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Transcript
UNIT 9 - ELECTROSTATICS Chapter 18 TYPES OF ELECTRIC CHARGE Static (Unit 9) Charge at rest Collection Dynamic Charge Such of charge static electricity (Unit 10) in motion : current as in electrical circuits ATOMS Structure Proton Positive charge object found in the nucleus Neutron Neutral object found in the nucleus Nucleus will also have a positive charge Electron Negatively charged object found orbiting the nucleus of an atom Can move from object to object or through a material. THE CHARGE OF AN ATOM If an atom has an equal number of protons and electrons it is considered electrically neutral. Ions (charged atoms) contain a different number of protons and electrons Fewer electrons – positively charged More electrons – negatively charged QUANTITY OF CHARGE Unit for charge – Coulomb (C) The charge on a single electron is −1.6 × 10−19 𝐶 and the charge on a single proton is +1.6 × 10−19 𝐶 To determine the total charge of an atom… Subtract the number of electrons from the number of protons = excess protons Multiply by +1.6 × 10−19 𝐶 EXAMPLE #1 A. Determine the charge on an object that has 4.0 × 1020 more electrons than protons. 4.0 × 1020 B. −1.6 × 10−19 = −64 𝐶 Determine the charge on an object that has 5.2 × 1017 protons and 2.8 × 1016 electrons. 5.2 × 1017 − 2.8 × 1016 = 4.92 × 1017 excess protons 4.92 × 1017 1.6 × 10−19 = .079 𝐶 EXAMPLE #2 An object carries an estimated negative charge of 2 Coulombs. Determine the number of excess electrons. # 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 # 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 1.6 × 10−19 = #𝐶𝑜𝑢𝑙𝑜𝑚𝑏𝑠 so… = 2 1.6×10−19 = 1.25 × 1019 electrons. WAYS TO CHARGE OBJECTS Friction The frictional charging process results in a transfer of electrons between the two objects that are rubbed together. Rubbing Charged objects. a balloon on your hair, feet across the carpet objects exert an electric force upon other Non-contact force (like gravity) WAYS TO CHARGE OBJECTS Friction Opposites attract and likes repel Since one object becomes negatively charged and the other positively charged, the two objects will cling together Charged objects and neutral objects are attracted to each other as well. WAYS TO CHARGE OBJECTS - FRICTION EXAMPLE #3 – WWW.PHYSICSCLASSROOM.COM CLASSIFICATION OF MATERIALS Conductors Materials with an ability to transfer charge from one object to another (or particle to particle) easily Charge Ex: is distributed evenly across the surface of the object copper, aluminum, water Insulators Materials Charge Ex: that resist the flow of charge is not evenly distributed across the surface wood, glass, rubber CLASSIFICATION OF MATERIALS Semiconductors Materials Often times based on temperature Exactly Ex: intermediate in the ability to carry charge 4 valance electrons silicon, germanium CHARGING BY INDUCTION Induction Redistribution of charge due to the presence of nearby charged objects. Charging without contact between objects Polarization – the positive and negative charges separate from each other. Law of Conservation of Charge – the overall charge in the system is the same before and after the charging process. CHARGING BY INDUCTION CHARGING BY CONDUCTION Conduction Charging Van by contact de Graaff generator Polarization – the positive and negative charges separate from each other. Law of Conservation of Charge – the overall charge in the system is the same before and after the charging process. CHARGING BY CONDUCTION ELECTRIC FORCE AND COLOUMB’S LAW The interaction between charged objects is a non-contact force that acts over some distance of separation. Depends on the two charges and the distance between them. Electric force is a vector quantity with both magnitude and direction. ELECTRIC FORCE AND COLOUMB’S LAW Direction is determined by opposites attract and likes repel. ELECTRIC FORCE AND COULOMB’S LAW Coulomb’s Law let’s us calculate the magnitude of the electric force between two charges: 𝑘𝑄1 𝑄2 𝐹= 𝑑2 𝑄1 d and 𝑄2 are the charges in Coulombs is the distance between the two charges k is Coulomb’s constant, which is 9.0 × 109 𝑁𝑚2 /𝐶 2 for air. + is repulsive force, and - is attractive force. EXAMPLE PROBLEM #1 Two balloons are each charged to -6.25 nC (−6.25 × 10−9 𝐶). They are held apart at 61.7 cm. Determine the magnitude of the electric force of repulsion between them. 𝐹 = (9.0×109 )(−6.25×10−9 )(−6.25×10−9 ) .6172 −7 9.23 × 10 𝑁 repulsive force. = 9.23 × 10−7 𝑁 EXAMPLE PROBLEM #2 Two balloons with charges of 3.37 µC (3.37 × 10−6 𝐶) and -8.21 µC (−8.21 × 10−6 𝐶) attract each other with a force of 0.0626 N. Determine the distance between the two charges. −0.0626 𝑑 2 𝑑 = = (9.0×109 )(3.37×10−6 )(−8.21×10−6 ) 𝑑2 (9.0×109 )(3.37×10−6 )(−8.21×10−6 ) −0.0626 = 1.99 𝑚 = 3.98 ELECTRIC FIELD Charged objects create electric fields – alterations in the space around them. Objects that enter the space will feel the charge, but the electric field exists regardless. If Q is the charge of the object and q is a test charge, the magnitude of electric field is 𝐸= 𝐹𝐸 𝑞 = 𝑘𝑄𝑞 𝑟2 𝑞 = The 𝑘𝑄 𝑟2 greater the test charge, the more force it will feel as a result of the same electric field. Direction depends on whether charges are +/- REVIEW PROBLEM #1 REVIEW PROBLEM #2 REVIEW PROBLEM #3 REVIEW PROBLEM #4 REVIEW PROBLEM #5 REVIEW PROBLEM #6 REVIEW PROBLEM #7 Calculate the charge of an object that has 1.4 × 1015 protons and 3.2 × 1015 electrons REVIEW PROBLEM #8 An object that has a charge of 0.5 C has how many more protons than electrons? REVIEW PROBLEM #9 Calculate the electric attractive force between 2 objects that have charges of 3.2 × 10−5 𝐶 and −5.2 × 10−6 𝐶 and are a distance of 0.25 meters apart. REVIEW PROBLEM #10 A test charge of 4.2 × 10−6 C experiences a force of 0.6 N when it is a distance of 0.1 m away from an object. A. What is the electric field intensity created by the object? B. What must be the charge of the object? C. What force would the test charge experience if it was twice as far away from the object? D. If it had twice the charge? ANSWERS 1. A,B,C (Y might be + or N, so Z could be anything) 2. A is rubber (insulator) and B is copper (conductor) 3. D (Polarization) 4. D (Protons don’t move!!!) 5. B,D,G 6. 0.294 C 7. −2.88 × 10−4 C ANSWERS CONTINUED 8. 9. 3.125 × 1018 excess protons 24 N attractive force 10. A. 1.43 × 105 N/C B. 1.59 × 10−7 C C. 0.15 N D. 1.2 N