1.6x10-19C/electron=6.25 x 1015 electrons. • First calculate the number of electrons added for every one electron = 6.25 x 1015/ 2.62 x 1024 = 2.385 x 10-9electron. • Hence for every 109 electrons already present the number added = 2.385 x 10-9 x 109 = 2.385 electrons

  24. Questions • Question 2

  25. Questions • Solution F2,7 F-4,7

  26. Questions • F7 = F2,7 + F-4,7

More Related