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Mastering Physics: Electric Charge and Field Fundamentals

Solidify your knowledge of electric charge and field concepts with effective strategies like reading before lecture, doing regular problems, and taking breaks. Covering topics such as static electricity, charge conservation, conductors, and Coulomb's Law.

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Mastering Physics: Electric Charge and Field Fundamentals

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  1. PHYSICS 2415 Suggested strategies: Read text before lecture Attend lecture Do problems! This is where what you learned from reading or from class gets solidified in your mind. Do not beat your head against a brick wall. If, after reasonable effort you’re stumped, go back to the chapter/section Spread your work out over 4-6 days a week, depending on demands of other classes Do not work more than ~2 hours or so at one time; then take a real break, at least an hour spent doing something completely different

  2. Chapter 21Electric Charge and Electric Field

  3. Units of Chapter 21 • Static Electricity; Electric Charge and Its Conservation • Electric Charge in the Atom • Insulators and Conductors • Induced Charge; the Electroscope • Coulomb’s Law • The Electric Field • Electric Field Calculations for Continuous Charge Distributions

  4. Units of Chapter 21 • Field Lines • Electric Fields and Conductors • Motion of a Charged Particle in an Electric Field • Electric Dipoles

  5. 21-1 Static Electricity; Electric Charge and Its Conservation Objects can be charged by rubbing

  6. 21-1 Static Electricity; Electric Charge and Its Conservation Charge comes in two types, positive and negative; like charges repel and opposite charges attract.

  7. ConcepTest 21.1aElectric Charge I 1)one is positive, the other is negative 2) both are positive 3) both are negative 4) both are positive or both are negative Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges?

  8. ConcepTest 21.1aElectric Charge I 1)one is positive, the other is negative 2) both are positive 3) both are negative 4) both are positive or both are negative Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges? The fact that the balls repel each other can tell you only that they have the same charge, but you do not know the sign. So they can be either both positive or both negative. Follow-up: What does the picture look like if the two balls are oppositely charged? What about if both balls are neutral?

  9. 21-1 Static Electricity; Electric Charge and Its Conservation Electric charge is conserved – the arithmetic sum of the total charge cannot change in any interaction.

  10. 21-2 Electric Charge in the Atom Atom: Nucleus (small, massive, positive charge) Electron cloud (large, very low density, negative charge)

  11. Elementary charge Charge on the electron: e = 1.602 x 10-19 C. Electric charge is quantized in units of the electron charge.

  12. 21-2 Electric Charge in the Atom Polar molecule: neutral overall, but charge not evenly distributed

  13. 21-3 Insulators and Conductors Conductor: Charge flows freely Metals Insulator: Almost no charge flows Most other materials Some materials are semiconductors.

  14. 21-4 Induced Charge; the Electroscope Metal objects can be charged by conduction:

  15. 21-4 Induced Charge; the Electroscope They can also be charged by induction, either while connected to ground or not:

  16. 21-4 Induced Charge; the Electroscope Nonconductors won’t become charged by conduction or induction, but will experience charge separation:

  17. 21-4 Induced Charge; the Electroscope The electroscope can be used for detecting charge.

  18. 21-4 Induced Charge; the Electroscope The electroscope can be charged either by conduction or by induction.

  19. 21-4 Induced Charge; the Electroscope The charged electroscope can then be used to determine the sign of an unknown charge.

  20. 1) 00 2) + – 3) – + 4) + + 5) – – 0 0 ? ? ConcepTest 21.2bConductors II Two neutral conductors are connected by a wire and a charged rod is brought near, butdoes not touch. The wire is taken away, and then the charged rod is removed. What are the charges on the conductors?

  21. 1) 00 2) + – 3) – + 4) + + 5) – – 0 0 ? ? ConcepTest 21.2bConductors II Two neutral conductors are connected by a wire and a charged rod is brought near, butdoes not touch. The wire is taken away, and then the charged rod is removed. What are the charges on the conductors? While the conductors are connected, positive charge will flow from the blue to the green ball due to polarization. Once disconnected, the charges will remain on the separate conductors even when the rod is removed. Follow-up: What will happen when the conductors are reconnected with a wire?

  22. 21-5 Coulomb’s Law Experiment shows that 1) the electric force between two charges is proportional to the product of the charges and 2) inversely proportional to the distance between them.

  23. 21-5 Coulomb’s Law Coulomb’s law: This equation gives the magnitude of the force between two charges. In vector form:

  24. 21-5 Coulomb’s Law Unit of charge: coulomb, C. The proportionality constant in Coulomb’s law is then: k = 8.99 x 109 N·m2/C2. Charges produced by rubbing are typically around a microcoulomb: 1 μC = 10-6 C.

  25. 21-5 Coulomb’s Law The proportionality constant k can also be written in terms of ε0, the permittivity of free space:

  26. 21-5 Coulomb’s Law The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if they are the same. Newton’s Third Law applies!

  27. 21-5 Coulomb’s Law Example: Three charges in a line. Three charged particles are arranged in a line, as shown. Calculate the net electrostatic force on particle 2 (the +3.0 μC in the middle) due to the other two charges.

  28. +4Q +Q 1 2 3 5 4 2R R 3R ConcepTest 21.4bElectric Force II Two balls with charges +Q and +4Q are separated by 3R. Where should you place another charged ball Q0 on the line between the two charges such that the net force on Q0 will be zero?

  29. +4Q +Q 1 2 3 5 4 2R R 3R ConcepTest 21.4bElectric Force II Two balls with charges +Q and +4Q are separated by 3R. Where should you place another charged ball Q0 on the line between the two charges such that the net force on Q0 will be zero? The force on Q0 due to +Q is: F = k(Q0)(Q)/R2 The force on Q0 due to +4Q is: F = k(Q0)(4Q)/(2R)2 Since +4Q is 4 times bigger than +Q, Q0 needs to be farther from +4Q. In fact, Q0 must be twice as far from +4Q, since the distance is squared in Coulomb’s law.

  30. 21-5 Coulomb’s Law Force is a vector quantity: Example: Calculate the net electrostatic force on charge q3 shown in the figure due to the charges q1 and q2.

  31. Coulomb’s Law

  32. 21-6 The Electric Field The electric field is defined as the force on a small charge, divided by the magnitude of the charge:

  33. 21-6 The Electric Field An electric field surrounds every charge.

  34. 21-6 The Electric Field For a point charge:

  35. 21-6 The Electric Field Force on a point charge in an electric field:

  36. 21-6 The Electric Field Example: Calculate the total electric field at point P due to both charges, q and -q. Hint: exploit symmetry! What direction would E point if both charges were equal, not opposite?

  37. 21-6 The Electric Field • Problem solving in electrostatics: electric forces and/or electric fields • Draw a diagram; show all charges, with signs, and electric fields and forces with directions. • Calculate forces using Coulomb’s law. • Add forces vectorally to get result. • Check your answer!

  38. -2 C -2 C 1 2 4 3 -2 C -2 C ConcepTest 21.9bSuperposition II What is the electric field at the center of the square? 5) E = 0

  39. -2 C -2 C 1 2 4 3 -2 C -2 C ConcepTest 21.9bSuperposition II What is the electric field at the center of the square? 5) E = 0 The four E field vectors all point outward from the center of the square toward their respective charges. Because they are all equal, the net E field is zero at the center!! Follow-up: What if the upper two charges were +2 C? What if the right-hand charges were +2 C?

  40. Questions?

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