Transcript Lecture32_Magnets

How rapidly is work being done
in an electrical circuit?
POWER =
WORK
TIME
What electrical unit is most closely
associated with work (energy)?
What electrical unit involves time?
What is the unit of POWER?
If an electric iron
draws 5 Amperes
from a 120v line,
at what rate is
it using power?
(1) 6 Watts
(3) 120 Watts
(5) 600 Watts
(2) 12 Watts
(4) 240 Watts
(6) 720 Watts
Which has a greater resistance,
a 100 Watt lightbulb or
a 25 Watt lightbulb?
1) the 25 W bulb
2) the 100 W bulb
3) the same for both
Which carries the greater current?
1) the 25 W bulb
2) the 100 W bulb
3) the same for both
A 1440 W microwave oven is
designed to operate using a
120 V power source. What is
the resistance of this appliance?
1) 0.1 
2) 10 
3) 72 
4) need to know the current
5) none of these
At age 67, in his autobiography,
Einstein wrote vividly of his recollection
of a new toy from his father:
a compass.
Thales of Miletus 600 BC
“Lodestone”
Lodestone first discovered
near the ancient city of
Magnesia in Asia Minor
The mineral ore is
called “magnetite”
Electrostatics
Magnetism
- attracts dust, hair,
plastic, paper scraps,
dried grass, leaves,
(small bits of anything)
- attracts only metals of
iron, nickel, & cobalt
- amber or glass charges
when rubbed
- lodestone always
exhibits this “charge”
- CONDUCTION can
charge other objects
- stroking (in one direction)
will magnetize metals
- induced charges are
long-lived
- induced magnetism is
short-lived
'When troops encountered gloomy weather or
dark nights, and the directions of space could not
be distinguished, they made use of the southpointing fish to identify the directions. A thin leaf
of iron is cut into the shape of a fish two inches
long and half an inch broad, having a pointed
head and tail. This is then heated in a charcoal
fire, and when it has become thoroughly red-hot,
it is taken out by the head with iron tongs and
placed so that its tail points due north. In this
position it is quenched with water in a basin, so
that its tail is submerged for several tenths of an
inch. It is then kept in a tightly closed box. To use
it, a small bowl filled with water is set up in a
windless place, and the fish is laid as flat as
possible on the water surface so that it floats,
whereupon its head will point south.'
Tseng Kung-Liang,
Compendium of Important Military Techniques
1044 AD
1296 Petrus Peregrinus (French physicist)
Any magnet has two “poles”
opposite ends at which all the magnetic
attraction seems to be concentrated.
like
poles
repel
unlike
poles
attract
How do the magnets exert forces on each other?
via the magnetic field
S
N
Magnetic Dipole
Electric Dipole
Magnetic Monopoles?
• Does there exist an isolated
magnetic charge?
– we would call this a magnetic monopole
– it would have a N or S magnetic
“charge”
• How might we find this isolated
magnetic charge?
Simple, right? Cut a bar magnet in half:
All you get
is a bunch
of little
magnets!
No attempt yet has been successful in
finding magnetic monopoles in nature
The North pole of a small magnet
(compass) points towards geographic
North because Earth’s magnetic
South pole is up there!!
Magnetic north is not
exactly geographic north.
The magnetic poles drift
and occasionally reverse polarity!
Magnetic declination
or “dip angle”
allows your lattitude
to be determined.
1813
Hans Christian Oersted
Danish physician and researcher
writes a philosophical
essay predicting a
connection between
electricity & magnetism
1820 Oersted discovers while
STATIC CHARGE has no effect
on a magnet (compass)
nearby CURRENT twists a
magnet sideways!
+
-
We saw that
A B-field must surround the
current carrying wire like
A current travels out toward you
through a conducting wire
The B-field surrounding this
wire is best represented by
A current travels out toward you
through a conducting wire
The B-field surrounding this
wire is best represented by
Looking down a length of wire, the magnetic
field lines show that the current must be
A. coming out of the screen at you.
B. zero.
C. going into the screen away from you.
The B-field at POINT
1. up.
3. left.
2. down.
4. right.
5. into the screen.
6.out of the screen.
7. is zero.
points
Current
Effect
0.001 Ampere
0.005 Ampere
mild shock
painful shock
<0.010 Ampere
0.010 Ampere
0.020 Ampere
0.030 Ampere
0.100 Ampere
0.200 Ampere
>0.200 Ampere
1.000 Ampere
Fatal?
no
no
tingles
no
paralysis of motor muscles
no
threshold of pain (cannot let go!)
no
breathing becomes labored
no
death due to fibrillation
seconds
no fibrillation, but severe seconds
burns and breathing stops
heart stops
seconds
serious burns
instantly
Galvani was correct in so far as our
central nervous system is in fact
a complex electrical circuit:
Neurons fire electrical pulses along synapses.
Muscle fibers contract in response to these signals.
Some Answers
QUESTION 1
(5) 600 Watts
QUESTION 2
Since P = V2/R, P  1/R.
P = IV
Greater resistance: (1) the 25 W bulb
QUESTION 3
The greater current: (2) 100 W bulb
The bulb with the smallest resistance must
draw the greater current!
Also: since P = IV, P  I.
QUESTION 4
(2) 10 
(120V )2 / 1440W  10
There can’t possibly be any way
nature distinguishes the wire’s top
from bottom, left side from right.
Imagine just rolling the wire on its side. Current still flows in the
same direction; the magnetic field must remain unchanged.
QUESTION 5
QUESTION 6
Which we’ll remember by a hand rule!
QUESTION 7
We certainly expect the magnetic field
to weaken with distance from the wire!
QUESTION 8
A. coming out of the screen.
To curling the fingers of your right hand in the direction of
the field lines, your thumb must point away from the screen.
QUESTION 9,10,11
6.out of the screen.
Just ride your closed hand along the curve in the wire, your
thumb pointing along the current’s direction. Your curled
fingers come out of the loop at all three points.