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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• protons have positive (+) charge• electrons have negative (–) charge• opposite charges attract; like charges repel• charge is always conserved (cannot be destroyed)• charge (q) is measured in coulombs (C)
Charles Augustin Coulomb1736 - 1806
Electric Charge is Quantized
qE mg
qmg
E
+
qE
mg
Charge on drop occurs in multiples of 1.60 x 10-19 C(±1e, ±2e, ±3e…).
1909: Robert Millikan
Transfer of Electric Charge
• charges move freely through conductors (typically metals)
• charges do not move freely in insulators (most other solids)
Electric charge can be transferred 3 ways:• 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 Law
Compare the gravitational and the electric forces between the proton and electron in a hydrogen atom.
k = 8.99 x 109 Nm2/C2
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)
• gravitational field (g)• electric field (E): (+) → (–)
F q E
EF
q
e
e
0
0
units are N/C
F m g
gF
m
g
g
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?
Electric Field due to a Point Charge
(a) What is the E-field strength at a distance of 0.45 m from a 8.2mC charge?(b) What force would be felt by a proton at this location?(c) What is the initial acceleration of the proton?
F kq q
r
EF
qkq q
r q
E kq
r
e
e
02
0
02
0
2
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
Electric Potential Energy (EPE)
PE m g hgrav PE q E delectric
g
E
ME KE PE PE PEgrav elastic electric
• 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?
EPE for a Pair of Charges
• Two alpha particles (two helium nuclei, 2 p+ each) have an EPE of 6.32 x10-19 J. What is the distance between the two particles?
PE q E d F r kq q
rr
PE kq q
r
elec e
elec
1 22
1 2
Potential Difference (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
" "
• voltage (V) is EPE per charge• 1 volt = 1 J/C• measured with a voltmeter or multimeter• voltage is like “electric pressure”
(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 that are separated by an insulator.
• Capacitance is measured in farads (F). 1 F = 1 C/V
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
Power Problems
If voltage = 120 V, what is the current through a 1200-W hairdryer?
At $0.05 / kW·hr, how much does it cost to operate a 240-W television for 24 hrs?