# Physics 9702 Doubts | Help Page 104

__Question 532: [Matter > Solids, Liquids and Gases]__There is one temperature, about 0.01 °C, at which water, water vapour and ice can co-exist in equilibrium.

Which statement about properties of the molecules at this temperature is correct?

A Ice molecules are closer to one another than water molecules.

B The mean kinetic energy of water molecules is greater than the mean kinetic energy of ice molecules.

C Water vapour molecules are less massive than water molecules.

D Water vapour molecules have the same mean speed as both ice and water molecules.

**Reference:**

*Past Exam Paper – June 2014 Paper 11 Q18*

__Solution 532:__**Answer: D.**

Since the kinetic energy (speed) of
the molecules depends on the temperature, all molecules will have the same mean
kinetic energy as the temperature is constant at 0.01 °C. [B is incorrect]

It is worth emphasising that molecules in a
solid and those in a liquid have similar spacing. The case of ice and water is
a special case. Ice is slightly less dense than water at 0 °C (it floats on
water), so its molecules are slightly further apart (not closer) than molecules
in water. [A is incorrect]

The mass of water molecules is the
same in all phase, only the density and volume change. [C
is incorrect]

__Question 533: [Waves > Stationary waves]__
Hollow tube is used to investigate
stationary waves. Tube is closed at one end and open at the other end.
Loudspeaker connected to a signal generator is placed near open end of tube, as
shown.

Tube has length L. Frequency of
signal generator is adjusted so that loudspeaker produces a progressive wave of
frequency 440 Hz. Stationary wave is formed in the tube. Representation of
stationary wave is shown.

Two points P and Q on stationary
wave are labelled.

**(a)**

(i) Describe, in terms of energy
transfer, difference between progressive wave and stationary wave

(ii) Explain how stationary wave is
formed in the tube

(iii) State direction of
oscillations of an air particle at point P

**(b)**On Fig label, with letter N, nodes of the stationary wave

**(c)**State phase difference between points P and Q on the stationary wave

**(d)**Speed of sound in tube is 330 m s

^{–1}.

Calculate

(i) wavelength of the sound wave

(ii) length L of tube

**Reference:**

*Past Exam Paper – June 2014 Paper 23 Q6*

__Solution 533:__**(a)**

(i) A progressive wave transfers
energy while in a stationary wave, there is no transfer of energy / keeps
energy with wave

(ii) A (progressive) wave / wave
from the loudspeaker reflects at the end of the tube. The reflected wave
overlaps with (another) progressive wave of the
same frequency and speed. Hence a stationary wave is formed

(iii) The direction of oscillations
of an air particle at point P is (side to side) along the length of the tube /
along axis of tube

**(b)**All 3 nodes must be clearly marked with N / clearly labelled the cross-over points

**(c)**Phase difference between P and Q = 0

**(d)**

(i)

Speed, v = fÎ»

Wavelength, Î» = 330 / 440 = 0.75m

(ii)

(Along the total length L
of the tube, there is 1Î» + ¼Î» = 5/4 Î» as
can be identified in the diagram)

Length of tube = (5/4) Î» = (5/4) x 0.75 = 0.94m

__Question 534: [Matter > Elastic and Plastic behaviour]__
Ductile material is stretched by a
tensile force to a point beyond its elastic limit. Tensile force is then
reduced to zero. Graph of force against extension is shown below.

Which area represents the net work
done on sample?

A X B
X + Y C Y + Z D Z

**Reference:**

*Past Exam Paper – November 2005 Paper 1 Q20*

__Solution 534:__**Answer: B.**

X, Y and Z represent the area under the graph in each of the sections
shown. The area under a force-extension graph of a material gives the work done
on the sample.

The behaviour of the material is represented by the curve with arrows.
So here, we need to find the area enclosed by the curve and the x-axis (extension
axis). This is X + Y.

__Question 535: [Current of Electricity]__In circuit shown, a light-dependent resistor (LDR) is connected to two resistors R

_{1}and R

_{2}. Potential difference (p.d.) across R

_{1}is V

_{1}and the p.d. across R

_{2}is V

_{2}. Current in the circuit is I.

Which statement about this circuit is correct?

A The current I increases when the light intensity decreases.

B The LDR is an ohmic conductor.

C The p.d. V2 increases when the light intensity decreases.

D The ratio V

_{1 }/ V

_{2}is independent of light intensity.

**Reference:**

*Past Exam Paper – June 2014 Paper 12 Q34*

__Solution 535:__**Answer: D.**

The resistance of an LDR decreases
as the light intensity increases.

The circuit is a series connection,
so the same current flows through all the components.

From Ohm’s law: Current I in circuit
= V / R

Thus, as the light intensity
decreases, the resistance of LDR increases and so current I decreases. [A is incorrect]

The LDR is not an ohmic conductor (it
does not follow Ohm’s law). [B is incorrect]

Since current I decreases as the light
intensity on the LDR decreases [which causes the resistance of LDR to
increase], V

_{2}(= IR_{2}) will also decrease. [C is incorrect]
The ration V

_{1}/ V_{2}= IR_{1}/ IR_{2}= R_{1 }/ R_{2}. So, the ratio is independent of the light intensity. The light intensity affects the current in the circuit, but the ratio does not depend on current.
In solution 533 how is the phase difference 0 ????????

ReplyDeleteIn solution 533 how is the phase difference 0 ???

ReplyDeletehow is [phase diff 0 in soultion 533 c

ReplyDeleteFor stationary waves, all points within any specific loop are in phase.

Deletefor solution 533 why is the direction of oscillation of air particle side to side and not up and down?

ReplyDeleteThe stationary wave formed is a sound wave (coming from the loudspeaker). Sound wave is longitudinal.

Deletehi I dont understand solution 534

ReplyDeletewhat exactly?

Delete