Download The Millikan Experiment: Determining the Elementary

The Millikan Experiment: Determining
the Elementary Charge
Textbook: 7.5
Homework: pg 364 # 1 – 5
Parallel plates capacitor
W  FE d
W  EE
W  q r
W  qV
q r  qV
 r  V
V

r
Millikan’s Oil Drop Experiment
Millikan’s Oil Drop Experiment
e  1.602 10
19
C
Ex. 2
• An oil drop of mass 4.95 x 10-15 kg is balanced
between two large, horizontal parallel plates
1.0 cm apart, maintained at a potential difference of
510 V. The upper plate is positive. Calculate the
charge on the drop, both in coulombs and as a
multiple of the elementary charge, and state
whether there is an excess or deficit of electrons.
Ex. 1
• Two large, horizontal metal plates are separated by
0.050 m. A small plastic sphere is suspended halfway
between them. The sphere experiences an upward
electric force of 4.5 x 10-15 N, just sufficient to
balance its weight.
(a) If the charge on the sphere is 6.4 x 10-19 C,
what is the potential difference between the
plates?
(b) Calculate the mass of the sphere.
Motion of charged particles in an
electric field
Textbook: 7.6
Homework: pg 368 # 1, 3, 5
pg 371 # 1, 2, 5
Motion of
charged
particles in
an electric
field
EE  EK
qV  EK2  EK1
1 2 1 2
qV  mv2  mv1
2
2
Ex. 3
• The cathode in a typical cathode-ray tube (Figure 4), found
in a computer terminal or an oscilloscope, is heated, which
makes electrons leave the cathode. They are then attracted
toward the positively charged anode. The first anode has
only a small potential rise while the second is at a large
potential with respect to the cathode. If the potential
difference between the cathode and the second anode is
2.0 x 104 V, find the final speed of the electron.
Mass of an electron = 9.1*10^-31kg
Charge of an electron = 1.6*10^-19 C
Ex. 4.
• An electron is fired horizontally at 2.5 x 106 m/s between
two horizontal parallel plates 7.5 cm long, as shown in
Figure 7. The magnitude of the electric field is 130 N/C. The
plate separation is great enough to allow the electron to
escape. Edge effects and gravitation are negligible. Find the
velocity of the electron as it escapes from between the
plates.
Magnetism
Textbook: 8.1 – 8.2
Homework: pg. 391 #1, 2, 7
•LEDs
•Incandescent lights
• What do we know about magnetism?
- They have poles
- Opposite poles attract, similar poles repel
- It is an action-at-a-distance force (therefore can be described in
terms of fields)
- One of the oldest known forces (magnetite was a naturally
occurring magnet)
- Is used for navigation (Earth’s iron core creates a magnetic field
aligned in the NS direction)
• What are some properties of magnets?
-
Things can be magnetized and demagnetized
When a magnet is broken in half it makes two smaller magnets
Some objects when magnetized maintain the field, others do not
Heating, dropping and striking a magnet can demagnetize it
A strong magnetic field can reverse the direction of a bar magnet
Domain Theory
• Ferromagnetic materials can be magnetized
(ex. Iron, nickel, cobalt)
• Ferromagnetic materials are made up of tiny
magnetic domains which act like miniature
magnets
• If, on average, the magnetic domains align
they create an induced magnet
Electromagnetism
• Oersted’s Principal: A moving electric current
induces a magnetic field
• The direction of the induced electromagnetic
field can be determined using the Right Hand
Rules.
RHR for straight conductor
RHR for solenoid
Relative permeability: is the ratio of the magnetic field strength for
a particular core material to the magnetic field strength in the absent
of the material.
Magnetic Force on Moving Charges
• Draw the magnetic fields of the permanent magnet and the
conductor.
• Determine the direction of the force on the conductor.
Show the labels of the magnetic poles, the magnetic field, and
the direction of force on the conductor
Describe the path of current through the conductor, brushes,
commutator, and coil by adding arrows. Identify the magnetic
polarity of the armature and the rotation direction of the
motor.
Magnetic Forces
Textbook: 8.2
Homework:
pg. 396 # 3 – 5
pg. 402 # 1 – 3 , 10
Magnetic Forces
• The magnetic force, FM [N], on a particle of charge
q [C] moving through a magnetic field strength B
[T, teslas] at a velocity v [m/s] is found by:
• Where  is the angle between B and v
Pg 396 # 2
• Determine the magnitude and direction of the
magnetic force on a proton moving
horizontally northward at 8.6 x 104 m/s, as it
enters a magnetic field of 1.2 T directed
vertically upward.
• An electron accelerates from rest in a
horizontally directed electric field
through a potential difference of
46 V. The electron then leaves the
electric field, entering a magnetic field
of magnitude 0.20 T directed into the
page (Figure 7).
(a) Calculate the initial speed of the
electron upon entering the
magnetic field.
(b) Calculate the magnitude and
direction of the magnetic force
on the electron.
(c) Calculate the radius of the
electron’s circular path.
Motion in Magnetic Fields
• A charged particle moving through a constant
magnetic field moves in a circular path
– J.J. Thomson and the e/m ratio
e/m = 1.76 x 1011 C/kg
– The Van Allen Belt (Aurora)
– Particle Accelerators
Magnetic Force on a Conductor,
Ampere’s Law
& Electromagnetic Induction
Textbook: 8.3, 8.4, 8.5
Homework: pg. 407 #1 – 5
pg. 414 #1 - 4, 6, 9
pg. 419 #1, 3, 8
FM on a Conductor in a Magnetic field:
• The magnetic force FM [N] on a conductor of
length l [m] carrying a current I [A] through a
magnetic field B [T] is:
F M  ILB sin 
– Where  is the angle between I and B
– RHR: Thumb points in direction of +ve I, fingers
in direction of B, palm pushes in direction of FM
Review - B
around a conductor:
• Magnetic field strength is proportional to
current:
B I
• Magnetic field strength is inversely
proportional to radius:
1
B
• Therefore:
r
• Ampère’s Law
Along any closed path through a magnetic
field, the sum of the products of the scalar
component of B, parallel to the path segment
with the length of the segment, is directly
proportional to the net electric current
passing through the area enclosed by the
path.
Ampere’s Law
B 
||
 0 I
0 I
B
2 r
•
•
•
•
Take a closed path in B
Add up B||l around path
Sum equals 0I
Permeability of free space
= 4 x 10-7 Tm/A
0
FM between TWO CONDUCTORS
• Two parallel straight conductors 5.0 m long
and 12 cm apart are to have equal currents.
The force each conductor experiences from
the other is not to exceed 2.0 x 10-2 N. What is
the maximum possible current in each
conductor?
Applications
• Coaxial Cable (see pg. 410)
– Electric Shielding
– Magnetic Shielding
• Definition of Ampere/Coulomb
– A current of 1 A in parallel wires 1 m apart in a
vacuum creates a force of 2 x 10-7 N per meter
– 1 C is the charge transported by 1 A in 1 s
Maglev Trains
• Opposite poles levitate
train
• Guide magnets change
polarity to push and
pull train
• Very little friction leads
to very high speeds
(World Record is 581
km/h)
Electromagnetic Induction
• A changing electric field (i.e. a current)
induces a magnetic field
• A changing magnetic field induces a current in
nearby conductors
• Examples:
– Electric Generators
– Transformers
– Guitar Pickups
Lenz’s Law
• The direction of an induced current is such
that it opposes the changing magnetic field
that created it
Magnitude of the magnetic field strength in the
core of the solenoid
NI
B
L
•
•
•
•
B is the magnitude of the magnetic
field strength in the core of the
solenoid, in teslas;
I is the current flowing through the
coil, in amperes;
L is the length of the solenoid, in
metres;
N is the number of turns on the coil.
Pg 418 # 3
• Two magnets are dropped through thin
metal rings (Figure 11). One of the rings
has a small gap.
• (a) Will both magnets experience a
retarding force? Explain your reasoning
• (b) Will your answers change if the rings
are replaced with long cylinders, one
with a long thin gap down one side?