Chapter 17: Electric Forces and Fields

1. Chapter 17: Electric Forces and Fields

2. Objectives • Understand the basic properties of electric charge. • Differentiate between conductors and insulators. • Distinguish between charging by contact, charging by induction, and charging by polarization.

3. Electric Charge • Ben Franklin: two kinds of charge, positiveand negative • opposite charges attract; like charges repel • Law of Conservation of Charge:it can’t be destroyed, total is constant • charge (q) is measured in coulombs (C) • electrons (–), protons (+) • Robert Millikan (1909): fundamental charge = +/– 1.60 x 10-19 C

4. Transfer of Electric Charge • charges move freely through conductors (typically metals, ionic solutions) • charges do not move freely in insulators (most other substances) Electric charge can be transferred 3 ways: • friction/contact • induction • polarization

5. Objectives • Calculate electric force using Coulomb’s law. • Compare electric force with gravitational force.

6. Coulomb’s Law Law of Universal Gravitation Coulomb’s Law k = 8.99 x 109 Nm2/C2

7. Which is Stronger, Fe or FG? • Compare the Fg and the Fe between the p+ and e- in a hydrogen atom (r = 53 pm).

8. Objectives • Calculate electric field strength. • Draw and interpret electric field lines. • Identify the properties associated with a conductor in electrostatic equilibrium.

9. Electric Fields • Field lines show direction and strength of force (represented by the line density) acting on a charge • E-field: (+) → (–) • units are N/C

10. Electric Fields The nucleus applies a force of 8.16 x 10-11N on the electron in a hydrogen atom. What is the electric field strength at the position of the electron? What is the orbital period of the electron, assuming it orbits in a circular path (it really doesn’t)?

11. Conductors in Electrostatic Equilibrium electrostatic equilibrium: no net motion of charge (a) The total electric field inside a conductor equals zero. (b) Excess charge resides on the surface. (c) E-field lines extend perpendicular to the surface. (d) Charge accumulates at points.

12. Chapter 18: Electric Energy and Capacitance

13. Objectives • Understand the concept of electric potential energy (EPE). • Calculate the DEPE when a charged particle is moved in a uniform electric field.

14. Electric Potential Energy (EPE) • uniform field only! • displacement in direction of the field g E

15. EPE Problems • What is the change in EPE if a proton is moved 2.5mm in the direction of a uniform 7.0 x1011 N/C electric field? • What is the change in EPE if an electron is moved in the same direction?

16. Potential Difference (Voltage) • voltage (V) is EPE per charge • 1 volt = 1 J/C • measured with a voltmeter • voltage is like an “electric pressure” that pushes charges • batteries, outlets, generators, etc. supply voltage (uniform field only)

17. Voltage Problems What voltage exists in a 3.5 x10-6 N/C electric field between two points that are 0.25 m apart?

18. Capacitors • Capacitors store EPE between two closely-spaced conductors (separated by an insulator). • Capacitance is measured in farads (F). 1 F = 1 C/V • Capacitors can discharge very quickly—producing short bursts of electrical current

19. Chapter 19: Electric Current and Electric Power

20. Electric Current Electric charges will flow between areas of different electric potential (voltage) • electric current (I): a flow of • electric charge • 1 ampere (A) = 1 C/s • measured with an ammeter • although electrons typically flow, current is • defined as direction of positive flow (+ → –) • drift speed of e– in Cu at 10 A is only 0.00025 m/s • 0.005 A is painful and 0.070 A can kill you

21. Electric Resistance • resistance (R): resistance • to electron flow • measured in Ohms (Ω) • V ↑, I↑ • R ↑, I ↓ A 2400-Ω resistor is attached to a 12-V power source. What is the current through the wire?

22. AC/DC • alternating current: electric field reverses periodically, current alternates direction(60 hz in USA) • direct current: field is constant, current is constant • batteries produce DC • electric generators can make AC or DC

23. Electric Power and Energy Consider the units of voltage: Electric power is transported at high voltage and low current to minimize “I2R loss.”

24. Power Problems An electric oven operates on a 240 V circuit (not the regular 120 V). How much current flows through the element in the oven if the power usage is 3200 W? At $0.06 / kW·hr, how much does it cost to operate a 280-W television for 2 hrs?

25. Objectives • To understand the concepts of series and parallel circuits. • To calculate the total resistance and current flowing through a circuit containing series and/or parallel circuits.

26. Series Circuit

27. Series Circuit • Resistors (or loads) “in series” just combine to make a larger resistance. • RT = R1 + R2 + R3 + … • In a series circuit, if V = 12 V, R1= 1 Ω, R2 = 2 Ω, and R3= 3Ω, what is RT and current? • What is the “voltage drop” across each resistor? • Holiday lights are often in series: if one bulb burns out, nothing works!

28. Parallel Circuit

29. Parallel Circuits • Resistors in parallel provide additional paths for current to flow, so resistance decreases. • 1/RT= 1/R1+ 1/R2+ 1/R3+ … • In a parallel circuit, if V = 12 V, R1 = 1 Ω, R2 = 2 Ω, and R3 = 3 Ω, what is RTand IT flowing through the entire circuit? What is the current in each resistor? What is the voltage drop across each resistor? • Household circuits are wired in parallel.

30. Voltage Drops • The current flowing through a resistor depends on the voltage drop “across” the resistor. • Series example: V = 12 V, R1 = 1 Ω, R2 = 2 Ω, and R3 = 3Ω • Parallel example: V = 12 V, R1 = 1 Ω, R2 = 2 Ω, and R3 = 3 Ω