Download november 2009 - The University of Sydney

93/08(a)
Semester 2, 2009
Page 1 of 11 pages
THE UNIVERSITY OF SYDNEY
PHYS1004
PHYSICS 1 (ENVIRONMENTAL & LIFE SCIENCES)
NOVEMBER 2009
Time allowed: THREE Hours
MARKS FOR QUESTIONS ARE AS INDICATED
TOTAL: 90 marks
INSTRUCTIONS
•
•
•
All questions are to be answered.
Use a separate answer book for each section.
All answers should include explanations in terms of physical principles.
DATA
Density of water
   103 kg.m3
Density of air
   kg.m3
Atmospheric pressure
=
1.01  10 Pa
g
=
9.80 m.s
=
6.022  10
=
8.854  10
=
8.99 × 10 N.m2.C-2
=
4 10
=
1.602  10
Magnitude of local gravitational field
Avogadro constant
N
A
Permittivity of free space

Coulomb’s law constant
K
Permeability of free space
5
1 atm
µ
Elementary charge
0
0
e
-2
23
mol
12
-1
F.m1
9
7
1
T.m.A
19
C
8
Speed of light in vacuum
c
=
2.998  10 m.s1
Planck constant
h
=
6.626  10
=
9.110  1031 kg
=
1.675  10
=
1.673  10
=
1.674  10
Rest mass of an electron
m
Rest mass of a neutron
m
Rest mass of a proton
m
Rest mass of a hydrogen atom
m
e
n
p
H
34
27
27
27
J.s
kg
kg
kg
Boltzmann constant
k
=
1.381  1023 J.K -1
Stefan-Boltzmann constant

=
5.671  10
Atomic mass unit
u
=
1.661  10
Energy equivalent of 1 atomic mass unit
E
=
931.49 MeV
-8
W. m
27
kg
-2 -4
K
93/08(a)
Semester 2, 2009
Page 2 of 11 pages
SECTION A
Question 1
(a)
Draw a labelled diagram showing the apparatus for a simple experiment to measure
the surface tension of a liquid. Explain the physical basis of the measurement and
write (in symbols) the equation that would be used to find the surface tension.
(b)
Why does water rise up a glass capillary tube inserted vertically into the water, while
mercury in a similar tube is depressed below the level of the liquid surrounding the
tube?
(5 marks)
Question 2
Consider a sample test apparatus such as those shown below.
(a)
Define shear stress. Use a diagram to illustrate your answer.
(b)
Describe the effect produced by a shear stress in:
(i)
a viscous liquid sample;
(ii)
a solid sample (where the shear stress is moderate in magnitude).
(5 marks)
93/08(a)
Semester 2, 2009
Page 3 of 11 pages
Question 3
The diagram below shows a cross-sectional view of a charge + q inside a cavity in a metal
spherical shell. The shell carries no net charge.
+q
(a)
Sketch the distribution of any charges on the sphere.
(b)
On the same diagram, sketch some representative electric field lines in the various
regions of the diagram, using them to indicate where the field is stronger or weaker (if
anywhere).
(c)
Someone now touches the outside surface of the spherical shell for a moment and then
moves away. Draw a new diagram showing some representative electric field lines.
Justify the differences (if any) between your two diagrams.
(5 marks)
93/08(a)
Semester 2, 2009
Page 4 of 11 pages
Question 4
Consider an experiment with a permanent magnet and a metal ring. The metal ring is
connected to a current meter. Initially the magnet is held above the ring, as shown in the
diagram, and the meter shows zero current. When the magnet is released and allowed to fall
through the ring, the needle on the meter is observed to move suddenly in one direction, then
in the other direction and then back to zero.
(a)
(b)
Carefully explain, using words rather than equations, what is happening in this
experiment.
In one part of this experiment, current in the metal ring is found to be flowing around
the ring from A to B. Explain when this happens and why the current is flowing in this
direction.
(5 marks)
93/08(a)
Semester 2, 2009
Page 5 of 11 pages
Question 5
(a)
In no more than a quarter of a page each, describe two key features of each of the
following nuclear processes:
(i)
(ii)
(b)
fission;
fusion.
The figure below shows a plot of the binding energy per nucleon of nuclei as a
function of their mass number. Copy this diagram into your answer book and show on
it:
(i)
(ii)
(iii)
where the most stable nuclei are to be found;
where nuclei most likely to undergo fission for a net energy gain are to be
found;
the region where fusion for a net energy gain may take place.
Briefly justify your choice in each case.
(5 marks)
Question 6
131
  process).
The radioisotope iodine-131 ( 131
53 I ) decays to xenon-131 ( 54 Xe ) via β-decay (the
The nuclear masses of
131
53
I and
131
54
Xe are 130.90666u and 130.90507u respectively and the
mass of an electron is 0.00055u . Assume that
131
53
I has a half life of 8.00 days.
(a)
Write down the nuclear reaction for this process.
(b)
Calculate the maximum possible kinetic energy of the   particle. Express your
answer in joules.
(c)
The isotope 131
53 I is used in the diagnosis and treatment of thyroid disorders. A patient
is injected with this isotope. Assuming that all the iodine is concentrated in her
thyroid and that it is only eliminated from her body over many months, what fraction
of 131
53 I will remain in her thyroid after 24 days.
(5 marks)
93/08(a)
Semester 2, 2009
Page 6 of 11 pages
SECTION B
(Please use a separate book for this section)
Question 7
A measurement is conducted to find the average density of an athlete’s body. As a first step,
the athlete’s mass is found to be 108 kg. The second part of the measurement is made using a
metal frame suspended from a spring balance and immersed in fresh water in a swimming
pool.
(a)
Unless he holds on to the frame, the athlete floats. But by holding on to the frame
underwater he totally submerges his body. Draw a labelled diagram to show the three
forces acting on the athlete’s body while he is underwater, and show in which
direction each force acts. State the origin of each of the three forces.
(b)
An initial reading of the spring balance is taken with the frame submerged, but
without the athlete. Then, with the athlete underwater and holding on to the frame, the
spring balance is found to read F less than the initial reading. In symbols, write an
equation relating the forces acting on the athlete’s body in this state.
(c)
If F  80 N , find the volume of the athlete.
(d)
Find the average density of the athlete.
(10 marks)
93/08(a)
Semester 2, 2009
Page 7 of 11 pages
Question 8
The diagram shows a Pitot tube that could be attached to the outside of an aeroplane and used
to measure its airspeed (i.e. the speed of air relative to the aeroplane). It consists of a central
tube with opening (point A) facing into the air flow, and an outer tube with holes (e.g. point
B) along the sides. The arrangement is connected to a U-tube manometer to measure the
pressure difference between point A and point B. Point A is a stagnation point, at which the
air is at rest relative to the Pitot tube. As the air passes point B it has a speed equal to the
airspeed of the plane.
(a)
Consider Bernoulli’s equation as applied to points A and B. Explain the reason(s) that
some term(s) can be omitted from the equation in this instance, and write down (in
symbols) the simplified equation.
(b)
The manometer contains mercury which has a density of 13.6 103 kg.m 3 . Write
down the equation relating the difference in pressure of the air in the two arms of the
manometer to the height difference h of the mercury. State the physical basis for this
relation.
(c)
The height difference h is 55 mm. Find the corresponding pressure difference in Pa.
(d)
If the density of the air is 1.2 kg.m 3 , find the airspeed of the aeroplane.
(10 marks)
93/08(a)
Semester 2, 2009
Page 8 of 11 pages
Question 9
The diagram below shows the charge in a thundercloud and the ground beneath it. The path of
a charged rain drop blown through the air is shown.
(a)
Copy the diagram and draw the electric field lines between the cloud and the ground.
(b)
Is the rain drop positively or negatively charged? Why?
(c)
Assume the cloud and the ground can be thought of as a parallel plate capacitor. The
base of the cloud is at an altitude of 1.0 km and has an area of 0.75 km 2 . Calculate the
charge stored in the cloud if the potential difference between the cloud and the ground
is 1.0  109 V
(d)
Calculate the electrostatic energy stored between the cloud base and the ground in this
situation.
Now the cloud moves without changing its height over a large plateau, 500 m higher than the
original ground surface, with the positive charges remaining directly beneath the cloud.
(e)
What is the effect on:
(i)
the potential difference between the cloud and the ground?
(ii)
the electrostatic energy stored between the cloud and the ground?
(10 marks)
93/08(a)
Semester 2, 2009
Page 9 of 11 pages
Question 10
The electric eel has cells, called electroplaques, each of which generate an emf of 0.15V
with an internal resistance of 0.25Ω .
(a)
If each row of electroplaques contains 5000 cells connected together in series, what is
the total emf and resistance of each row?
(b)
If there are 140 such rows connected in parallel, what is the net emf and resistance of
all the electroplaques?
(c)
A circuit is created by the water presenting a resistance to the net emf source created
by the eel’s electroplaques. If the current through the water is 1A , calculate the
resistance of the water.
(d)
How much power is dissipated in each individual electroplaque?
(10 marks)
93/08(a)
Semester 2, 2009
Page 10 of 11 pages
Question 11
(a)
Light of a single wavelength λ is shone onto a metal surface and, as a result, electrons
are ejected from the surface. Does the maximum kinetic energy of the ejected
electrons increase, stay the same or decrease if:
(i)
(ii)
(iii)
λ is increased?
the intensity of the incident light is increased?
the exposure time is increased?
Briefly explain each answer.
(b)
A proton, a deuterium nucleus, and an electron have the same (non-relativistic) kinetic
energy. List the particles in order of increasing de Broglie wavelength (starting with
shortest wavelength). Briefly explain your reasoning.
(c)
Draw a carefully labelled energy level diagram for a hydrogen atom, showing the first
4 energy levels. Also, show the upper limit of the energy levels. Hint: Use an equation
for the Equation Sheet to give relative energy levels.
(d)
Consider a hydrogen atom in its first excited state (n = 2). A photon from an external
source is absorbed by the atom and causes the atom to ionise. Suppose the photon had
exactly the right amount of energy to ionise the excited atom and no extra.
(i)
(ii)
(iii)
Draw an arrow on your diagram to represent this transition.
What was the energy of the photon in eV? Hint: Use your results from part (c).
In which part of the electromagnetic spectrum (radio wave, microwave, infrared, ultra-violet, X-ray or gamma ray) was this photon?
(10 marks)
93/08(a)
Semester 2, 2009
Question 12
The polonium isotope,
210
84
Page 11 of 11 pages
Po , undergoes alpha decay to a daughter lead ( Pb ) nucleus with a
half life of 138.4 days. This means that the process has a decay constant of 5.8 108 s 1 . The
kinetic energy of the alpha particle produced is 5.30MeV which is equal to 8.50 1013 J .
The atomic mass of 210
84 Po is 209.982848u . The atomic mass of the daughter lead nuclide of
this decay is 205.974440u . The mass of the helium nucleus is 4.002605u .
(a)
Write down the nuclear equation for this decay of polonium to lead.
(b)
How many neutrons and protons are there in the daughter nuclide?
(c)
3
A pellet containing 210
84 Po is placed near a radiation counter which registers 2.0 10
counts in 20.0s . The counter picks up 20% of all the emitted alpha particles.
Calculate the activity of the pellet expressed in becquerel.
(d)
The pellet is put inside some biological tissue of mass 0.50 g. Assuming that all the
emitted alpha particles are stopped within the tissue, calculate the absorbed dose,
expressed in gray (Gy), after 50 minutes of exposure. This is a very short time
compared to the half life of 210
84 Po and you can ignore changes in the activity of the
pellet over the exposure.
(e)
Briefly explain what effect this radiation has on the tissue.
(10 marks)
This is the end of your questions