Post on 27-Jan-2016
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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
F Gm m
rG 1 22
F kq q
re 1 22
Law of Universal Gravitation
Coulomb’s Lawk = 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
• Field lines show direction and strength of force (represented by the line density) acting on a charge
• E-field: (+) → (–)• units are N/C
F q E
EF
q
e
e
0
0
Electric Fields
The nucleus applies a force of 8.16 x 10-11N on the electron in a hydrogen atom. (a) What is the electric field strength at the position
of the electron?(b) What is the orbital period of the electron,
assuming it orbits in a circular path (it really doesn’t)?
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)
PE m g hgrav PE q E delectric
g
E
• uniform field only!• displacement in direction of the field
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
PE m g h
PE
mg h
grav
grav
PE q E d
PE
qE d
vo ltagePE
qE d
V E d
electric
electric
electric
" "
(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
• Capacitors can discharge very quickly—producing short bursts of electrical current
CQ
VPE Q Velectric
12
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 ↓
IV
R
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
J
CV
J C V
J
s
C V
s
C
sV
W A V
P I Velec
Consider the units of voltage:
E I V telec
Electric power is transported at high voltage and low current to minimize “I2R loss.”
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?
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?
• What is the “voltage drop” across each resistor?• 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? What is the voltage drop across 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