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Transcript
What Are Electric Charges and How Do They Interact ? (Chapter 26) Where do charges come from ? • • • Charge is a intrinsic property of all basic building blocks of the universe. These building blocks are known as particles. There are three fundamental particles that make up all of the matter in our world – Protons: Positive (+) charge. Form the nucleus of atoms. – Neutrons: Neutral (0) charge. Form the nucleus of atoms with Protons. – Electrons: Negative (-) charge. They orbit the nucleus. Other particles (neutrinos, muons, tauons, mesons, pions, etc…) can only be found under exotic conditions: radioactive materials, nuclear reactors, particle accelerators, inside of stars and other astronomical bodies. An Atom Charge quantization • A macroscopic object has net charge Proton weighs 1800 times more than electron but has the same amount of charge! Neutrons have same mass as protons. • Where Np and Ne are the number of protons and electrons contained in the object. • The process of removing an electron from the electron cloud of an atom is called ionization. • An atom that is missing an electron is called a positive ion. Its net charge is q = +e. Charge Conservation • The total amount of charge is always conserved! • In other words, in a closed system (like the universe), the sum of the positive and negative charges is always the same! • Charges move around OR can be created/destroyed in balanced pairs. Example of charge creation and destruction in nuclear reactions: The net charge is unchanged by the reactions. PET (Positron Emission Tomography) Scanner Because protons and neutrons are much heavier than electrons, they do not move. Charge is transferred by the motion of electrons. An atom is a neutral object (no. electrons= no. protons). An ion is an atom that has lost or received an extra electron. EXAMPLE: Table salt when dissolved in water creates negative Cl and positive Na ions. Insulators • The electrons in an insulator are all tightly bound to the positive nuclei and not free to move around. • Charging an insulator by friction leaves patches of molecular ions on the surface • These patches are immobile. Chemistry: Insulators have a nearly full valence shell Conductors • Metals: the outer atomic electrons are weakly bound to the nuclei. • Valence electrons become detached from their parent nuclei and are free to wander about through the entire solid. • Electrons form negatively charged “liquid” permeating an array of positively charged ion cores. Chemistry: Metals are atoms with a nearly empty valence shell What else besides metals is a conductor??? How Do you Charge Objects?? • Friction (rubbing plastic rod with wool) • Transfer (touching a charged rod to a piece of metal) • INDUCTION: Charging without physical contact Polarization Force: Metals The Electric Dipole Dipoles are created whenever a charged object is placed near atoms or molecules. An electric dipole is a charge neutral object where one side has +q positive charge and the opposite side has -q negative charge. Charge Polarization: Insulators Coulomb’s Law: Force Between Two Point Charges In SI units K = 8.99 × 109 N m2/C2. The Electric Force (~Gravity) • The electric force between two charges is – Attractive if they have opposite charges – Repulsive if they have the same charge Limitations of Coulomb’s Law • Coulomb’s Law applies to point charges. OR • Size of the charged objects is very small compared to the distance separating them. The Superposition Principle • Electric forces like other forces can be superimposed. Coulomb’s Law: Example What is total force on q2 ??? Equilteral triangle- all sides 1m Solution.... ! F32 = ? "1ˆ 3 % ! " ˆ " r32 = cos(60 )i ! sin(60 ) ĵ = $ i ! ĵ ' m 2 & #2 r32 = 1m "1ˆ 3 % $# 2 i ! 2 ĵ '& m ! q3q2 ! F32 = ke 3 r32 = 9 ( 10 9 Nm 2 / C 2 (3C)(3C) r32 (1m)3 ! F32 = 4 ( 1010 iˆ ! 3 ĵ N ( ( ) ) ˆ ! q1q2 ! 1im 9 2 2 F12 = ke 3 r12 = 9 ( 10 Nm / C (6C)(3C) r12 (1m)3 ! F12 = 1.6 ( 1011 Niˆ ! ! ! F2 = F32 + F12 ( ) What is a Field ???? Fields • Fields are mathematical functions that have a value at all points in the space that they occupy • They describe some physical quantity that is changing from point to point. • The physical quantity at each point is either an ordinary number or a vector. What are examples of fields encountered in everyday life ???? Scalar Field: Each point has a numerical value • Temperature Map: Each location on the map has temperature (scalar quantity). Vector Field: Each point has both a magnitude and direction • Example: Wind Velocity Vector Field Example of a Vector Field: The Gravitational Field The Electric Field Source charges: alter the space around them by creating an electric field. Probe charge: a single point charge in the electric field that experiences a force exerted by the field. Probe charge q experiences an electric force Fon q due to other charges. The electric field is DEFINED by Units of E ~ N/C Comparison Between Gravity and Electricity Field of Source Mass: Field of Source Charge: s Force of Source Mass on Probe Mass: Force of Source Charge on Probe Charge: p Positive Charge Negative Charge