Exam grades.
Frequency
40
35
30
25
20
Frequency
15
10
5
0
45
55
65
75
85
• Mean = 83.6/ stdev = 9.7
• Median = 85
95
Announcements
• Homework 4 posted today
– Sorry
Midterms
Extra office hours 2-4 Duane F 619 to
discuss exam
Reading Finish chapter 38
Today.
• The Electron
– Thompson (e/m)
– Millikan (just e)
• Alpha particles
• Ruterford and the atomic nucleus.
Cathode Rays
Glass tube with a good (but not too good) vacuum inside.
Two electrodes sealed into the tube.
When you apply a
large voltage a
beam of particles
goes between the
electrodes.
- Hundreds of volts +
Eventually, shown
to be particles.
What are they?
Cathode Rays
Glass tube with a good (but not too good) vacuum inside.
Two electrodes sealed into the tube.
+
-
They are charged
particles
Which way are charges
going?
- Hundreds of volts +
How can we tell?
Cathode Rays
If we apply a magnetic field into the board which of
the following will happen to the cathode rays?
XXX
XXX
XXX
- Hundreds of volts +
𝑭𝑩 = 𝑞𝒗 × 𝑩
+
-
a) Both will deflect
upwards
b) Both will deflect
downwards
c) + go up and – go
down
d) - go up and + go
down
Cathode Rays
-
- Hundreds of volts +
Put an object into the
beam and look for a
shadow.
Cathode Rays
If we apply a magnetic field into the board which of
the following will happen to the cathode rays?
XXX
XXX
XXX
- Hundreds of volts +
-
How many forces are
present on the
particles?
a)
b)
c)
d)
1
2
3
4
Cathode Rays
If we apply a magnetic field into the board which of
the following will happen to the cathode rays?
XXX
XXX
XXX
Really 2 that matter
-
The magnetic field curves
them into a circle
𝑭𝑬 = 𝑞𝑬
𝑭𝑩 = 𝑞𝒗 × 𝑩
𝑚𝑣
𝑟=
𝑞𝐵
Cathode Rays
Now apply a magnetic field and a perpendicular
electric field?
X
X
X
𝑩
𝑬
XXX
XXX
Really 2 that matter
-
𝑭𝑬 = 𝑞𝑬
𝑭𝑩 = 𝑞𝒗 × 𝑩
You can tune this to find 𝑣
𝑒
Then you can find for the electron
𝑚
(Noble for J.J. Thompson 1906)
Millikan oil drop experiment.
How many forces are important for this experiment?
a) 1
b) 2
c) 3
d) 4
Gravity, friction, electric
Millikan oil drop experiment.
Which of the following has the smallest terminal velocity?
a) Person
b) Elephant
c) Ant
d) Dog
Millikan oil drop experiment.
Again 3 forces
Terminal velocity gives size (temperature and pressure)
Density gives mass
Use electric field to measure charges
Millikan oil drop experiment.
83 drops, 58 published.
Some cosmetic surgery in his data but ended with
good measurement of e = 1.6x10-19 C
Another particle discovered +
charge.
alpha
𝑞
Quickly it was known that
𝑚
electron.
Is it a 𝐻2+ ion or a 𝐻𝑒 ++ ?
=
1
of that for the
2
Another particle discovered +
charge.
alpha
Wait until many alpha’s produced and look at the
spectrum:
Helium spectrum
Alpha particles are emitted with
high energy
Now we have a positive charge to probe the
internal structure of matter.
Alpha particles
All heavy elements
want to decay
Helium is
very stable
Radioactivity goes this
way.
Rutherford shot alpha particles at atoms and he figured
out that a tiny, positive, hard core is surrounded by
negative charge very far away from the core.
• One possible model:
Atom is like a solar system:
electrons circling the nucleus
like planets circling the sun…
• The problem is that accelerating
electrons should radiate light
and spiral into the nucleus:
*Elapsed time: ~10-11 seconds
Why don't planets spiral into the
sun?
A. Well, they do, but very, very slowly.
B. Because planets obey quantum mechanics,
not classical mechanics.
C. Because planets obey classical mechanics,
not quantum mechanics.
D. Because gravitational forces work differently
than electrical forces and there is no such a
thing as gravitational radiation.
E. Because planets are much bigger than
electrons.
Answer is A. Gravitational radiation is much, much weaker
than electromagnetic radiation.