# Physics 9702 Doubts | Help Page 243

__Question 1124: [Electric field]__
Two conducting layers of a liquid
crystal display of a calculator are 8 μm apart. A 1.5 V cell is connected
across the conducting layers when the calculator is switched on.

What is the electric field strength
between the layers?

A 1.2 × 10

^{–5}V m^{–1}
B 0.19 V m

^{–1}
C 12 V m

^{–1}
D 1.9 × 10

^{5}V m^{–1}**Reference:**

*Past Exam Paper – June 2013 Paper 12 Q30*

__Solution 1124:__**Answer: D.**

Electric field strength, E = V / d =
1.5 / (8×10

^{-6}) = 1.9×10^{5 }V m^{-1}

__Question 1125:__

__[Simple harmonic motion]__

A spring is hung from a fixed point.
A mass of 130 g is hung from the free end of the spring, as shown in Fig. 3.1.

Fig. 3.1

The mass is pulled downwards from
its equilibrium position through a small distance d and is released. The mass
undergoes simple harmonic motion.

Fig. 3.2 shows the variation with
displacement x from the equilibrium position of the kinetic energy of the mass.

Fig. 3.2

**(a)**Use Fig. 3.2 to

(i) determine the distance d through
which the mass was displaced initially, [1]

(ii) show that the frequency of
oscillation of the mass is approximately 4.0 Hz. [6]

**(b)**(i) On Fig. 3.2, draw a line to represent the total energy of the oscillating mass. [1]

(ii) After many oscillations,
damping reduces the total energy of the mass to 1.0 mJ.

For the oscillations with reduced
energy,

1. state the frequency,

2. using the graph, or otherwise,
state the amplitude. [2]

**Reference:**

*Past Exam Paper – November 2007 Paper 4 Q3*

__Solution 1125:__**(a)**

(i) d = 0.8cm

{12 small squares after
0.5cm. 20 small squares à 0.5. So, 12 small squares à (12/20) x 0.5 = 0.3cm. So, d =
0.5+0.3 = 0.8cm}

(ii)

{Frequency can be found
from angular frequency, which can be calculated from the kinetic energy}

Maximum kinetic energy = 2.56mJ

v

_{max}= ωa
Maximum kinetic energy = ½ mω

^{2}a^{2}or ½ mω^{2}(a^{2}– x^{2})
2.56×10

^{-3}= ½ (0.130) ω^{2}(0.8×10^{-2})^{2}
ω = 24.8 rad s

^{-1}
f = ω / 2π = 4.0Hz (3.95Hz)

**(b)**(i) A line parallel to the x-axis at 2.56mJ is drawn

(ii)

1. Frequency: 4.0Hz

{Detailed explanations for this part of the question is available as Solution 261 at Physics 9702 Doubts | Help Page 42 -

*http://physics-ref.blogspot.com/2014/12/physics-9702-doubts-help-page-42.html*}

__Question 1126: [Diffraction grating]__
Which electromagnetic wave
phenomenon is needed to explain the spectrum produced when white light falls on
a diffraction grating?

A coherence

B interference

C polarisation

D refraction

**Reference:**

*Past Exam Paper – June 2011 Paper 11 Q26 & Paper 13 Q25*

__Solution 1126:__**Answer: B.**

When light falls on a diffraction
grating, it is diffracted at each slit. The diffracted waves from the slit
interference both constructively and destructively to produce a pattern of
bright and dark fringes.