REFERENG LIBRANY
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1973英文中學會考試題預習專欄
4o M 50% V PA E M
物理科
Sound
(廿八)
Physics (28)
1. Wave motion
Energy can be transported
from one place to another by several means. The conduction of heat from a place of high temperat -ure to a place of low temperature is an example. There is another way of transporting energy, namely by wave motion. Wave is a kind of disturbance. Broadly speaking, Wave can be classified as (1) mechanical wave & (ii) electro- magnetic wave. • The first type of wave requires t the presence of a
material medium to assist its propagation. The latter can propagate in a vacuum. Sound wave is a mechanical wave while light is an electromagnetic wave. A wave of any kind can be represent ed by a waveform as shown in fig 1
displacement
AA
Hime
It is a sine wave.. Each point in the wave can be considered as a particle vibrating in a certain direction. If the vibration is parallel to the direction of propagation of the wave, (which is from left to right in the fig.) The wave is called a longitudinal wave. If the particle vibration is perpendicular to the propagat- ion of the wave, the wave is call- ed a transverse wave. Sound wave is a longitudinal wave while electromagnetic wave is transverse
•
wave called a The path ABCD is cycle. The time taken for 1. complete cycle (1.e. the time at the point D) is called the period of the wave motion, The number of cycles per second is called the frequency of the wave motion. The unit of frequency is in hertz (hz). It is obvious that period. X frequency - 16
The distance travelled in one period of time is called the wave- length (A). It is in units of metre
The wave velocity is in m/s. The fundamental relation relating frequency (f), wavelength Xand velocity (v) is that
properties of velocity of sound
wave:
(1) velocity of sound wave is
proportional to the square root of the absolute temperat-
ure.
(2) of sound wave through
a gas is proportional to pressure (p), of the gas and inversely proportional to the gas density (d).
i.e.
and
v.
is the ratio of the specific
heat at constant pressure to the
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S.H. at constant volume.
It is a constant for a particular gas. From the perfect gas equation,
FV = RT
for 1 mole of a gas if R is the gas constant per mole.
If M is the molecular weight of the gas, then for a fixed mass of a gas,
RT
RT
M
RT
Properties of sound wave
When sound wave is incident on a wall (in an empty room) echo can be heard. This proves that sound wave is reflected from the wall. Indeed the laws of reflect- ion for light also holds for sound wave. When sound wave
travels from medium of density d to a medium of density do, refract
-ion occurs.
since the velocity
.
of sound wave depends on the den- sity of the medium, the refractive index of a medium can be determin- ed if the velocity of sound is. known in the medium and in the medium in which the sound wave originally traverses. Suppose sound wave travels from a medium
to
in which its velocity is V.
a second medium in which is velocity is V2. Then the refract- ive index
Since the frequency of the wave is unchanged in going from one medium to another medium, the refractive index of medium 2 can be written as
1N2
21. Az
Where A & A2 are the waveleng- ths of the wave in media 1 & 2 respectively. Finally consider: sound wave travelling towards an opening. If the opening is large compared to the wavelength of the wave, the wave emerging on the other side of the opening is of the same wave form as the incident wave. But if the opening is small compared to the wavelength of the wave, the emergent wave becomes circular
wave
开
| emergent
wave
1.e. spreading out in all direct- ions. This phenomenon is called diffraction.
Reflection, refraction and diffraction are three general properties of any wave motion.
4. Vibrating string
Consider a string of length l fixed at both ends. If the mid- point of the string is slightly depressed and then released, the string vibrate in a mode as shown in fig 3.
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The end points of the string are called nodes. They are fixed points. The points labelled A
They are called anti nodes. represent the maximum displacement This mode of vibration is called the fundamental mode.
The wavelength of the wave set up along the string which is called the stationary wave as they are reflected at the nodes is equal to 2.
For a vibrating string, the wave velocity was found to be equal to the square root of the ratio of the tension in the string to the linear density (ie. mass per unit length) of the string,
T in neuton and m in kg/m. Using
入
This is the frequency for the fundamental mode or first harmor -ic). If the string is plucked at the mid-point, the mode of vibrat ion is called the second harmonic or the first overtone
K
Hence, wavelength
£ 2 = √
If the string is plucked at a point one-third from one end, the mode of vibration is called the 3rd harmonic the second over- tone..
f3=2 /
0.
In general, for the nth harmonic the frequency is
n
Solution to exercise 7.
The p.d. between anode and cathode 400 V.
mass of electron beam =
kg
charge of electron beam
18x
70-5
coulomb
(a) K.E. of the beam on reaching
the anode
αν 18 x 10 x 400
72 x 10
(b)
JMV2
qv
* 105 v
72 x 10
14400 = 120 m/s. (c) The number of electrons
reaching the anode
18 x 10 5
1.6 x 10
X 1015
1.13 x 1015
180
16 x 10
(d) The anode current
1
2. From the figure, it is obvious that the beam will be deflect- ed upward. If the beam is to travel in a straight line, the magnetic field must be applied. in a direction as determined by the Fleming's left hand rule. Bearing in mind that the
current in the rule is the direction of motion of positive charges which is opposite to the electron beam direction, the field must be applied into the paper so that the motion of the bear is downward and this downward movement cunteracts the upward deflection of the beam due to the electric field.
3. By Fleming's left hand rule,
the deflection is upward. (Note that particle is positively charged. The direction motion is the same as th
conventional current.
Wavelength:
visible light
4 x 10-5
rays:
7.5
10 cm.
gamma rays: 10 penetration power:
visible light X-rays
Production mechanism:
gamma
rays
Visible light produced when an electron jumps from a higher orbi to a lower orbit. The orbits in- volved are outer orbits.
V-rays, produced when an elect- rons jumps from a higher orbit into an inner orbit (e.g. from M shell to K shell or from M to I etc.)
gamma rays: emitted when a nucleus goes from an excited to a more stable state.
Exercise 8.
state
(a The distance between succes sive crests of a certain wave is 30 cm and the wave travels cm. in 2 sec. What is the wavelength, the velocity of
Wave and the frequency of
Wave?
(b) If the wave travels from the present medium to another medium which its velocity becomes 14 cm/s, What is the frequency, wavelength of the wave in this medium? What is the refractive index of the medium? Sketch the change in waveform of the wave. Calculate the velocity of sound in coal gas at 20°C, given that the velocity of sound in coal gas is 500 m/s at 0°0.
3. Calculate the velocity of
sound in hydrogen at 2000 from the following data. Velocity of sound in air at 0°C - 331 m/s.
Density of air at S.T.P.
1.293 gm/
Density of hydrogen at S.T.
0.0896 gmy
The string of a sonometer is stretched by a force of 16 kgf When the length of the part of the string vibrating is 0.3 m., the string has a frequency of 480 ha What emitted by the string (a) when the length is altered to 0.5 m. (b) when the tension is halved (c) when the length is made 50 cm. & the tension is halved at the
same time?
5. One end of a long thin wire is fixed and the other end passes over a pulley and is attached to a heavy
load. If the load is 250 gm. and the string is plucked at a certain point in the string, 8 segments are formed in 420 cm. Length of the string. If the mass of this length of wire is 1.8 gm. What is the frequency of the note emitted by the string?
18-10
0.01
18 x 10
A
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