Combined Science (Physics)

2015 HKDSE Physics &

Combined Science (Physics)

Report on Assessment

Y.T.SZETO

Manager (Physics), HKEAA

8 & 15 Oct 2015

1

Overview

Paper Physics CS(Phy)

1A (MC)

Mean: 17 out of 33 (i.e.52%)

(2014: 18 out of 33)

Mean: 8.5 out of 22 (i.e.39%)

(2014: 9 out of 22)

1B

(2014: ~>50%)

^~<50%

~30%

(2014: ~40%)

2

(2014: ~>50%)

^~<50% N.A.

SBA ^~>70%

^(~2014)

^~<70%

^(~2014)

Candidature ALL: 13 184 SCH: 12 106

ALL: 1 063 SCH: 946

2

Marking & Grading

On-Screen Marking (OSM) panels

Physics CS(Phy) 1B-1: Q.1, 4, 5, 6

1B-2: Q.7, 8, 9 1B-3: Q.2, 3, 10

1B-1: Q.1, 2, 3, 4 1B-2: Q.5, 6, 7

--- 2A: Astronomy (21%)

2B: Atomic World (66%) 2C: Energy (87%)

2D: Medical Physics (26%)

---

SBA marks stat. moderated with both Mean and SD

adjusted (outlining cases reviewed by Supervisors) 3

Marking & Grading

Expert Panel (Chief Examiners etc.) determine level boundaries/cut scores based on

Level descriptors /

Group Ability Indicator (GAI) / Viewing student samples.

CS(Phy) graded by

Common items / Viewing student samples.

Endorsement by Senior Management/Exam Board Note: GAI is calculated from Physics candidates’ actual

awards obtained in 4 core subjects CEML.

4

Results

Level 5** 5+ 4+ 3+ 2+ 1+

Percentage 2.7% 27.1% 49.4% 72.7% 90.0% 97.9%

Level 5** 5+ 4+ 3+ 2+ 1+

Percentage 0.8% 7.1% 19.2% 45.2% 73.5% 92.0%

5

Physics

CS(Phy)

Cut score difference = 43 marks

Cut score difference = 38 marks

No. of MC 28 22 18/17 13 10 8/7

No. of MC 17/16 14 12/11 9 7 5

Paper 1A

Physics (33 MC)

>70% 50%-70% <50%

2 17 14

E a s y D i f f i c u l t

CS (Phy) (22 MC)

>70% 50%-70% <50%

0 4 18

E a s y D i f f i c u l t

6

PHYSICS MC

Topic (No. of Qu.) Average

% correct

No. of Qu.

< 50% correct

Heat & Gases (3) 66% 0

Force & Motion (8) 56% 3

Wave Motion (9) 53% 3

Electricity &

Magnetism (10) 48% 6

Radioactivity (3) 44% 2

7

CS(PHY) MC

Topic (No. of Qu.) Average

% correct

No. of Qu.

< 50% correct

Heat & Gases (3) 52% 2

Force & Motion (6) 28% 6

Wave Motion (8) 34% 7

Electricity &

Magnetism (5) 37% 3

8

9

lower 27%

top 27%

(1 0.497) (0.771 0.285)

10

11 12

13 14

15 16

Observations

Most candidates were competent in handling calculations (e.g. Paper 1B Q.2 & 10).

However, it seems that their grasp of

fundamental physics concepts was not strong.

Quite weak or careless in handling/converting units or scientific notations.

Omitting subtle precautions / procedures of experiments even though they are simple but unfamiliar.

Weaker candidates (Levels 1 & 2) tend to give up answering some of the questions. They also performed poorly in Paper 2.

17

Points to note

~70% of Paper 1 (Physics) with questions from core part.

Accept answers using g = 9.81 or 10 m s ^-2 .

Method marks ‘M’ awarded to correct formula / substitution

In general, numerical ans. with 3 sig. fig.

Answer marks ‘A’ awarded to correct numerical answer in correct unit within tolerance range.

18

Points to note

Equating Electives using Paper 1

(Total = 40 (MC) + 40 (Qu.) = 80 each elective) Before equating: Mean 29 to 37 / SD 15 to 19 After equating: Mean 37 to 41 / SD 15 to 16

2A Astronomy: n

2B Atomic World: n

2C Energy: n

2D Medical Physics: ~ unchanged

19

Points to note

From 2014 Exam onwards:

PHY no. of MC = 33 CS(PHY) no. of MC = 22

* Syllabuses trimmed with effect in 2016 Exam Student samples of performance (Levels 1 to 5) available in October (HKEAA website).

SBA Conference on 21 Nov 2015

SBA Online Submission in Jan/Feb 2016 All SBA tasks adopt 0 – 20 mark range.

20

Mr. WONG KW Mr. TAM YW

Q.1

• (a) (i) Well performed.

(ii) unsatisfactory.

(b) unsatisfactory.

- Reasoning is not clearly shown.

- confusion between experimental

value and actual value.

Q.4

• (a) Satisfactory.

Description is not precise enough.

• moving forwards / moving forwards with acceleration / moving forwards with uniform acceleration( —).

• Moving with – 0.5 m s^-2 ( u)

Q.4

(b) (i) magnitude only.

(ii) Well performed!

(c) Common mistakes:

direction of friction!

excess forces shown(inertia, upward force)

Q.4

• (d) Difficult question. Unsatisfactory.

Concepts involved:

F = ma

resolution of mg

direction of friction changes

Q.5

• Performance is satisfactory .

• Refer to the question: diagram/ equation / description are required, so marks are given to diagram / equation / description.

• Common mistakes:

incorrect setup /

using the diagram properly

Q.6

• (a) (b) (i) Satisfactory!

• Part (c) Poor performance.

Common mistakes:

• (b) (ii) confusion between D ^and T

• (c) explanation is not clear! Cannot relate the

reasoning behind (b) (ii) and (c).

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2015 DSE PHYSICS/

COMBINED SCIENCE (PHYSICS) 1B-3: Q.2, 3 & 10

Mr. Y.H. MUI

Mr. W.C. NG

QUESTION 2

Marking Scheme Performance/Common

Errors

(a)

210 atm u (1.0 u 10⁴ cm³) = 2.0 atm u V V = 1.05 u 10⁶ cm³

[1M]

Volume available = 1.05 u 10⁶ 1.0 u 10⁴ = 1.04 u 10⁶ (cm³)

[1A]

(b)(i) V₀ = 1.04 u 10⁶ cm³Ǹ 60

= 17333 | 17300 (cm³) (per minute) [1A/1M]

- Accept ans. without considering residual volume, i.e. 1.05 u 10⁶ (cm³)

- If using pV = nRT , SI units must be used.

- Applied the equation pV = nRT without first converting the data into S.I. units,

Accept ans. from (a) Ǹ 60 for 1M V₀ = 17500 (cm³) if residual volume

not considered

.

QUESTION 2

Marking Scheme Performance/Common

Errors (ii) V' : total volume of air at this depth/in this

situation

V' = 4.60 u 10⁵ cm3 [1M]

Volume available = 4.60 u 10⁵ - 1.0 u 10⁴ = 4.50 u 10⁵ (cm³)

Length of time : =

[1M]

= 26.0 (min.) [1A]

-

Accept method/ans. without considering residual volume, i.e. V'

= 4.60 u 10⁵ cm³

Length of time = 26.3 min

- Did not change the temperature from Celsius scale to Kelvin scale.

17333 10 50 .

4 u ⁵

QUESTION 2 (SAMPLE 1)

1M 1A

9 9

QUESTION 2 (SAMPLE 2)

9 ^1M

9

9

1M

1M

1A

QUESTION 2 (SAMPLE 3)

0M 1A

8

9

QUESTION 2 (SAMPLE 4)

9 ^1A

8 8

8

0 M

1 M

9 0 A

QUESTION 3

Marking Scheme Performance/Common

Errors

(a)(i)

^{- v}B correctly drawn with label

(roughly A OB).

[1A]

v_B² = 2gh = 2 u 9.81 u (8 cos30q) [1M]

v_B = 11.7 m s^1 (11.65) (or 11.77 for g = 10 m s^-2) [1A]

- Accept equation with incorrect h such as 8 m / 8 sin30 q/

8 Ǹ cos30q

- Solve the problem by using an equation of motion instead of conservation of energy to find

v_B

.

QUESTION 3

Marking Scheme Performance/Common Errors (b)(i)

= 11.7 cos30q ǘ 1.25 [vx = v_B cos30q ][1M]

= 12.6 m (12.74283)

(or 12.7 to 12.8 m for g = 10 m s^-2) [1A]

- 1M for using v_x = v_B cos30q

(ii)

u = v_y = v_B sin30q = 5.83 m s1 [1M]

y = 0.38 m (0.414 to 0.352 m) [1M]

(or 0.455 to 0.4375 m for g = 10 m s^-2) Platform C is 0.38 m below B.

[1A]

(c) Total mechanical energy is the same / unchanged.

[1A]

- 1M for able to resolve v_B into v_B sin30q

- Accept ut r

- Most candidates were able to apply correct equations to find the answers to (b)(i)(ii) although a few got vx, vy

and v_B mixed up.

- Accept “No”, “No change”, “No energy lost”, “PE → KE”.

y B

QUESTION 3 (SAMPLE 1)

9

1M 1A 1A

9 9

QUESTION 3 (SAMPLE 2)

1M 1A

9 9

QUESTION 3 (SAMPLE 3)

9 9 9

1M 1M

1A

9 ^1A

QUESTION 3 (SAMPLE 4)

9 1 A

8 8

0M

0A

QUESTION 3 (SAMPLE 5)

8

1M 0A

QUESTION 3 (SAMPLE 6)

8 ^1M _1M

0A

9 ^1A

QUESTION 10

Marking Scheme Performance/Common

Errors (a)

Mass deficit

= (2.014102 + 3.016049) u – (4.002602 + 1.008665) u [1M]

= 0.018884 u

Energy released = 0.018884 u 931 MeV

= 17.58 (MeV) [1A]

[Or Energy released = 0.018884 u 1.661 u 10^{–27 u} c²

= 2.823 u 10^–12 J or 17.64 MeV]

- Candidates only did well in (a) - A few candidates had difficulty

in dealing with the units eV and J

(b)(i)

To overcome the (electrostatic) repulsion [1A]

between the two (positive) nuclei and becomes electrical potential energy (of the two nuclei). [1A]

-Did not know that the energy used for overcoming the repulsion between the two positive nuclei became electrical potential energy.

-Some wrongly thought that it turned into kinetic energy, heat or nuclear energy.

QUESTION 10

Marking Scheme Performance/Common

Errors (b)(ii)

High temperature enables them to have sufficient K.E.

(to overcome electrical repulsion between their nuclei) [1A]

(iii)

Kinetic energy becomes electrical P.E.

E_p = 2 ǘ ½ m (c_rms)² = 2 ǘ

0.4 MeV =

[1M]

T = 1.545 u 10⁹ K i.e. order of magnitude 10⁹ (K) [1A]

-

Accept “high” KE/speed.

Only “KE” NOT accepted.

-Accept without “u2”

0.4 MeV = 6.4 ǘ 10^14 J - Most candidates failed to relate the average kinetic energy of the nuclei to the large amount of work done needed.

A few even wrongly employed E = 17.58 MeV in (a) in their calculations.

QUESTION 10

Marking Scheme Performance/Common

Errors (b)(iii)

Alternative method:

[1M]

T = 5.56 u 10⁹ K

i.e. order of magnitude 10⁹ (K) [1A]

- correct equation with 10^15 - accept without “u2”

QUESTION 10 (SAMPLE 1)

1M 1A

9 9

QUESTION 10 (SAMPLE 2)

0 A 1 A

9

8

QUESTION 10 (SAMPLE 3)

9 ^{1 A}

8 8

0 M

0 A

QUESTION 10 (SAMPLE 4)

8 8

0 A 0 A

QUESTION 10 (SAMPLE 5)

8

0 A

THANK YOU!

Paper 2

Section A :Astronomy and Space Science

Mr. LEE WK & Mr. YING PC

Q.1 Multiple-choice questions

A B C D 1.1

^{33.29%* 29.21%}

22.73% 12.89%

1.2 14.73% 5.18%

52.23%* 26.10%

1.3 ^6.51%

^19.66%

^15.13%

^56.66%*

1.4 ^12.37%

^{46.18%* 26.20%}

^13.50%

1.5

^{33.29% 40.19%*}

15.25% 9.49%

1.6 8.32% 14.60%

59.46%* 15.74%

1.7

^{45.85%* 21.24%}

15.48% 15.22%

1.8 ^10.88%

^19.53%

^16.22%

^51.36%*

* : key ; Red colour : most favourable distractor

A*

B

C D

MCQ 1.1

A satellite is orbiting the Earth at a distance h from the Earth’s surface. What is the gain in gravitational potential energy of the satellite in the orbit with respect to the Earth’s surface ?

m = mass of the satellite R = radius of the Earth

g = acceleration due to gravity on the Earth’s surface

¸¹

¨ ·

©

§

 h R mgh R

2

¸¹

¨ ·

©

§

 h R mgh R

¸¹

¨ ·

©

§  R h mgh R

2

¸¹

¨ ·

©

§  R h mgh R

P.E =- ^ீெ௠

ோ ܽ݊݀݃ ൌ^ீெ

ோ^మ

MCQ 1.4

A (1) and (2) only B* (1) and (3) only C (2) And (3) only D (1), (2) and (3)

Which of the following observations by Galileo contradict with the geocentric model of the universe ?

(1) the discovery of satellites of Jupiter (2) the retrograde motion of Mars

(3) the changing phase of Venus

(2) was not a discovery by Galileo.

MCQ 1.5

Sun

Earth Mars

When the Earth lines up with the Sun and Mars as shown, how does Mars appear to move across the night sky as viewed from Earth ?

A Mars moves from west to east against the background stars.

B* Mars moves from east to west against the background stars.

C Mars does not move against the background stars.

D The movement of Mars cannot be determined because the east and west directions are not known.

MCQ 1.7

absolute magnitude

apparent magnitude

star X 2.8 4.7

star Y 5.4 3.2

According to the information given above, which of the following about stars X and Y is/are correct ?

(1) Luminosity of star X is greater than that of star Y.

(2) A telescope collects more energy per unit area per unit time from star X than from star Y.

A* Only (1) is correct B Only (2) is correct

C Both (1) and (2) are correct D Both (1) and (2) are incorrect

The larger the value of magnitude, the fainter the star appears.

Q.1 Structured question

Figure 1.1 shows a distant binary star system viewed by an observer on Earth who is also on the orbital plane of the two stars. The system consists of stars 1 and 2 with masses m₁ and m₂ respectively orbiting in uniform circular motion about their centre of mass O under their mutual gravitational force. They move with the same period in two orbits of radii r₁ and r₂ with orbital speeds v₁ and v₂ respectively.

observing along the line of sight!

observer on Earth

positive direction

O

m₁ r₁ r2 m2

v₁

v2 ǘ

Q.1 Structured question

By finding the radial velocity v_r of the two stars inferred from the Doppler shift (Δλ) of the hydrogen-alpha line (H_D) observed on Earth, astronomers are able to deduce the masses of the stars. Assume that the centre of mass O of the binary system is stationary with respect to the observer. Figure 1.2 shows the radial velocity curves for the two stars. The direction moving away from the observer is taken to be positive velocity.

Q.1 Structured question

(a) (i) What does it mean by radial velocity ݒ_௥of a star observed on Earth ? (1 mark)

(ii) Identify which point, A, B, C or D, marked on the radial velocity curve corresponds to the orbital position of star 1 (in solid line) at the instant shown in Figure 1.1. (1 mark)

a(i) radial velocity is the component of the star’s velocity along the observer’s line of sight / velocity along the observer direction / the line joining the star and the observer.

1A

“Move away or toward” NOT accepted

(ii) Point D

1A

Candidates’ performance in (a) was fair. Most of them failed to state the concise meaning of radial velocity of a star observed on Earth.

Q.1 Structured question

(b) Find, from Figure 1.2, the orbital speed ݒ_ଵ of star 1 and calculate its orbital radius ݎ_ଵ. Using a similar method, or otherwise, find the orbital radius ݎଶ of star 2. (4 marks)

For the calculation of orbital radii in (b), candidates were able to use the information given in the graph.

v1 = 180 km s^1

60 60 40

π π 2

2 _{1 1}

1 u u

r T

v r (Period T = 40 hr)

r1 = 4.125 × 10⁶ km or 4.125 × 10⁹ m

From figure, v2 = 120 km s^1; and by ratio or similar calculation gives r2 = 2.75 × 10⁶ km or 2.75 × 10⁹ m

1A 1M 1A 1A

1M for the correct expression T v₁ 2πr¹

Q.1 Structured question

(c) By considering the circular motion of star 1, calculate the mass

m2of star 2. (2 marks)

 ] [ 2

 ) ( 2 )

(

₁

2 1 1 1 2 2 1

2 1

2 1

T r

v r m m T r r

m Gm

 Z

Therefore, m2 = 5.57 ǘġ 10³⁰ kg

1M

1A

Many candidates had difficulties in understanding the binary stars system and therefore failed to set up a correct equation of motion in (c).

Some candidates even treated the circular motion of star 1 as if it was a single star.

Q.1 Structured question

(d) A spectrometer can only measure change of wavelength larger than 0.5 nm. Explain whether this instrument is suitable to measure the Doppler shift Δλ of the hydrogen-alpha line (λ₀ = 656.28 nm) of the two stars. (2 marks)

nm 656.28

nm 5 . ' 0

O O c v_r

Ÿvr = 228.3 km s^1 > 180 km s^1 ;

Or ₈

3

10 3

10 180

u u '

c v_r O

O Ÿ'O = 0.394 nm < 0.5 nm ; therefore not suitable.

Accept using 120 km s^1 , 'O = 0.263 nm < 0.5 nm

1M

1A

Most candidates managed to give correct explanations concerning the suitability of using the spectrometer in (d).

Paper 2

Section B : Atomic World

Mr. LEUNG NC & Mr. LAW MW

Q.2 Multiple-choice questions

A B C D 2.1 21.23 6.05 32.13 40.05*

2.2 41.69* 12.32 35.16 10.35 2.3 14.17 7.92 15.70 61.73*

2.4 13.24 66.01* 10.74 9.13 2.5 27.64 8.80 47.39* 15.86 2.6 10.09 27.79 43.30* 17.72 2.7 42.10* 18.68 23.67 15.17 2.8 16.29 35.29* 14.37 33.65

* : key ; Red colour : most favourable distractor

Q.2 Multiple-choice questions

2.1 A beam of α-particles with the same initial kinetic energy are scattered by a heavy nucleus N. In the figure, if P is a possible path for one of the α-particles, which of the paths, Q, R and S, is/are possible for these α-particles ?

A. Q and R only (21.23%) B. R and S only ( 6.05%)

C. Q only (32.13%)

*D. S only (40.05%)*

* : key ; Red colour : most favourable distractor

Q.2 Multiple-choice questions

2.2 Which of the following provides experimental evidence for discrete energy levels in atoms ?

(1) the spectrum of a sodium discharge tube (2) the spectrum of a tungsten filament lamp

(3) the diffraction of electrons by atomic spacing in crystals

*A. (1) only (41.69%)*

B. (3) only (12.32%)

C. (1) and (2) only (35.16%) D. (2) and (3) only (10.35%)

Q.2 Multiple-choice questions

2.5 A photocell is connected to a d.c. source as shown.

Monochromatic light is incident on cathode C of the photocell so that photoelectrons are emitted. The maximum kinetic energy of photoelectrons reaching anode A depends on

(1) the kind of metal making the cathode surface.

(2) the voltage of the d.c. source.

(3) the intensity of the monochromatic light used.

A. (1) only (27.64%) B. (3) only ( 8.80%)

*C. (1) and (2) only (47.39%)*

D. (2) and (3) only (15.86%)

Q.2 Multiple-choice questions

2.6 Aurora Borealis (Northern lights) are often observed in skies at high latitudes. When energetic electrons from outer space collide with the oxygen atoms in the upper atmosphere, the oxygen atoms are excited. The

subsequent emission of light is usually green light of wavelength 558 nm. The minimum speed of these energetic electrons is of order of magnitude

A. 10

²

m s

^-1

(10.09%) B. 10

⁴

m s

^-1

(27.79%)

*C. 10

⁶

m s

^-1

(43.30%)*

D. 10

⁸

m s

^-1

(17.72%)

ଵ

ଶ

݉ݒ

²

ൌ

^௛௖

O v | 8.8u10

⁵

Q.2 Multiple-choice questions

2.7 Which of the following can increase the resolving power of a transmission electron microscope (TEM) ?

(1) increasing the anode voltage in the electron gun (2) decreasing the aperture of the magnetic objective

lens

(3) increasing the separation between the magnetic projection lens and the fluorescent screen

*A. (1) only (42.10%)*

B. (2) only (18.68%)

C. (1) and (3) only (23.67%)

D. (2) and (3) only (15.17%)

T ൌ ͳǤʹʹO V increases o ܦ O decreases o T decreases.

The energy levels E

_n

of an electron in a hydrogen atom from the Bohr model can take the form below:

eV where n = 1, 2, 3, …

(a) B ohr’s idea was sometimes criticized by some physicists as semi- classical and semi-quantum. Point out ONE classical aspect in the

Bohr model. (1 mark)

Q.2 Structured question

2

6 . 13 E

_n

 n

- the electron is considered as a particle revolving around the nucleus in definite orbits/circular motion; or

- the centripetal force is provided by the Coulomb force - the electron’s motion obeys Newton’s laws of motion

(b) From the energy point of view, state the physical meaning of a

hydrogen atom being in its ground state. (1 mark)

(c) If the minimum energy required to ionize a hydrogen atom in the ground state is E, express the minimum momentum p of a photon for ionizing such a hydrogen atom in terms of E and another physical

constant. (2 marks)

Q.2 Structured question

Well answered!

- lowest energy level or most stable state State 13.6 eV only

NOT accepted.

Accept 13.6 eV is required to ionize hydrogen atom.

c p E

c h hc

p

˜ 1

O O

^1M

1A

(d) Hydrogen atoms in the ground state are bombarded by electrons each with kinetic energy 12.9 eV.

(i) Show that these hydrogen atoms can be excited at most to the third excited state (i.e. n = 4). (2 marks)

Q.2 Structured question

eV 2 0.85

4 3.6 1

4  

E

eV 0.544 52

3.6 1

5  

E

'E = E4  E1 = 0.85 – (13.6) = 12.75 eV

'E = E

5

 E

1

= 0.544 – (13.6) = 13.056 eV 12.75 eV < 12.9 eV < 13.06 eV,

therefore at most to the 3

^rd

excited state (n = 4).

1M : 'E

1A : 'E

Q.2 Structured question

∆E E

₁

 E

4

.

(ii) For a hydrogen atom excited to the third excited state (n = 4), what is the de Broglie wavelength of the orbiting electron in the atom ? Given: the orbital radius r

_n

of the electron in a hydrogen atom from the Bohr model is equal to 0.053 n

²

(unit: nm), where n = 1, 2, 3, ...

(2 marks)

Q.2 Structured question

O mv n r nh mvr

_n

nh Ÿ 2 π

_n

π 2

When n = 4, 2 π(0.053)(4

²

) = 4 O Therefore, O = 1.33 nm

1M : deduce/apply n O = 2Sr

n

Alternative method

1M : Using centripetal force to find v or mv.

1 5 2

2 2

0

s m 10 49 . π 5

4

1 _

u !

˜ v

r mv r e

H

n

O ^{= 2} Sr

E4



(iii)

Copy the energy level diagram below to your answer book and

draw arrow(s) to illustrate all possible transitions leading to emission of photons by these excited hydrogen atoms.

(2 marks)

Q.2 Structured question

Deduct 1A each for

- wrong or incomplete arrows - omitting any one transition

Accept more than 6 correct transitions

Paper 2

Section C : Energy and Use of Energy Mr. LEE WK & Mr. YING PC

Q.3 Multiple-choice questions

A B C D 3.1 32.99* 14.56 25.30 26.87

3.2 7.67 22.04 19.53 50.29*

3.3 16.32 67.00* 12.05 4.09 3.4 58.51 4.99 20.61* 15.60 3.5 19.61 28.08* 26.21 25.75 3.6 39.55 20.66 28.42* 10.98 3.7 15.53 18.29 46.62* 18.75 3.8 31.90* 39.98 23.80 4.05

* : key ; Red colour : most favourable distractor

MCQ 3.1

) 600 5 . 2 ( 4 S

²

P

lux 75

60 cos 600

60 ) cos 5 . 2 ( 60 4

) cos 0 . 5 ( 4

3 3 2 2

q q

q S

S

P P

*

favourable distractor

MCQ 3.4

% 7 . 50 16 250

50



favourable distractor

*

MCQ 3.5

Y will absorb less visible light

Y will absorb more IR

favourable distractor

*

MCQ 3.6

(2)

U

X

A T

X

U

Y

A T

Y

t

Q ' '

(1)

U N d

Y Y

X

! N Ÿ U

_X

! U

N

(3)

U

_X

! U

_Y

Ÿ Δ T

_X

 Δ T

_Y

favourable distractor

*

MCQ 3.8

favourable distractor

*

Q.3 Structured question

Q.3 Structured question

Part (a)(i) was well answered.

Accept in text form / sub. with

incorrect order of magnitude Accept 84 to 85 s

Q.3 Structured question

NOT accept:

• Heat generated from the air- conditioner

• There are people / electrical appliances / furniture without mentioning heat / heat capacities

Quite a number of them stated that longer cooling time was due to ‘heat lost to surroundings’ but not the fact that thermal energy is being removed.

(a)(ii) revealed that many candidates were weak in explaining principles of physics.

Q.3 Structured question

Most were able to work out the average electrical power input.

Q.3 Structured question

Most were able to work out the COP.

1M for the ratio (b)(i) from Ans

80 . 6

1A for the working principle of heat pump / the meaning of Q_C AND W / heat flow vs work done by the air-conditioner 1A for Q_C not

necessarily equal to W / small W can lead to large Q_C

However, many did not fully understand the concept that 1 J of electrical energy consumed by the air-

conditioner/compressor can remove 3.24 J of thermal

energy from the room.

Q.3 Structured question Q.3 Structured question

In (c)(i), quite a number of the candidates made mistakes in the direction of the flow of refrigerant. They were not familiar with the working principle of a RCAC and the functions of different components in it.

As a result, many failed to state in which component the refrigerant had the highest temperature.

condenser evaporator

Q.3 Structured question

Part (c)(ii) was generally well answered despite some wrong statements such as reversing the direction of electric

current could convert a cool-only air-conditioner into an RCAC.

condenser evaporator

Paper 2

Section D: Medical Physics Mr. LEUNG NC & Mr. LAW MW

HKDSE 2015

Multiple Choice

Qn. 1 2 3 4 5 6 7 8

A 18.2% 13.1% 21.5% 23.9% 17.9% 8.2% 50.1% 60.2%

B 47.0% 14.5% 21.5% 13.4% 63.8% 16.0% 15.7% 7.4%

C 11.2% 27.5% 30.6% 33.8% 12.1% 57.6% 15.1% 8.0%

D 23.3% 44.6% 26.2% 28.7% 6.1% 18.0% 18.9% 24.4%

Qn. 4.1

Answer : B (47.0%) Best distractor: D (23.3%) Mix upfar point and near point.

X

Y

W Z

best distractor

Qn. 4.2

X

Y

W Z

Answer : D (44.6%) Best distractor: C (27.5%)

May have problem in the word anesthesia.

best distractor

Qn. 4.3

Answer : D (26.2%)

Best distractors: C (30.6%)

Tricky option (1). Poor understanding in (2) & (3) on theory of piezoelectric effect.

best distractor

Qn. 4.4

Answer : D (28.7%) Best distractor: C (33.8%)

Mix up acoustic impedance and reflection coefficient.

best distractor

Q.4 Structural question

Part (a)(i) was well answered. Many failed to deduce the gain of pressure in (a)(ii). May not understand the meaning of the ratio of area.

Q.4 Structural question Q.4 Structural question

In (i), a significant number of candidates do not know how to read 60 phons from the graph. Some candidates were unable to state concisely the physical significance of the loudness curve being higher at both low and high frequencies. Some simply describe the graph (e.g. higher dB in higher and lower frequency ends).

Q.4 Structural Question

Q.4 Structural question

Not many candidates managed to identify the correct loudness curve in (b)(ii) with appropriate explanations. Many candidates know that it is curve C, but the explanation is not appropriate. Some simply say that the curve C is higher than the normal ear, which is also correct for the curve B.

They do not use the information especially significant in kHz range.

Q.4 Structural Question

Part (c) was in general well answered although a few of them were not familiar with logarithmic operations.