2B
2006-AL
PHY
Candidate Number
Center Number
Seat Number
PAPER 2
(SECTION B)
NLSI LUI KWOK PAT FONG COLLEGE
S. 7 MOCK EXAMINATION 2006
PHYSICS A-LEVEL PAPER 2
Question-Answer Book 2B
Time allowed: 3 hours
This paper must be answered in English.
INSTRUCTIONS FOR SECTION B
1. Answer THREE questions from this section.
Write your answers in this Question-Answer
Book.
2. Each question carries 16 marks.
3. Write your Candidate Number, Center Number
and Seat Number in the spaces provided on the
cover of this Question-Answer Book.
4. Supplementary answer sheets and graph papers
will be supplied on request. Write your Candidate
Number on each sheet and fasten them with
string inside this book.
Question
No.
Marker’s
Use Only
Examiner’s
Use Only
Marker No.
Examiner No.
Marks
Marks
Total
Checker’s Use Only
Checker No.
Total
2006-AL-PHY 2B-1
Section B
Answer any THREE questions from this question. Write your answers in the spaces provided in
this question-answer book.
1.
(a) An object is traveling at a constant speed v in a circle.
(i) Using Newton’s law of motion, show that there should be a net force acting on the
object.
(ii) By drawing a suitable diagram, or otherwise, show that the net force acting on the
mv2
. Show also the direction of this force.
r
(iii) Explain why this net force acting on the object will not increase the speed of the
object.
(7 marks)
(b) A man can maintain stationary on the wall of a rotor without sliding down.
object is
Figure 1
(i)
By considering all the forces acting on the man, explain whether there is any net
force acting on the man or how the forces are being balanced.
(ii) If the maximum friction between the man and the wall of the rotor is equal to N,
where  is a constant and N is the normal reaction from the wall, find the minimum
angular speed of the rotor so that the man will not slide down the wall.
(4 marks)
(c) Explain why it is dangerous to drive a car at high speed round a bend. To minimize the
danger, what can be done
(i) to the design of the road,
(ii) to the design of the car?
(5 marks)
2.
(a) State Huygens’ principle. With the aid of diagrams, use the principle to explain
(i) how the direction of propagation of a plane wave is related to the wavefront;
(ii) why plane waves refract as they pass from one medium to another.
(7 marks)
(b) (i) State the conditions necessary for the formation of a stationary wave.
(ii) Describe FOUR contrasting features of progressive and stationary waves.
(6 marks)
(c) Describe, with the aid of a diagram, the production of acoustic resonance in a measuring
cylinder with the help of a tuning fork, an open pipe and water.
(3 marks)
2006-AL-PHY 2B-2
3.
(a) In the early years of the last century, Madam Curie used the illustration below to show
how the three radiations emitted from radium travelled in air in a uniform magnetic field.
X
Y
radioactive
source
Z
lead
shielding
Figure 3.1
(i) Identify the radiations X, Y and Z. State the direction of the magnetic field and
explain why X all have the same length but Z have different curvatures.
(ii) Give THREE reasons to explain why it is difficult, if not impossible, to take a
photograph of a cloud chamber which is like Fig. 3.1 above.
(5 marks)
(b) Radium-226 mainly undergoes -decay at a half-life of 1600 years and changed into
Radon-222 with a release of 4.88 MeV of nuclear energy. The decay process can be
represented by the following equation:
226
88
(i)
Ra  222
86 Rn    
The associated  radiation only shares 0.01 MeV of the energy released. Explain, in
details, how will the remaining 4.87 MeV of nuclear energy released be distributed
between the outgoing  particle and the recoiling daughter nucleus.
(ii) Describe, with the aid of a diagram, an experiment by which you can confirm the
radiations emitted from Radium-226 consist of  particles and  radiation.
(iii) Hong Kong has an abundant resource of granite in which Radium-226 can be found
and Radon-222 will further decay into Polonium-218 by emitting an  particle with
a half-life of 3.8 days.
222
86
Rn  218
84 Po  
With reference to the two equations listed, account for the potential hazards of using
granite as a building material and suggest TWO methods to reduce the risk of those
hazards.
(11 marks)
2006-AL-PHY 2B-3
4.
(a) A student uses a flame probe to investigate the variation of electric potential in the region
around a positively charged sphere. The probe, in the form of a small flame at the point of
a needle, is connected to an electroscope calibrated to measure potentials.
Figure 4.1
(i) Briefly explain why the electroscope reading is a measure of the potential at the
point where the probe is situated.
(ii) The experiment has to be performed with the charged sphere remote from the floor
and neighbouring walls. Explain briefly why.
(4 marks)
(b) (i)
Describe, with the aid of a labelled diagram, an experiment that can be used to show
that the electric field strength between the two oppositely charged parallel
conducting plates shown in Fig. 4.2 is uniform.
A
X
B
conducting plates
d
+
EHT power supply
E
insulating stands
Figure 4.2
(ii) Draw a diagram to show the electric field lines and equipotential lines between the
plates.
(iii) Give the definition of the potential difference between plate A and plate B.
(iv) Hence, derive an equation for the electric field strength between the two plates.
(8 marks)
2006-AL-PHY 2B-4
(c) A light conducting sphere is suspended from a fixed point X by a long insulating thread.
It is placed between two metal plates A and B in Fig. 4.2. The sphere is now made to
touch plate A. Describe the subsequent motion of the metal sphere. What will happen if
the power supply is suddenly disconnected from the two charged plates?
(4 marks)
5.
(a)
clamp-on
pulley
wooden
blocks
sticker
ruler
load
Figure 5.1
In an experiment to find the stress-strain curve of a material, the material is made into
long and thin wires and assembled as shown in the Fig. 5.1.
(i) Explain why the wires should be long and thin.
(ii) State and explain two precautions in the experiment.
(3 marks)
Figure 5.2
Figure 5.3
(b) In Fig. 5.2, the graphs are produced by four materials, A, B, C and D. Briefly explain
which one is
(i) the stiffest,
(ii) the strongest, and
(iii) the most flexible.
(3 marks)
2006-AL-PHY 2B-5
(c) In Fig. 5.3, the graphs are produced by two materials P and Q. Briefly explain which one
is
(i) ductile, and
(ii) brittle
Suggest one material for each of them.
(3 marks)
Bernoulli’s Principle is used in aircraft to lift it up and allows it to fly. Explain, with
an aid of diagram, the action of the air flow around the aerofoil so as to provide the
lifting force.
(ii) Bernoulli’s Principle is also applicable in car racing to provide a downward force
(d) (i)
instead of the lifting force. Explain briefly why the downward force is required on
fast moving racing cars (Fig. 5.4). Explain, with an aid of diagram, how the
downward force can be acted by considering the set up of front and rear wings.
rear
wing
Figure 5.4
(7 marks)
END OF PAPER
2006-AL-PHY 2B-6
Solutions:
1.
(a) (i)
v2
B
A
1/2
v1
When the object moves from A to B, there is a change in the direction of
the velocity. Thus the object experiences acceleration.
1/2
According to Newton’s second law, there is a net force acting on the 1/2
object.
1/2
(ii)
1
Consider a small body (i.e. a particle) moving with constant speed v in a
circle of radius r. It travels from A to B in a short interval of time  t.
arc AB = v  t -------- (1)
1/2
By geometry, arc AB = r  -------- (2)
1/2
From (1) and (2),
v  t = r 
or
v
r
 =  t ------ (3)
 
Change of velocity = vB  v A
 v = XZ  arc XZ (if  is very small)
1/2
= v 
1/2
v
= v (  t)
r
a=
v
v2
=
t
r
As v = r  ,
a=
r 2
= 2r
r
Direction: As  t is very small, A and B are nearly coincide.
 arc XZ  vA and thus pointing to O.
1/2
The acceleration is directed radially inward and it is called centripetal 1/2
acceleration.
(iii) As the net force acting on the object is perpendicular to the displacement, 1/2
the work done on the object is zero. So there will be no change in K.E. and 1/2
thus the speed does not change.
2006-AL-PHY 2B-7
(b) (i)
f
N
1
W
As the man does not slide down the wall, friction f is balanced by the 1/2
weight W of the man.
As the man is undergoing circular motion in the rotor, the normal reaction 1/2
from the wall of the rotor acting on the man is the net force accounting for
the required centripetal force.
(ii) N = m2r
max f = N = m2r
For the man to remain on the wall,
max f  W
1/2
1/2
m2r  mg
thus  
1/2
g
r
the minimum angular speed =
g
r
1/2
(c) It is dangerous to drive round at a high speed as the centripetal force depends
1
mv 2
on friction. If the speed is too high that fmax <
, skidding occurs.
r
Moreover, reaction on the two wheels are not the same and overturning may 1
also occurs at high speed.
(i) The danger can be minimized by using banked track. The centripetal force 1/2
will depend on friction and the normal reaction on the wheels.
1/2
The friction of the road can be increased by using special material on the road 1/2
surface.
(ii) The C.G. of the car can be made lower and the distance of the wheels can 1
be made wider. Good tyres providing sufficient friction should be used.
1/2
2006-AL-PHY 2B-8
2.
(a)
Huygens’ principle states that each point on the existing wavefront of a wave 1/2
acts as a source of secondary wavelets.
The plane tangential to these wavelets is the new wavefront.
(i)
1/2
At several points on the initial wavefront, draw spheres of radius r = ct, 1/2
representing the distance travelled by the secondary wavelets in time t (c =
velocity of wave). The new wavefront displaces a distance ct from the initial 1/2
wavefront. Thus, from the diagram the wave always propagates at right 1/2
angles to the wavefront.
secondary wavelet
seconday sources
ct
constructed wavefront
ct
ct
rays showing
1
the direction of propagation
(ii) The incident wavefront has just reached the boundary between the two media
at point A. Points A and B act as sources of secondary wavelets. The wave 1
from B, travelling at speed v1, in medium 1, moves a distance v1t to point B’.
Within the same time, the wave from A, travelling at a slower speed v2 in 1
medium 2, moves a smaller distance v2t to point A’. The new wavefront in
medium 2 is given by the line through A’B’ which has clearly been deflected, 1/2
refraction of waves results.
secondary wavelet
Normal
seconday sources
incident ray
v1 t
cructed wavefront
B
incident wavefront
medium 1 A
medium 2
v2 t
A‘
B’
refracted wavefront
1
refracted ray
(b) (i)
A stationary wave is formed when there is superposition of
two waves of nearly equal amplitude
and equal frequency
travelling in opposite directions.
2006-AL-PHY 2B-9
½
½
½
½
6
(ii)
progressive wave
stationary waves

Waveform advances as time 
goes on.
Waveform does not advance.

Energy is transmitted along the 
direction of travel of the wave.

Particles within one wavelength 
have different phases.

All particles are vibrating.
Energy is confined within the ½ + ½
stationary wave.
All particles between two ½ + ½
adjacent nodes are in phase.
Some particles (at nodes) have ½ + ½
no vibration.

All vibrating particles have the 
same amplitude.

½+½
Different
particles
have ½ + ½
different
amplitudes,
in
particular,
amplitude
is
maximum at anti-nodes.
(any four)
(c)
1
The open pipe with one end inside water is raised up slowly. A tuning fork is sounded 1/2
just above the mouth of the pipe. Stationary wave is formed inside the pipe.
4l
1/2
At a certain length of air column when  n 
(n = 1, 3, 5, … ), a loud sound is
n
heard. This is called resonance, which is due to the fact that the sound waves sent by the 1/2
tuning fork inside the pipe are reflected from the water surface with node and antinode
at the bottom and the top of the pipe respectively.
1/2
2006-AL-PHY 2B-10
3.
(a) (i)
X is  radiation, Y is  radiation and Z is  radiation.
1
The direction of B-field is into the paper.
½
 particles emitted from radium nuclei have the same kinetic energy. ½
Their ranges in air is nearly the same hence they will leave tracks of
nearly the same length.
 particles emitted from radium nuclei have different kinetic energies
½
and therefore different initial velocities.
Since Bev 
mv 2
mv
, therefore  particles of different initial
r
r
Be
½
velocities will have different curvatures in air.
Tracks of  rays are nearly invisible. The ionization power of  rays is ½
extremely weak.
(ii)
1. To make an observable deflection of  radiation, an extremely
strong magnetic field is needed.
½
2. The ionization power of  particles is very weak. Since the mass
½
of a  particle is very small, its path in air can easily be twisted by
½
air particles.
(5)
(b) (i)
Let M be the mass of Rn-222 and m be the mass of  particle.
Conservation of momentum: MV = mv
Ratio of K.E. =
1
2
1
2
MV 2 V m
4
 

2
v M 222
mv
½
1
As the mass of Rn-222 is much larger than that of  particle,  particle
will share most of the energy released.
K.E. of Rn-222  4.87 
4
= 0.086 MeV
226
K.E. of  particle  4.87
(ii)
222
= 4.78 MeV
226
2006-AL-PHY 2B-11
½
½
½
(drawing)
2 cm
GM tube + scaler
½
radium source
½
(labeling)



Place a piece of paper between GM tube and the source. If there is a
significant decrease in count rate, it can be sure that the source emits
 particles.
Place a few mm thick aluminium plate between GM tube and the
source. If there is no significant change in count rate, this shows that
the source does not emit  radiation.
Place a few cm thick lead block between GM tube and the source. If
the count rate decreases to background count rate, this shows that
the source emits  radiation.
½
½
½
½
½
½
(7)
(iii) Potential hazards:



Although the half-life of radon-222 is short, the half-life of
radium-226 is long. Radiation from granite can exist for years.
Radon is a colourless and odourless gas. It can leak out from the
cracks of granite and go into the air.
If we inhale the radon gas, it may undergo  decay in our lungs.
½
½
½
½
The  particles emitted will directly damage our tissues and cause ½
cancer.
½
Methods to reduce risk:
1.
2.
½
Ensure that a room has good ventilation.
Granite walls or floors should be covered with wall papers or tiles ½
to reduce the chance of radon gas going out into the air.
(4)
2006-AL-PHY 2B-12
4.
(a)
(i)
When the needle is inserted into a point in the field with a positive potential,
negative charge is induced in the needle,
which is readily neutralized by the positive ions in the flame,
while positive charge appears on the leaf of the electroscope.
The needle will always become neutral and does not affect the original field
and potential. The electroscope remains positively charged and is at the
same potential as the uncharged probe since they are connected.
The closer the needle to the charged body, the higher is its potential and the
greater will be the positive charge induced in the electroscope and so the
greater the deflection.
(ii)
(b) (i)
1/2
1/2
1/2
1/2
1/2
1/2
It is to avoid the potential variation around the space of the charged sphere 1/2
from disturbing by the floor and neighbouring wall which have been 1/2
earthed, as negative charge will be induced on the surfaces of them.
(4)
Apparatus needed:
Long insulating rod with a
small strip of metal foil at one end.
Procedures:
1. Touch the metal foil with one of the plates.
2. Note the angle of deflection.
3. Move the rod around the space between the plates to see if there is any
1/2
1/2
1/2
1/2
change in the deflection.
1/2
4. If the deflection is constant, the force acting on the foil is constant. The
E-field is uniform.
1/2
insulating rod
1/2
labelling
metal foil
1/2
(ii)
1/2
labelling
1/2
Electric field lines
drawing
Equipotential lines
2006-AL-PHY 2B-13
(iii)
The external work done in taking an unit positive charge from A to B.
(iv)
dW = qE dx
 dW  qE  dx
W = qEd
W E
V

q d
E
(c)
V
d
1/2
1/2
1/2
1/2
1/2
1/2
(8)
On touching the positively charged plate A, electrons leave the sphere 1/2
causing it to become positively charged.
The force of repulsion (due to the positive plate) and the force of attraction 1/2
(due to the negative plate) move the sphere towards the negative plate.
1/2
On contact, the sphere becomes negatively charged and will move back to 1/2
the positively charged plate to repeat the motion again.
1/2
This oscillatory motion of the sphere will continue as long as the plates are 1/2
charged.
If the power supply is suddenly disconnected from the two charged plates,
the oscillatory motion of the sphere will cease after some time. This is 1/2
because the charge on the plates have been neutralized by the oscillating 1/2
sphere.
(4)
2006-AL-PHY 2B-14
5.
(a) (i) The wire is long so that the extension is more significant for the same stress. 1/2
The wire is thin so that the stress is more significant for the same force 1/2
applied.
(ii) Other precautions :
- put some foam boards on the floor
- to prevent floor surface and standard weight from damaging
- measure the thickness of the wire at various positions along the wire
- to reduce the random error in the thickness of the wire
- wear protective goggles
- to protect the eyes and faces
- warn the others when the wire is about to break
1/2
1/2
1/2
1/2
- to prevent hurting the others
(any two)
(b)
(i) A is the stiffest material as the slope is the greatest, i.e. the Young Modulus 1
is the greatest.
(ii) C is the strongest as the breaking stress is the greatest.
1
(iii) D is the most flexible as the deformation of the material is the greatest 1
under the same stress.
(c)
(i) Q is ductile as it has long range of plastic deformation.
1
(ii) P is brittle as it breaks over a very short range of deformation, it has no 1
plastic deformation.
Q is nylon.
P is glass.
1/2
1/2
(d) (i)
1
When the aircraft is flying to the left, speed of air flow on side A is faster than 1/2
the speed on side B. According to Bernoulli’s Principle, pressure on side A is
smaller than that on side B. This pressure difference results in a net force 1/2
pointing upward. This is the lifting force.
1/2
(ii) A downward force can press the car on the track
so as to increase the friction between the tyres and the road.
Therefore, the racing car can turn round corners at a high speed.
2006-AL-PHY 2B-15
1/2
1/2
1/2
1/2
label
1/2
diagram
The wings of the racing car are upside down as that of the aerofoils. The
streamlines around the wings is shown in the diagram when the racing car is 1/2
moving to the left,. Speed of air flow is faster on the bottom of the wing than
that on the top. According to the Bernoulli’s Principle, pressure on the top is 1/2
higher than that on the bottom. A net force acting downward is resulted from 1/2
this pressure difference.
2006-AL-PHY 2B-16
1/2