• 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 = fλ 

Wavelength, λ = 330 / 440 = 0.75m

(ii)

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

Length of tube = (5/4) λ = (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 & June 2016 Paper 11 Q21

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 R₁ and R₂. Potential difference (p.d.) across R₁ is V₁ and the p.d. across R₂ is V₂. 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 V₁ / V₂ 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], V₂ (= IR₂) will also decrease. [C is incorrect]

The ration V₁ / V₂ = IR₁ / IR₂ = R₁ / R₂. 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.