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93英文中學會考試題價智為體
物理科(廿六)
Physics (26)
A.C. dynamo.
An a.cz dynamo is a device
which generates alternating
current,i.e. a current which
changes direction periodically.
1.
The essential feature of an a.c. dynamo is as shown in fig.
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A rectangular coil is rotated between a horse-shoe magriet by some mechanical device. The arms of the coil are connected to 2 slip rings. The resistance R is the output load. i.e. the a.c. generated a dissipated in R.. Because of the rotation of the coil, the lines of flux of the magnetic field linking with the coil change. According to the Faraday's law of en induction, an enf is induced in the coil. Suppose the coil is rotated clock÷ wisey.
The side A is moving upward & the side B is moving downward. By appling Fleming's right hand rule, the direction of the induced current is inward in the arm A and outward in B. Thus the induced current appears to flow from slip ring b, through and to the slip ring a. The slip. ring b acts as the positive pole while slip ring a acts as the
a cell.
negative pole of alge
If the coil has turned thro' 180°. the positions of A & B are interchanged. This time current flows outward in A & inward in B. The induced current in the exter hal circuit flows in the direction aRb. Thus a acts as the positive pole while B as the negative pole: of a cell. The polarity of the induced current has changed when the coil has turned 180°, starting from the position when the plane of the coil is horizontal. In this manner, the current flowing through R changes its direction periodically. Such a current is called alternating. The variation of the induced emf with time is shom in fig. 2.
Included EMF:
NA
The loop labelled positive means
that the induced emf is positive
and the loop labelled, negative is
the induced emf after the coil
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An ac. has tumed through 1800. dynamo is a very good example of the conversion of energy.
2. Rectification of A C
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The transmission of a.c. is more convenient than the t transmiss -103 of d.o. But in many cases, dic must be used. It is there- fore desirable to convert the a.c. transmitted along cables into d.c. This process is called rectificat- jon. A very common method to convert an a.c. to d.c. is to use a thermionic diode. A schematic diagram of a thermionic diode is given in fig 3...
F is the filament which heats the cathode C & A is the anode. If the anode is maintained at a poten -tial positive with respect to the cathode, then electrons emitted from C will be accelerated towards: A. Conversely, if A is maintained negative relative to C, then electrons emitted from C will not travel toward A and the region between C&A is charge-free. The diode is said to be non-conducting in this case. As electrons are emitted from C, some are repelled by their neighboura and return to C. The most energetic ones are able to travel to A. This is the case when the anode voltage is not very positive. As the anode vol- tage is increased, the number of the electrons emitted arriving at anode A is increased. When the anode voltage has reached a thre- shold value, the number of elect- rons reaching the anode is con- stant with time. If an ammeter (reading currents in unit of milliampere) is used to measure the anode current, it will be observed that the anode current increases rapidly with the increa se of anode voltage until the thre
shold voltage is attained. At this voltage, the anode current is constant. A circuit diagram for the above experiment and a curve of anode current against anode voltage called the character -istic curve of the thermionic diode is given in fig 4 & 5.
anode saturation current
<fig+)
anade current (MA)
threshold
anode voltage
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日三廿月四年三七九一层公年二十六國民觀中育教儒曲
The threshold voltage is the vol- tage at which the anode current is constant. The space saturat- ion current varies with temperat- ure of the filament. The higher the filament temperature, the greater is the anode saturation current.
To rectify the a.c. using a thermionic diode, the circuit employed is given in fig 6.
felle
output
When the anode voltage is positive relative to the cathode (e.g. în the first half of a cycle of the A.C. source as given in the upper curve in fig 7.
ENF
Time
Tems
Time
electrons emitted from the cathode: travel towards the anode. The diode is conducting, the same current as the input is obtained at the output. When the anode is negative relative to the cathode: (in the 2nd half of the cycle), the diode is nonconducting. No current can be obtained from the output. This half of the supply is chopped off. After this half cycle, the anode is again positive as the input is positive, the diode is conducting again and the output is obtained. Repeating in this manner, the output current (or enf) obtained as a function of time is represented by the lower curve in fig 7. Only first half cycle in each period is obtained in the output. The rectifier is called a half-wave rectifier.
Moderm physics.
Atomic physics.
An atom is known to consist of a nucleus which contains protons and neutrons, with elect- rons moving outside the nucleus. A proton is a positively charged particle having nearly the same mass as a neutron which is neutral. Thus the nucleus of an atom is positively charged. Protons and neutrons are called nucleons.
The charge on a proton is of the same. magnitude as that on an electron, but the sign is opposite to it. The mass of a proton is very much greater than that of an electron. It was found that a protons mass is about 1840 times greater than an electron mass. The number of protons are called the atomic number Z, of the atom.
The mass number of an atom is the number of nucleons inside the nucleus. It should be noted that an atom is electrically neutral, i.e. the. number of electrons outside the
fig
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nucleus is equal to the number of
protons inside the nucleus. This
intum implies that the atomic
·
number is equal to the number of electrons in the atom. An atom. is said to be in a ground state if the electrons are filled up in order. It is known that the electrons outside the nucleus of an atom move in different shells. The direction of motion of an electron in a particular shell is not fixed. Each shell can only accommodate a limited number of electrons. The shell nearest to the nucleus can accommodate only. 2 electrons. It is called the K-shell, the shell next to the K shell can accommodate only 8 electrons. It is called the I- shell. The shell next to the I shell is called the M shell and can accommodate 18 electrons...etc The shells beyond the M shell are N,O,P,Q shells and so on. The number of electrons (which can be) contained in each cell increases as well go out from the nucleus. In modern theory, the energy of each shell is thought to be different and the electron in a particular shell is said to be in a particular energy level. A ground state atom is thus the atom which has the lowest possible energy which all its shells filled up
in the order K, L, M, L,O,P,Q. When an electron in an energy level is excited to a higher energy level, e.g, from the M shell to the N shell, the atom is said to be in an excited state. The electron jumping to the higher energy level will rapidly return to a lower energy Level (i.e. from N to M) in order to make the atom stable. In doing so, the extra energy between the two energy levels is emitted as light. light photon emitted is of wave length which falls in the visible spectrum. This is the origin of the visible light.
The
The general notation, of an atom X of atomic number Z and mass number A is za • The
difference A - Z is the neutron number, 1ve. the number of neutrons in the nucleus.
eig.1.
aHe
This means that
helium atom has D protons and 2. electrons. The number of neutrons in the nucleus 4 -2 = 2.
14
e.g. 2. N In the nitrogen aton
there are 7 protons in the nucleus and 7 electrons outside of the nucleus. The number of neutrons in the nucleus is 14 - 7 = 7.
Note that a nucleus may not contain any neutrons. e. hydrogen ¡H
Atoms which have the same chemical properties but different masses are called isotopes.
e.g.3. Hydrogen has 3 isotopes, The ordinary hydrogen‚H, deuterium H2 and tritium ¡H3.
The properties of these 3 isotopes are basically the same, but they are of different masses. A triti- um nucleus contains 1 proton and 2 neutrons. A deuterium nucleus contains 1 proton and 1 neutron while the nucleus of the ordinary hydrogen contains only 1 proton. Their chemical properties are basically the same because they all have one valence electron.
Exercise 6
1. How many neutrons are there in
60 29Ni
29Cu
58
If the nucleus of an atom
absorba a neutron, what happens to its mass number? Its atomic number?
3. If a proton inside a nucleus absorbs an electron, what happens to the mass number? The atomic number?
4. If inside a nucleus, 2 protons and 2 neutrons gombine to form
a helium nucleus which then
leaves the nucleus, what happen -s to the nucleus? Is the mass number changed? The atomic number changed?
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