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Advanced Physics Chapter 16 Electric Charge and Electric Field Electric Charge and Electric Field • • • • • • • • • • 16-1 Static Electricity 16-2 Electric Charge in the Atom 16-3 Insulators and Conductors 16-4 Induced Charge: the Electroscope 16-5 Coulomb’s Law 16-6 Solving Problems Involving Coulomb’s Law and Vectors 16-7 The Electric Field 16-8 Field lines 16-9 Electric Fields and Conductors 16-10 Electric Forces in Molecular Biology (DNA) Static Electricity Static Electricity = ? Electric charge = ? • Types • Interactions • Giving an object a net electric charge Static Electricity Law of conservation of electric charge—the net amount of electric charge produced in any process is zero • This means = ? Electric Charge in the Atom • So what are the electric charges in an atom? • What happens if we “charge” an atom? • What is a polar molecule? Insulators and Conductors • What are insulators and conductors? • What are examples of insulators and conductors? • Can you charge insulators and/or conductors? – Explain how. • What is a semiconductor? Induced Charge: the Electroscope • What is an electroscope? • Can you use an electroscope to find the type of charge on an object? Induced Charge: the Electroscope • How do you charge an electroscope by conduction? • What charge does it get? Induced Charge: the Electroscope • How do you charge an electroscope by induction? Induced Charge: the Electroscope • How do you charge an electroscope by induction? • What is grounding? • What charge does it get? Coulomb’s Law • Where: F = (kQ1Q2)/r2 – F = force between two charged objects (1 & 2) – k = Coulomb's constant (9 x 109 N*m2/C2) – Q = net charges on objects (Coulomb) • 1 electron has the charge of 1.6 x 10-19 C – r = distance between objects Coulomb’s Law F = (kQ1Q2 2 )/r • k can be represented by k = 1/(40) • Where 0 = permittivity of free space – 8.85 x 10-12 C2/N*m2 • You will see 0 again later Coulomb’s Law F = (kQ1Q2 2 )/r • What is the relationship between F, Q’s and r? Coulomb’s Law F = (kQ1Q2)/r2 • If the force is between two objects with the same charge the force is repulsive and positive • If the force is between two objects with the opposite charges the force is attractive and negative Solving Problems Involving Coulomb’s Law and Vectors • The net force acting on an object is the sum of all the forces acting on it • For charged objects, these forces can be calculated using Coulomb’s Law and remembering that force vectors have direction • It is usually better to ignore signs and just deal with direction of force when looking at net force acting on an object Solving Problems Involving Coulomb’s Law and Vectors Example • Three charged particles Q1, Q2, and Q3 are arranged in a straight line. They have charges of –8.0C, 3.0C and – 4.0C respectively. Q1 is 0.3 m from Q2 and 0.5 m from Q3. Find the net force on Q3 Answer • 1.5 N toward Q1 Solving Problems Involving Coulomb’s Law and Vectors Example • Three charged particles Q1, Q2, and Q3 are arranged where Q1is 52 cm east of Q2 and Q3 is 30 cm north of Q2 They have charges of –86.0C, 50.0C and 65.0C respectively. Find the net force on Q3 Answer • 282.36 N at 64.6° north of east or 25.4° east of north The Electric Field Electric field—property of all charged objects • the vector sum of the force exerted on a test charge when it is by charged object divided by the charge on the test charge Test charge must be ______ and _______. Why? The Electric Field Electric field E = F/q' • Why is the electric field not just determined by the force on the test charge? • What is the unit for electric field strength? The Electric Field Electric field E = F/q' So…F = Eq' So…F = kQq‘/r2 = Eq' So…E = kQ/r2 for an electric field generated by charge Q The Electric Field Electric field When the electric field is due to more than one charge, the total field at any point is the vector sum of the individual fields (superposition principle) E = E1 + E2 + ….. Field Lines Electric field • How do you diagram electric field? – Positive charge • How do the lines tell an electric field’s magnitude and direction? Field Lines Electric field • How do you diagram electric field? – Negative charge Field Lines Electric field • How do you diagram electric field? – Two negative charges Field Lines Electric field • How do you diagram electric field? – Two positive charges Field Lines Electric field • How do you diagram electric field? – One negative and one positive charge Field Lines Electric field • How do you diagram electric field? – Between two parallel plates of opposite charge Electric Fields and Conductors • The electric field inside a good conductor is zero in the static situation (when charges are at rest) • Any net charge on a good conductor distributes itself on the surface • The direction of the electric field is perpendicular to the surface of the conductor Electric Fields and Conductors • For a hollow sphere with a charge in it; there is an electric field due to net charges on inside and outside surface so there is an electric field inside and outside but not within the metal Electric Fields and Conductors • For a hollow box inside an electric field; there is no electric field because of the accumulation of charges along its surface (safety in a car) Electric Forces in Molecular Biology (DNA) • What are the four bases in DNA? – Thymine, cytosine, adenine, and guanine • How do they match up? – Thymine and adenine – Cytosine and guanine • What holds the two DNA strands together? – Hydrogen bonds