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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. - PowerPoint PPT Presentation
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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
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