Chapter 17: Electric Forces and Fields

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.

Electric Charge Ben Franklin: two kinds of charge, positive and 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

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

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

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

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

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

Electric Fields E-field lines show direction and strength of force (by line density) acting on a small charge E-field: (+) → (–) applet units are N/C

Electric Fields The nucleus applies a force of 8.16 x 10-11 N on the electron in a hydrogen atom. What is the electric field strength at the position of the electron? What is the acceleration of an electron in a 2.5 x 103 N/C electric field? What is the acceleration of a proton in the same field?

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.

Chapter 18: Electric Energy and Capacitance

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

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

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?

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)

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

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

Electric Current and Electric Power Chapter 19: Electric Current and Electric Power

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

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?

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

Electric Power and Energy Consider the units of voltage: P = IV = I2R. Electric power is transported at high V and low I to minimize “I2R loss” (high I causes too much friction and heat).

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 $0.0599 / kW·hr, how much does it cost to watch a 2-hour movie on a 280-W big-screen television?

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.

Series Circuit

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 = 3 Ω, what is RT and current? Holiday lights are often in series: if one bulb burns out, nothing works!

Parallel Circuit

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 RT and IT flowing through the entire circuit? What is the current in each resistor? Household circuits are wired in parallel.

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 = 3 Ω Parallel example: V = 12 V, R1 = 1 Ω, R2 = 2 Ω, and R3 = 3 Ω