Transcript Lecture 33 - USU Department of Physics

Physics of Technology
PHYS 1800
Lecture 33
Introduction
Electromagnetism
Section 0
Lecture 1
Slide 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electromagnetism
Lecture 33 Slide 1
PHYSICS OF TOF
ECHNOLOGY
- PHYS 1800
PHYSICS
TECHNOLOGY
ASSIGNMENT SHEET
Spring 2009Spring
Assignment
Sheet
2009
Date
Day
Lecture
Chapter
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 2
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
No Class
8
5-8
5-8
9
9
9
10
10
10
No Classes
11
11
12
12
13
9-12
13
14
9-12
14
15
15
16
17
17
17
1-17
18 (not on test)
21 (not on test)
Homework Due
-
6
7
8
-
9
10
11
No test week
12
Fall 2004
* = Homework Handout
*Homework Handout
Physics of Technology—PHYS 1800
Spring 2009
Electromagnetism
Lecture 33 Slide 2
Physics of Technology
PHYS 1800
Lecture 33
Introduction
Electromagnetism
Section 0
Lecture 1
Slide 3
Magnetism and Currents
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electromagnetism
Lecture 33 Slide 3
Magnetic Effects of Electric Currents
• Oersted discovered that a compass needle was
deflected by a current-carrying wire.
– With the wire oriented along a north-south line, the compass
needle deflects away from this line when there is current
flowing in the wire.
Introduction
Section 0
Lecture 1
Slide 4
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electromagnetism
Lecture 33 Slide 4
Magnetic Effects of Electric Currents-Right Hand Rule
• The magnetic field produced by the current is perpendicular
to the direction of the current.
• The magnetic field lines produced by a straight, currentcarrying wire form circles centered on the wire.
– The right-hand rule gives the direction of the field lines: with the
thumb in the direction of the current, the fingers curl in the
direction of the field lines produced by that current.
– The effect gets weaker as
the compass is moved
away from the wire.
Introduction
Section 0
Lecture 1
Slide 5
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
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Electromagnetism
Lecture 33 Slide 5

Magnetic Effects of Electric Currents-Right Hand Rule
• Two parallel current-carrying wires exert an attractive force on
each other when the two currents are in the same direction.
– The force is proportional to the two currents (I1 and I2) and
inversely proportional to the distance r between the two wires:
F 2 k I1I2

l
r
where k  1107 N/A2
– One ampere (A) is the amount of current
flowing in each of two parallel wires
separated by a distance of 1 meter that
produces a force per unit length on each
wire Introduction
of 2 x 10-7 Section
N/m. 0 Lecture 1 Slide 6
INTRODUCTION TO Modern Physics PHYX 2710
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Electromagnetism
Lecture 33 Slide 6
Magnetic Effects of Electric Currents-Right Hand Rule
Two long parallel wires carry currents of 5 A and 10 A in
opposite directions as shown. That is the magnitude of the
force per unit length exerted by one wire on the other?
a)
b)
c)
d)
e)
2.0 x 10-6 N/m
5.0 x 10-6 N/m
2.0 x 10-4 N/m
50 N/m
1000 N/m
F 2k I1 I 2

l
r
2 110 7 N/A 2 5 A 10 A 
 Introduction Section 0 Lecture 1 Slide 7
0.05 m 


 2.0 10  4 N/m
INTRODUCTION TO Modern Physics PHYX 2710
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Spring 2009
Electromagnetism
Lecture 33 Slide 7
Magnetic Effects of Electric Currents-Right Hand Rule
Two long parallel wires carry currents of 5 A and 10 A in
opposite directions as shown.
What are the directions of the forces on each wire?
a)
b)
c)
d)
e)
The wires exert an attractive
force on each other.
The wires exert a force repelling
each other.
Each wire exerts a force on the
other in the direction of the
other wire’s current (the red
arrows shown).
Each wire exerts a force on the
other in the direction opposite to
Lecture 1 Slide 8
theIntroduction
other Section
one’s0 current.
The wires exert no force on
each other.
INTRODUCTION TO Modern Physics PHYX 2710
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The wires repel each other.
Physics of Technology—PHYS 1800
Spring 2009
Electromagnetism
Lecture 33 Slide 8
Magnetic Effects of Electric Currents-Right Hand Rule
Two long parallel wires carry currents of 5 A and 10 A in
opposite directions as shown.
What is the total force exerted on a 30-cm length of the 10-A
wire?
a)
b)
c)
d)
e)
2.0 x 10-6 N
3.0 x 10-6 N
2.0 x 10-5 N
6.0 x 10-5 N
2.0 x 10-4 N
F 2k I1I 2

 2.0104 N/m
l
r
FIntroduction

Section 04 Lecture
Slide m
9
F   l  2.010
N/m10.30

 l 
 6105 N
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
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Spring 2009
Electromagnetism
Lecture 33 Slide 9
Physics of Technology
PHYS 1800
Lecture 33
Introduction
Electromagnetism
Section 0
Lecture 1
Slide 10
Magnetic Forces
INTRODUCTION TO Modern Physics PHYX 2710
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Electromagnetism
Lecture 33 Slide 10
Magnetic Forces
• Magnetic forces are exerted by magnets on other magnets, by
magnets on current-carrying wires, and by current-carrying wires on
each other.
– The force exerted by one wire on the other is attractive
when the currents are flowing in the same direction and
repulsive when the currents are flowing in opposite
directions.
– The magnetic force exerted on a moving charge of an electric current is
perpendicular to both the velocity of the charges and to the magnetic
field.
– This force is
proportional to the
quantity of the charge
and the velocity of the
moving charge and to
the strength of the
magnetic field:
F  IlB

Introduction
Section 0
Lecture 1
Slide 11
F  qvB
INTRODUCTION TO Modern Physics PHYX 2710
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Spring 2009
Electromagnetism
Lecture 33 Slide 11
Magnetic Forces
Two long parallel wires carry currents of 5 A and 10 A in
opposite directions as shown.
What is the strength of the magnetic field produced by the 5-A
wire at the position of the 10-A wire?
a)
b)
c)
d)
e)
2.4 x 10-6 T
2.0 x 10-5 T
1.2 x 10-5 T
1.2 x 10-4 T
2.4 x 10-4 T
F  6 10 5 N  IlB
where l  0.30 m is the length of the 10A wire,
B is due to the 5A current,
and I is the 10A current.

Introduction

Section 0
F
6 10 5 N
B 
Il 10 A 0.30 m 
 0.00002 N/A  m
Lecture 1
Slide 12
INTRODUCTION TO Modern Physics PHYX 2710
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 2 10 5 T
Physics of Technology—PHYS 1800
Spring 2009
Electromagnetism
Lecture 33 Slide 12
Magnetic Forces
• For this relationship to be valid, the velocity must be perpendicular
to the field.
• This actually defines the magnetic field as the force per unit charge
and unit of velocity:
F
B
qv 
units: 1 tesla (T) = 1 N/Am
• If the index finger of the
right hand points in the
direction of the velocity
of the charge, and the
middle finger in the
direction of the magnetic
Section 0 Lecture
field, thenIntroduction
the thumb
indicates the direction of
the magnetic force acting
on a positive charge.

1
Slide 13
INTRODUCTION TO Modern Physics PHYX 2710
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Electromagnetism
Lecture 33 Slide 13
Magnetic Forces
•
The force on a moving positively charged
particle is perpendicular to the particle’s
motion and to the magnetic field, just as the
force on a current is perpendicular to the
current and to the field.
– The force on a negative charge is in the
opposite direction of the force on a positive
charge: q  -q.
•
Because the force is perpendicular to the
velocity of the particle, the force does no
work on the particle.
– It cannot increase the particle’s kinetic
energy; it only serves to change the direction
of the particle’s motion.
– It provides a centripetal acceleration.
– If the charge is moving perpendicular to a
Introduction
Section
0 Lecture
1 Slidewill
14
uniform
magnetic
field,
the particle
follow a circular path.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electromagnetism
Lecture 33 Slide 14
Magnetic Forces
Two long parallel wires carry currents of 5 A and 10 A in
opposite directions as shown.
What is the direction of the magnetic field produced by the 5-A
wire at the position of the 10-A wire?
a)
b)
c)
d)
e)
f)
Perpendicular to the plane of the
page and into the page
Perpendicular to the plane of the
page and out of the page
Upward
Downward
Inward toward the other wire
Outward away from the other wire
Introduction
Section 0
Lecture 1
Slide 15
Perpendicular to plane of page
and into page
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electromagnetism
Lecture 33 Slide 15
Magnetic Forces
A straight wire with a length of 15 cm carries a current of
4 A. The wire is oriented perpendicularly to a magnetic
field of 0.5 T. What is the size of the magnetic force
exerted on the wire?
a)
b)
c)
d)
e)
0.3 N
0.48 N
0.6 N
1.0 N
2.0 N
l  15 cm  0.15 m
I4A
B  0.5 T
F  IlB
The direction
of this
force
will1 beSlide 16
Introduction
Section
0 Lecture
perpendicular to both the current in the
wire and to the magnetic field, as
described by the right-hand rule.
INTRODUCTION TO Modern Physics PHYX 2710
 (4 A)(0.15 m)(0.5 T)
 0.3 N
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009

Electromagnetism
Lecture 33 Slide 16
Physics of Technology
PHYS 1800
Lecture 33
Introduction
Electromagnetism
Section 0
Lecture 1
Slide 17
Current Loops
INTRODUCTION TO Modern Physics PHYX 2710
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Electromagnetism
Lecture 33 Slide 17
Magnetic Effects of Current Loops
• When a currentcarrying wire is
bent into a circular
loop, the magnetic
fields produced by
different segments
of the wire add to
produce a strong
field near the
center of the loop.
Introduction
Section 0
Lecture 1
Slide 18
INTRODUCTION TO Modern Physics PHYX 2710
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Physics of Technology—PHYS 1800
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Electromagnetism
Lecture 33 Slide 18
Magnetic Effects of Current Loops
• The magnetic field produced by a current loop is identical
to one produced by a short bar magnet (a magnetic dipole).
– In fact, in an external magnetic field, a current loop will
experience a torque just as a bar magnet would.
Introduction
Section 0
Lecture 1
Slide 19
INTRODUCTION TO Modern Physics PHYX 2710
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Electromagnetism
Lecture 33 Slide 19
Magnetic Effects of Current Loops
• Consider a rectangular loop:
– Each segment of the
rectangular loop is a straight
wire.
– The force on each segment is
given by F=IlB.
– Using the right-hand rule, you
can verify that the loop will tend
to rotate in the direction
indicated.
– The forces on the two ends of
the loop produce no torque
about center of the loop,
because their lines of action
pass through the center of the
loop.
– TheIntroduction
forces on
the 0other
two
Section
Lecture
1 Slide 20
sides combine to produce a
torque that tends to line up the
plane of the loop perpendicular
to the magnetic field.
INTRODUCTION TO Modern Physics PHYX 2710
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Spring 2009
Electromagnetism
Lecture 33 Slide 20
A current-carrying rectangular loop of wire is placed in an
external magnetic field as shown. In what direction will this loop
tend to rotate as a result of the magnetic torque exerted on it?
a)
b)
Clockwise
Counterclockwise
End view
Introduction
0 Lecture 1 Slide 21
The loop will
rotateSection
counterclockwise.
The forces on the long arms are outward and
because they do not share a common line of
action, impart a counterclockwise torque on
the loop.
INTRODUCTION TO Modern Physics PHYX 2710
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Electromagnetism
Lecture 33 Slide 21
• Since the magnetic forces on the loop segments are
proportional to the electric current flowing around the
loop, the magnitude of the torque is also proportional to
the current.
Thus,
the torque on a
current-carrying coil can be
used for measuring electric
current.
An electric meter consists of
a coil of wire, a permanent
magnet, and a restoring
spring to return the needle to
zero when there is no current
flowingIntroduction
throughSection
the coil.
0 Lecture 1
Slide 22
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Electromagnetism
Lecture 33 Slide 22
• This torque is also the basis of operation for electric motors.
– The current must reverse directions every half turn to keep the coil
turning.
– This can be achieved by using alternating current, or by using a
reversing direction of dc current with a split ring commutator.
One design for a simple
dc motor consists of a
wire-wound rotor
mounted on an axle
between the pole faces of
a permanent magnet.
• The split ring causes the
current to reverse
directions every half turn,
thus keeping
the coil
Introduction
Section 0 Lecture
turning the same
direction.
•
1
Slide 23
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Electromagnetism
Lecture 33 Slide 23
• The magnetic field produced
by a coil of wire will be
stronger than one produced
by a single loop carrying the
same current.
– The magnetic field produced
by each loop all add
together.
– The resulting field
strength is proportional
to the number of turns
N that are wound on
the coil.
– The torque on the coil,
when placed in an
external magnetic field,
is also proportional to
Section 0 Lecture 1 Slide
bothIntroduction
the current
and
the number of turns in
the coil.
24
INTRODUCTION TO Modern Physics PHYX 2710
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Electromagnetism
Lecture 33 Slide 24
Can we utilize the similarities between a current-carrying coil of
wire and a magnet?
•By winding a coil
around a steel needle or
nail, the magnetic field
produced is enhanced.
•The nail then behaves
like a magnet that is
stronger
than
most
natural magnets.
Introduction Section 0 Lecture
•This
is
an
electromagnet.
1
Slide 25
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Electromagnetism
Lecture 33 Slide 25
Physics of Technology
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Lecture 33
Introduction
Electromagnetism
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Slide 26
Faraday’s Law
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Electromagnetism
Lecture 33 Slide 26
Faraday’s Law: Electromagnetic Induction
• We have seen that an electric current produces a magnetic field.
 Can magnetic fields produce electric currents?
• Faraday tried, at first unsuccessfully, to detect a current in a coil
as a result of a current in a nearby coil.
– The primary coil was connected to a battery to produce a current.
– The secondary coil was connected to a galvanometer, a device to
detect magnitude and direction of current.
Introduction
Section 0
Lecture 1
Slide 27
INTRODUCTION TO Modern Physics PHYX 2710
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Electromagnetism
Lecture 33 Slide 27
Physics of Technology
Next Lab/Demo:
Electric Circuits
Magnetism
Thursday 1:30-2:45
ESLC 46
Ch 13 and 14
Next Class:
Friday 10:30-11:20
BUS
Slide 28318 room
Read Ch 14
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
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Electromagnetism
Lecture 33 Slide 28