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02 AL Physics/Essay Marking Scheme/P.1
PLK VICWOOD K.T. CHONG SIXTH FORM COLLEGE
02’ AL Physics: Essay
Marking Scheme
1.
(a) Shot put (parabolic)
½
Neglect air resistance, the shot is only acted upon by its weight (mg) whose magnitude
is constant and its direction is always towards the ground. As a result, the vertical
component of the motion is under constant acceleration towards the ground while the
horizontal component is unaffected (i.e. uniform motion).
½
1
1
Satellite around the earth (uniform circular motion)
½
The satellite is pulled by the earth (i.e. its own weight mg’) and this force is constant and
its direction is always towards the centre of the earth/motion.
The whole of this force is used for centripetal acceleration or merely changing its
direction of motion, the (tangential) speed of the satellite remains unchanged.
½
½
½
½
Mass-spring system (simple harmonic motion)
½
The force (kx) originates from the compression/extension (x) of the spring is always
directed towards the equilibrium position or the centre of motion. The magnitude of
the force and thus the acceleration are proportional to the displacement of the mass
from the equilibrium position. Hence this force is restoring and so an oscillatory
motion results.
½
½
½
½
8
(b) (i)
W
1+1
R
R’
W
W’
R
R’
W’
: weight of the boy
: gravitational force acting on the earth by the boy
: reaction force acting on the boy by the ground
: action force acting on the ground by the boy
The reaction R gives a resultant upward force (R-W) that enables the boy to
accelerate vertically.
(ii) When the fuel in a rocket burns, a stream of gas is produced and then escapes
at high velocity through the exhaust nozzle. The exhaust gases are pushed
backwards by the rocket.
Therefore a reaction acts on the rocket in the opposite (upward) direction and
it is this force that overcomes its weight and enables it to accelerate.
(c) Since there is no external force acting on the system, the linear momentum of the
system is conserved.
1
3
1
1
1
2
02 AL Physics/Essay Marking Scheme/P.2
Since the collision is elastic and a gold nucleus is much more massive than an alpha
particle, the kinetic energy of the alpha particle becomes the electric potential energy of
the system and then changes back to its kinetic energy when it rebounds.
2.
1
1
3
(a) (i)
3 cm
Probe
receiver
6 cm
transmitter
1
3 cm
Metal
plates
Arrange the metal plates such that two slots of approximately 3 cm wide are
formed with a separation of about 6 cm.
½
Interference occurs between the two wave-trains diffracted from the two slots
that act as two coherent sources. The receiver detects the maxima and minima
of the interference pattern as it is moved around.
1
½
Constructive and destructive interference occur wherever the path difference of
1
the microwaves from the slots is n and (n + ) respectively.
2
(accept other reasonable experimental setup).
(ii) - blooming of lenses
- testing of the flatness of a surface
(1½ mark for explanation including the path difference, the rays that interfere
and the kind of interference results)
(accept any other reasonable answers)
(b) The radar sends out a narrow microwave beam, which is swept continuously through
360 by a rotating aerial. The pulses reflected from land, other ships and buoys are
shown on a CRO (Plan Position Indicator), which has the time base origin in the centre
of the screen and represents the ship. This assists safe navigation in fog and at night.
Sonar is similar to radar but employs ultrasonic waves (i.e. sound waves of frequency
above 20 kHz) which are sent towards the bottom of the sea. It makes use of the
principle of reflection to measure the depth of the sea (i.e. in echo sounding) and to
detect shoals of fish.
(c) (i) These energy sources cannot be exhausted.
Tidal energy can be harnessed by building a barrage (barrier) containing water
turbines and sluice gates, across the month of a river, large gates are opened
during the incoming (flood) tide, allowing the water to pass until high tide,
when they are closed. On the outgoing (ebb) tide, when a sufficient head of
water has built up, small gates are opened letting the potential energy of the
trapped water drive the turbines and generate electricity.
1
4
½
1½
2
1
½
½
1
½
½
4
1
½
1
½
1
4
02 AL Physics/Essay Marking Scheme/P.3
(ii) The tidal range has to be considered together with the demand, both domestic
and industrial.
The initial and maintenance costs are great.
There are important environmental implications for nearby mudflats, water
supply, sewage disposal, shipping, fishing and wild life.
3.
(a) (i) A system of natural frequency f0 is being driven by a driving force of frequency
f. The system, oscillating with frequency f, is said to perform forced oscillation.
ANY
TWO
@1
2
1
1
If f is close to or equal to f0 the system is in resonance with the driving force.
And the amplitude of the system becomes very large.
1
½
The driving force and the displacement of the resonating system have a phase
1
difference of a quarter of an oscillation, i.e. 90 or T (or close to 90)
4
(accept graphical presentation).
½
4
(ii)
Amplitude
of forced
vibration
Light damping
Heavy damping
Natural
frequency
1+1
Driving
frequency
The natural frequencies of (the suspension of) a car should not equal any that
may be produced by the forces experienced in its motion. Also there should
be sufficient damping so that no large amplitude vibration is produced.
(b) (i) In the production of musical sounds from air columns in wind instruments
resonance occurs, in many cases, between the vibrations of air columns and of
small vibrating reeds.
1
1
4
1½
The resonant frequency depends on the length of the vibrating air column
1
(f  ) in general the shorter the air column is, the higher the resonant
L
frequency results. The vibrating air gives rise to the damping.
½
(ii) When a radio circuit is tuned by making its natural frequency for electrical
oscillations equal to that of the incoming radio signal. The resonant frequency
depends on the values of the capacitance and the inductance in the circuit
1
(f 
), the smaller the capacitance/inductance is, the higher the resonant
LC
frequency results. The resistance (usually comes from the inductor) gives rise
to the damping.
½
1
½
2½
½
½ 2½
02 AL Physics/Essay Marking Scheme/P.4
(c)
E2
E2
Photon of
frequency
(E2-E1)/h
Electron
E1
E1
Excited atom
Stimulating
photon of
frequency
(E2-E1)/h
E2
Spontaneous
emission
E2
Stimulating
photon
Stimulating
photon
Electron
E1
E1
Excited atom
½
Stimulated
emission
Laser light is emitted when a photon from a spontaneous emission between two
energy levels induces stimulated emission of more photons.
Laser light is monochromatic because all the photons have the same energy
(E2 – E1) and hence the same frequency [(E2 – E1)/h].
Laser light is also coherent because all the photons/waves are in phase,
therefore it is intense.
4.
Frequency
(E2 - E1)/h
½
1
1
3
(a) (i)
When a current-carrying conductor is placed with its current perpendicular to a
uniform magnetic field as shown, forces will act at right angles to the charge
carries according to Fleming’s left-hand rule.
1
The charge carriers will be pushed upwards, thus increasing their concentration
towards the top of the conductor.
1
As a result, a p.d. (and an electric field) is produced across the top and bottom
of the conductor. According to the diagram, the side X of the conductor
develops a negative potential that indicates the charge carriers take a negative
sign while a positive potential implies the charge carriers take positive sign.
1
1
1
5
02 AL Physics/Essay Marking Scheme/P.5
(ii)
I
I
½+½
0
V
0
(I)
(b)
V
(II)
For a tungsten filament lamp, the I-V graph bends over a V and I increase,
indicating that a given change of V causes a smaller change in I at larger value
of V. That is, the resistance (V/I) of the tungsten wire filament increases as the
current raises its temperature and makes it white-hot. It is ohmic only for
small V and I.
½
½
½
For a semi-conductor diode, the I-V graph shows that current passes when the
p.d. is applied in one direction that is almost zero when its acts in the opposite
direction. A diode thus has a small resistance if the p.d. is applied one way
round but a very large resistance when the p.d. is reversed. It is thus
non-ohmic.
½
½
½
4
(i) If the normal to the coil makes an angle  with the field at time t when the flux
 linking each turn is given by
 = BA cos 
= BA cos t
½
½
By Faraday’s law, the e.m.f.  induced in N turns is
d
d
 =  ( N) =  N ( BA cos t )
dt
dt
d
=  BAN (cos t )
dt
= BAN  sin t
1
½
½

0
t
1
1
5
(ii) With the soft iron cylinder, a stronger magnetic field results and therefore a
larger induced e.m.f. is produced for the same angular speed.
The soft iron cylinder is laminated so that the eddy current induced in it is
minimized, power loss is kept to a small value.
1
1
2
02 AL Physics/Essay Marking Scheme/P.6
5. (a)
(b)
Atomic number is a number allotted to an element in the Periodic Table, and it equals to
the number of protons in an atom of that element.
½
Mass number of an atom is the number of nucleons in the nucleus.
½
Isotopes are atoms that have the same number of protons (atomic number) but different
number of neutrons (therefore different mass number).
1
(i) An ionized isotope will experience a downward electric force (QE) due to the
uniform electric field and a upward magnetic force (Q v B) due to the uniform
magnetic field
½
The ions, with a particular speed v, would pass straight through when the
electric and magnetic forces acting on it are balanced,
½
QvB = QE
E
v =
B
2
½
½
½ 2½
(ii)
photographic
P
plate
speed
selector
a beam of
ionized
isotopes
½
When entering the magnetic field, all ions describe circular arcs and strike the
photographic plate P. For particles of mass M, the radius r of the path is given
by
Mv 2
r
Mv
r =
B1Q
QvB1 =
(c)
½
½
½
½
If B1 is constant, r is directly proportional to M (assuming Q is the same for all
ions). When ions with different masses are present each set produces a
definite line and from their positions the (relative) masses can be found.
1
1 4½
(i) Any THREE of the following:
cosnic rays, radioactive minerals, radon in the atmosphere, potassium-40 in the
body, X-rays from television screens, etc. (1 mark for the first answer and @½
for the other two answers)
(accept any other reasonable answer).
2
02 AL Physics/Essay Marking Scheme/P.7
(ii) The background count rate is recorded when no radioactive source is present.
Arrange the GM tube (connected to a scaler or ratemeter) a few centimeters
from the radioactive source as shown.
½
½
1
Record the reading of the ratemeter. Place a magnet between the source and
the GM-tube such that the magnetic field is vertically downward (or upward).
½
½
This time the reading of the ratemeter would drop but it does not fall to the
background value. This indicates that  rays, which is not deflected by a
magnetic field, is present.
½
½
Move the GM-tube along an arc to either side. The reading decreases to a
value near the background count rate when the GM-tube is moved clockwise.
There is reading (above the background count rate) when the GM-tube is
moved anticlockwise showing that alpha particles are present.
(accept any other reasonable answer).
½
½
5