Transcript Introduction to Modern Physics PHYX 2710

Physics of Technology
PHYS 1800
Lecture 28
Introduction
Electricity and Charge
Section 0
Lecture 1
Slide 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 1
No area of
physics has had
UNIT THREE
a greater
impact
Electricity
Magnetism
on theandway
we
live than the
study of
electricity and
magnetism.
Introduction
Section 0
Lecture 1
Slide 2
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 2
PHYSICS OF TOF
ECHNOLOGY
- PHYS 1800
PHYSICS
TECHNOLOGY
ASSIGNMENT SHEET
Spring 2009Spring
Assignment
Sheet
2009
Date
Day
Lecture
Feb 16
M
Presidents Day
17
Tu
Angular Momentum (Virtual Monday)
18
W
Review
19
H
Test 2
20
F*
Static Fluids, Pressure
Feb 23
M
Flotation
25
W
Fluids in Motion
27
F*
Temperature and Heat
Mar 2
M
First Law of Thermodynamics
4
W
Heat flow and Greenhouse Effect
6
F*
Climate Change
Mar 9-13
M-F
Spring Break
Mar 16
M
Heat Engines
18
W
Power and Refrigeration
20
F*
Electric Charge
Mar 23
M
Electric Fields and Electric Potential
25
W
Review
26
H
Test 3
27
F*
Electric Circuits
Mar 30
M
Magnetic Force Review
Apr 1
W
Electromagnets
3
F
Motors and Generators
Apr 6
M
Making Waves
8
W
Sound Waves
10
F*
E-M Waves, Light and Color
Apr 13
M
Mirrors and Reflections
Introduction
Section
0 Lecture 1 Slide 3
15
W
Refraction and Lenses
17
F*
Telescopes and Microscopes
Apr 20
M
Review
22
W
Seeing Atoms
24
F
The really BIG & the really small
INTRODUCTION TO Modern Physics PHYX 2710
May
1
F
Final Exam: 09:30-11:20am
Chapter
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No Classes
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21 (not on test)
Homework Due
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No test week
12
Fall 2004
* = Homework Handout
*Homework Handout
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 3
Physics of Technology
PHYS 1800
Lecture 28
Electricity and Charge
Introduction
Section 0
Lecture 1
Slide 4
Charge
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 4
Describing Motion and Interactions
Position—where you are in space (L or meter)
Velocity—how fast position is changing with time (LT-1 or m/s)
Acceleration—how fast velocity is changing with time (LT-2 or m/s2)
Force— what is required to change to motion of a body (MLT-2 or kg-m/s2 or N)
Inertia (mass)— a measure of the force needed to change the motion of a body (M)
Energy—the potential for an object to do work. (ML2T-2 or kg m2/s2 or N-m or J)
Work is equal to the force applied times the distance moved. W = F d
Kinetic Energy is the energy associated with an object’s motion. KE=½ mv2
Potential Energy is the energy associated with an objects position.
Gravitational potential energy PEgravity=mgh
Spring potential energy PEapring= -kx
Momentum— the potential of an object to induce motion in another object (MLT-1 or kg-m/s)
Introduction
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Lecture 1
Slide 5
Angular Momentum and Rotational Energy— the equivalent constants of motion for rotation (MT-1 or
kg/s) and (MLT-2 or kg m/s2 or N)
Modern Physics PHYX 2710
Pressure— forceINTRODUCTION
dividedTOby
the area over which the force is applied (ML-1T-1 or kg/m-s or N/m2 or Pa)
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 5
What are the major subfields in Physics?
• Classical Physics (pre 20th century)
•
•
•
•
Mechanics → forces, motion
Thermodynamics → heat, temperature
Electricity and magnetism
→ charge, currents
Optics
→ light, lenses, telescopes
• Modern Physics (20th century)
• Atomic and nuclear → radioactivity, atomic power
•
•
•
•
Quantum mechanics
→ basic structure matter
Particle physics
Condensed matter → solids and liquids, computers, lasers
Relativity, Cosmology → universe, life!
}
Introduction
Section 0
Lecture 1
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INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 6
State of Physics cira 1895
Conservation Laws
Electricity & Magnetism • Energy
Statistical Mechanics
• 3 Laws of Thermodynamics
• Kinetic Theory
Maxwell Equations (c 1880)
• Gauss’ Law
•Faraday’s Law
•Ampere’s Law
•No magnetic monopoles
• Linear & Angular Momentum
Mechanics (Gravity)
Newton’s Laws (c 1640)
1-Law of inertia
2-F=ma
3-Equal and opposite reactions
Introduction
Section 0
Lecture 1
Slide 7
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 7
Chapter 12
Electrostatic Phenomena
Introduction
Section 0
Lecture 1
Slide 8
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 8
What does
lightning have in
common...
Introduction
Section 0
Lecture 1
... with hair on a dry
winter day?
Slide 9
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 9
Effects of Electric Charge
• Hair seems to have a mind of its own when combed on
a dry winter day.
• What causes the hairs to repel one another?
• Why does a piece of
plastic refuse to leave your
hand after you peeled it off a
package?
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Slide 10
• Why do you get a slight
shock after walking across
carpet and touching a light
switch?
Charge
Lecture 28 Slide 10
Effects of Electric Charge
• All these phenomena involve different materials rubbing
against one another.
– Electrostatic effects can be demonstrated by rubbing plastic or
glass rods with different furs or fabrics.
– Small wads of dry, paperlike material called pith balls are light
enough to be strongly influenced by electrostatic forces.
– When a plastic rod, vigorously rubbed with cat fur, is brought
near the pith balls, at first the pith balls are attracted to the rod
like bits of iron to a magnet.
– After contacting the rod, the pith balls
dance away from the rod.
– They are now repelled by the rod and
also by each other.
Introduction
Section 0
Lecture 1
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INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 11
Effects of Electric Charge
• A repulsive force must be acting between the two pith balls
after they have been in contact with the rod.
– Perhaps the balls have received something (call it electric charge)
from the rod that is responsible for the force we observe.
– This charge was somehow generated by rubbing the rod with the cat
fur.
– The force that is exerted by one stationary charge on another is
called the electrostatic force.
Introduction
Section 0
Lecture 1
Slide 12
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 12
• Experiments with different materials indicate that there are two
types of charge.
• An electroscope consists of two metallic-foil leaves suspended
from a metal post inside a glass-walled container.
– If the foil leaves are uncharged, they will hang straight down.
– If a charged rod is brought in contact with the metal ball on top, the
leaves immediately spread apart and stay apart, even if the rod is
removed.
– If an object of the same charge as the
original rod is later brought near the
metal ball, the leaves will spread
farther apart.
– An object with the opposite charge will
make the leaves come closer together.
– A larger charge produces a larger
effect.
Introduction
Section 0
Lecture 1
Slide 13
Like charges repel each other,
and unlike charges attract
each other.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
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Lecture 28 Slide 13
• Benjamin Franklin introduced the names positive and negative for
the two types of charge.
• He also proposed that a single
fluid was being transferred
from one object to another
during charging.
– A positive charge resulted from
a surplus of the fluid, and a
negative charge resulted from
a shortage of the fluid.
– Franklin arbitrarily proposed
that the charge on a glass rod
when rubbed with silk be called
positive.
Introduction Section 0 Lecture 1 Slide 14
Like charges
repel each other,
and unlike charges attract
each other.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 14
• Franklin’s model comes surprisingly close to our modern view.
• When objects are rubbed together, electrons may be
transferred from one object
to the other.
– Electrons are small, negatively
charged particles present in all
atoms and, therefore, in all
materials.
– A negatively charged object has
a surplus of electrons, and a
positively charged object has a
shortage of electrons.
– The atomic or chemical properties
of materials dictate which way the
electrons flow when objects are
rubbed together.
Introduction
Section 0
Lecture 1
Slide 15
Like charges repel each other,
and unlike charges attract
each other.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 15
Conductors and Insulators
• Different materials behave differently in the
presence of electrostatic forces.
– Charge can readily flow through conductors:
• metals, like copper, silver, iron, gold; our bodies
– Materials that do not ordinarily permit charge to flow are
insulators:
• plastic; glass; ceramics; other nonmetallic materials
– Charge flows much more readily through several miles
of copper wire than through the few inches of insulating
ceramic material.
Section 0 Lecture 1 Slide 16
– Introduction
Semiconductors
are intermediate between a good
conductor and a good insulator.
• Their importance to modern technology is enormous.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 16
Can you charge an object without actually touching
it with another charged object?
• Charging by induction involves
the conducting property of
metals:
– Charge a plastic rod with cat
fur and bring the rod near a
metal ball mounted on an
insulating post.
– The electrons in the metal ball
are repelled by the negative
rod.
Introduction Section 0 Lecture 1 Slide
– There
is a negative charge
buildup on the side opposite
the rod, and a positive charge
on the near side.
17
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 17
Can you charge an object without actually touching it with
another charged object?
• To charge the ball by
induction, now touch the ball
with your finger on the side
opposite the rod.
– The negative charge flows
from the ball to your body,
since it is still repelled by the
negative rod.
– If you now remove your
finger and then the rod, a net
Introduction Section 0 Lecture 1 Slide 18
positive
charge is left on the
ball.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 18
Charging By Induction
• Charging by induction illustrates the mobility of charges
on a conducting object such as the metal ball.
– The process will not work with a glass ball.
– Charging by induction is an important process in machines
used for generating electrostatic charges, and in many
other practical devices.
– It also explains some of the phenomena associated with
lightning storms.
Introduction
Section 0
Lecture 1
Slide 19
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 19
Charging By Induction
Why are insulators attracted to charged
objects?
• Recall that the pith balls were
attracted to the charged rod
before they were charged
themselves.
– Electrons are not free to move
in the insulating material of the
pith balls.
– However, within each atom or
molecule, charges can move.
– Each atom becomes an electric
dipole: the center of the
Introduction charge
Section 0 isLecture
1 Slide 20
negative
slightly
displaced from the center of the
positive charge.
– The material is polarized.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 20
Charging By Induction
• Since the negatively charged surface is closer to the rod
than the positively charged surface, it experiences a
stronger electrostatic force.
– The overall effect is that the pith ball is attracted to the
charged rod, even though the net (total) charge on the pith
ball is zero.
– After the ball comes in contact with the charged rod, some
of the charge on the
rod is transferred to the
pith ball.
– The pith ball is then
positively charged like the
rod, and so is repelled by
the rod.
Introduction
Section 0
Lecture 1
Slide 21
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 21
Polarization
• Polarization explains why small bits of paper or styrofoam
are attracted to a charged object such as a sweater rubbed
against some other material.
• Electrostatic precipitators used to remove particles from
smoke in industrial smoke stacks use this property.
– Polarized particles are attracted to charged plates in the
precipitator, removing them from the emitted gases.
Introduction
Section 0
Lecture 1
Slide 22
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 22
Introduction
Section 0
Lecture 1
Slide 23
Charging on the
James Webb Space Telescope
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 23
Physics of Technology
Next Lab/Demo:
Electric Charge
Electric Circuits
Thursday 1:30-2:45
ESLC 46
Ch 12 and 13
Next Class:
Wednesday 10:30-11:20
BUS
Slide 24318 room
Read Ch 13
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Charge
Lecture 28 Slide 24