Chapter 1 The Early Experiments
Example 1.8 a) The particles shown in the margin are in the same medium and the numbers are
their relative charges. Arrange the particles from most attractive to most repulsive. The relative forces can be obtained from F∝ qq 12(^2) /r
because both k and
in Equation 1.3 ε
are constant in this example.
The units of q and r are not im
portant so long as they do not
change from one system to the next.
∝∝ ∝ ∝
22
2
2
ac
bd
(+1)(-2)
(+3)(+2)
(+3)(-3)
(-3)(-2)
F
= -2; F
= +0.67; F
= -0.56; F
= +1.
(1)
(3)
(4)
(2)
1+
2-
1nm
3+
3-
4nm
a)
3+ 3-
2+ 2-
3nm 2nm
b) c) d)
Exam
ple 1.
Negative values are attractive, so the order
from most attractive to most repulsive is
(most negative)
a, c, b, d
(most positive)
b) Arrange the systems in order of energy of interaction.
Use E
q∝
q 12
/r for the reasons outlined in Part a to find the relative energies
∝∝ ∝ ∝ac
bd
(+1)(-2)
(+3)(+2)
(+3)(-3)
(-3)(-2)
E
= -2; E
= +2; E
= -2.25; E
= +
(1)
(3)
(4)
(2)
(a) (b)
ZnS-coating onend of tube glowswhere cathode
ray strikes
Cathode
Anode
Hole inanode Evacuated tube
(+) (+)
Negatively chargedmetal plate
Positively chargedmetal plate
(-) (+)
(-) (-)
Spot is moved byas beam is deflected
D
Spot wherebeam strikes
PowerSupply PowerSupply
D
Figure 1.2 Schematic of a cathode-ray tube (a) In the absence of an electric or
magnetic field, the ‘ray’ travels in
a straight line from the cathode to the end of the tube where it strikes ZnS, causing it to glow. (b) The ‘ray’ is bent toward the + plate by an electric field. The amount of deflection is given by
Δ.
(most attractive)
c , a, b, d
(most repulsive)
The charges in system
have the lowest potential (interaction) energy. a
- ATOMIC STRUCTURE
J. J. THOMSON AND THE CATHODE RAY (1897) By the late 1800’s, electricity had been observed and studied by many scientists, but its origin and nature were not yet understood. One observation made during this period was that radiation was produced by the applicati
on of a high voltage across two metallic plates
sealed in an evacuated glass tube like the one
shown in Figure 1.2. The rays were named
cathode rays because they appeared to originat
e at the cathode or negatively charged plate.
Cathode rays are invisible by themselves, but
when they strike certain materials, like glass
or zinc sulfide (ZnS), they cause the material
to glow. When a zinc sulfide coating was
applied to the walls opposite the cathode, the surface glowed brightly where the ‘cathode rays’ struck (Figure 1.2a). From the position of the glow, it was clear that the ‘cathode rays’ traveled in straight lines. The apparatus is now called a cathode ray tube or CRT. In 1897, the British physicist J. J. Thomson explor
ed the nature of the ‘rays’ by passing them
through external electric and magnetic fields (Figure 1.2b). He made the following observations and conclusions:
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