Download Electrostatics, Electric Fields, and Electric Potential

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
m1  m2
FG  G 
r2
q1  q 2
Fe  k 
r2
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
Fe  q 0  E
Fe
E
q0
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
PE grav  m  g  h
PE electric   q  E  d
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
PE grav
 gh
• 1 volt = 1 J/C
m
• measured with a
PE electric   q  E  d
voltmeter
PE electric
• voltage is like an
 E  d
q
“electric pressure”
DPE electric
that pushes charges
" voltage" 
  E  Dd
q
• batteries, outlets,
(uniform field only)
generators, etc. supply
DV   E  Dd
voltage
PE grav  m  g  h
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
Q
C
DV
PE electric  21 Q  DV
• A capacitor can discharge very
quickly—makes a short burst of
electrical current
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 ↓
V
I
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
Consider the units
of voltage:
J
V
C
J  C V
J C V C

 V
s
s
s
W  A V
Pelec  I  V
E elec  I  V  t
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