Transcript ppt - plutonium

Physics II:
Electricity & Magnetism
Sections 21.9 to 21.10
Tuesday
(Day 14)
Warm-Up
Tues, Feb 10
 Each charge on the next slide is ±q. What will happen to the lines if a 3rd
charge of +q is added to the (1) right side and (2) left side?
 Place your homework on my desk:
 “Foundational Mathematics’ Skills of Physics” Packet (Page 16)
 Web Assign 21.5 - 21.7
 For future assignments - check online at www.plutonium-239.com
Field Example #1: Each charge below is ±q. What will
happen to the lines if a 3rd charge of +q is added to the
(1) right side and (2) left side?
Essential Question(s)
 WHAT PRIOR FOUNDATIONAL MATHEMATICS’ SKILLS ARE
NECESSARY IN PHYSICS II?
 HOW DO WE DESCRIBE THE NATURE OF ELECTROSTATICS
AND APPLY IT TO VARIOUS SITUATIONS?
 How do we describe and apply the nature of electric fields in and
around conductors?
 How do we describe and apply the concept of induced charge and
electrostatic shielding?
 How do we describe and apply the concept of electric fields?
Vocabulary
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Static Electricity
Electric Charge
Positive / Negative
Attraction / Repulsion
Charging / Discharging
Friction
Induction
Conduction
Law of Conservation of Electric
Charge
 Non-polar Molecules
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Polar Molecules
Ion
Ionic Compounds
Force
Derivative
Integration (Integrals)
Test Charge
Electric Field
Field Lines
Electric Dipole
Dipole Moment
Foundational Mathematics
Skills in Physics Timeline
Day
Pg(s)
Day
Pg(s)
Day
Pg(s)
Day
Pg(s)
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21
2
13
14
7
4
12
17
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8
3
22
23
8
5
13
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18
9
4
24
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6
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WHAT PRIOR FOUNDATIONAL MATHEMATICS’ SKILLS ARE NECESSARY IN PHYSICS II?
Agenda
 Review “Foundational Mathematics’ Skills of Physics” Packet (Page
16) with answer guide.
 Discuss
 Electric Fields and Conductors
 Motion of a Charged Particle in an Electric Field
 Work on Web Assign
Field Example #2: Each charge below is ±5q. What will
happen to the lines if a 3rd charge of +q is added to the
(1) right side and (2) left side?
Field Example #3: Each charge below is ±10q. What will
happen to the lines if a 3rd charge of +q is added to the
(1) right side and (2) left side?
Section 21.9
 How do we describe and apply the nature of electric fields
in and around conductors?
 How do we explain the mechanics responsible for the absence of
electric field inside of a conductor?
 Why must all of the excess charge reside on the surface of a
conductor?
 How do we prove that all excess charge on a conductor must reside
on its surface and the electric field outside of the conductor must
be perpendicular to the surface?
Section 21.9
How do we describe and apply the concept of
induced charge and electrostatic shielding?
What is the significance of why there can be no electric
field in a charge-free region completely surrounded by
a single conductor?
21.9 Electric Fields and Conductors
The static electric field inside a conductor is
zero – if it were not, the charges would move.
The net charge on a conductor is on its
surface.
Charge ball suspended in
a hollow metal sphere
Observations
The hollow sphere had a charge on
the outside.
The charged ball still had a charge.
Conclusions
The charged ball on the inside
induces an equal charge on the
hollow sphere.
21.9 Electric Fields and Conductors
The electric
field is
perpendicular
to the surface
of a
conductor –
again, if it
were not,
charges
would move.
Charge ball placed into a
hollow metal sphere
Observations
The hollow sphere had a charge on
the outside.
The charged ball no longer had a
charge.
Conclusions
The charge resides on the outside of
a conductor.
Applications of E-fields and conductors:
Faraday Cages

Faraday cages protect you from lightning because there is no electrical field inside the metal cage
(Notice (1) it completely surrounds him and (2) the size of the gaps in the fence (it is not a solid piece
of metal).
Section 21.10
How do we describe and apply the nature of
electric fields in and around conductors?
How do we determine the direction of the force
on a charged particle brought near an uncharged
or grounded conductor?
Section 21.10
How do we describe and apply the concept
of induced charge and electrostatic
shielding?
How do we determine the direction of the force
on a charged particle brought near an uncharged
or grounded conductor?
Section 21.10
How do we describe and apply the concept
of electric field?
How do we calculate the magnitude and
direction of the force on a positive or negative
charge in an electric field?
How do we analyze the motion of a particle of
known mass and charge in a uniform electric
field?
Electron accelerated by an electric field
 An electron is accelerated in the uniform field E
(E=2.0x104N/C) between two parallel charged
plates. The separation of the plates is 1.5 cm. The
electron is accelerated from rest near the negative
plate and passes through a tiny hole in the positive
plate. (a) With what speed does it leave the hole? (b)
Show that the gravitational force can be ignored.
[NOTE: Assume the hole is so small that it does not
affect the uniform field between the plates]
Electron accelerated by an electric field
(a) With what speed does it leave the hole?
F qE

F  ma  a 
m
m
v 2  v02  2ax  v  2ax
F  qE
0
 qE 
v  2   x
 m
1.60 x 10 C2.0 x 10
2
9.1 x 10 kg 
-19
v
v  1.0 x 10 7 m s
31
4
N

C 0.015 m 
Electron accelerated by an electric field
(b) Show that the gravitational force can be ignored.
FE  qE


FE  1.60 x 10-19 C 2.0 x 10 4 N C
FE  3.5 x 1015 N
FG  mg


FG  9.1 x 10-31 kg 9.8 m
s2


FG  8.9 x 10-30 N
Note that FE is 1014 times larger than the FG.
Also note that the electric field due to the electron
does not enter the problem since it cannot exert a
force on itself.
Applications of an electron accelerated by an
E-Field: Mass Spectrometer


Mass Spectrometers are used to separate isotopes of atoms.
The charged isotopes (a.k.a. ions) are accelerated to a velocity by the parallel plates (located from S
to S1)
Projectile Motion of a Charged Particle:
Electron moving perpendicular to E
 Suppose an electron is traveling with a
speed, v0 = 1.0x107m/s, enters a uniform
field E at right angles to v0. Describe the
motion by giving the equation of its path
while in the electric field. Ignore gravity.
F
eE
F  ma  a   
m
m
F  qE  eE
eE 2
eE 2


x
t
y  v0 y t  ayt  
2
qmv0
2m
0
1
2
2
This is the equation of a
parabola (i.e. projectile motion).
2
2


x
x
x
x
2
2
1

t
   2

t


x  v0 x t  2 ax t
 v0  v0
v0 x v0
0
Electrons moving perpendicular to E:
The discovery of the electron: J.J. Thomson’s
Experiment
 J. J. Thomson’s famous experiment that allowed him to discover the electron.
Applications of an electron moving
perpendicular to E: Cathode Ray Tube (CRT)
 Television Sets & Computer Monitors (CRT)
Applications of an electron moving
perpendicular to E: Mass Spectrometer


Mass Spectrometers are used to separate isotopes of atoms.
The charged isotopes (a.k.a. ions) are accelerated to a velocity by the parallel plates (located at the & + plates)
Applications of an electron moving
perpendicular to E: e/m Apparatus
 e/m Apparatus
Applications of an electron moving
perpendicular to E: e/m Apparatus
 e/m Apparatus
Summary
 Using your kinematic equations, determine the equation that relates y to v0, g,
, and x?
 HW (Place in your agenda):
 “Foundational Mathematics’ Skills of Physics” Packet (Page 17)
 Web Assign 21.12 - 21.14
 Future assignments:
 Electrostatics Lab #3: Lab Report (Due in 1 class)
How do we use Coulomb’s Law and the principle of superposition to determine the force that acts between point charges?