Physics 9702 Doubts | Help Page 158
Question 782: [Current
of Electricity]
In circuit shown, the resistance of
the thermistor decreases as temperature increases.
Which graph shows variation with
Celsius temperature θ of potential difference V between points P and Q?
Reference: Past Exam Paper – November 2013 Paper 13 Q37
Solution 782:
Answer: C.
V is the p.d.
across the fixed resistor.
From Kirchhoff’s
law, the sum of p.d. across the thermistor and the p.d. across the fixed
resistor is equal to the e.m.f. of the d.c. supply.
Ohm’s law: V =
IR
As the temperature
increases, the resistance of the thermistor decreases, and hence the p.d.
across it also decreases since V = IR. Thus, as temperature increases, the p.d.
V across the fixed resistor increases (so that the sum of p.d.’s is always
equal to the e.m.f.). [A and B are incorrect]
But the
resistance of a thermistor does not vary linearly with temperature. So, the
increase in the p.d. V with the temperature is not linear.
Question 783: [Measurement
> Uncertainty]
Diagram shows an experiment to
measure speed of a small ball falling at constant speed through a clear liquid
in a glass tube.
There are two marks on the tube. Top
mark is positioned at 115 ± 1 mm on the adjacent rule and the lower mark at 385
± 1 mm. The ball passes the top mark at 1.50 ± 0.02 s and passes the lower mark
at 3.50 ± 0.02 s.
Constant speed of the ball is
calculated by (385 – 115) / (3.50 – 1.50) = 270 / 2.00 = 135 mms-1.
Which expression calculates
fractional uncertainty in the value of this speed?
Reference: Past Exam Paper – June 2011 Paper 11 Q5 & Paper 13 Q4
Solution 783:
Answer: A.
Speed v = Distance s / Time t
Δv / v = (Δs / s) + (Δt / t)
Fractional uncertainty of speed = Δv / v
Distance s = 385 – 115 = 270 mm
Δs = ± (1 + 1) = ± 2 mm
Time t = 3.50 – 1.50 = 2.00s
Δt = ± (0.02 + 0.02) = ± 0.04 s
Question 784: [Kinematics]
Diagram shows a laboratory
experiment in which a feather falls from rest in a long evacuated vertical tube
of length L.
The feather takes time T to fall from
top to the bottom of the tube.
How far will the feather have fallen
from the top of the tube in time 0.50 T?
A 0.13 L B 0.25 L C
0.38 L D 0.50 L
Reference: Past Exam Paper – June 2012 Paper 11 Q8 & Paper 13 Q9
Solution 784:
Answer: B.
Since a vacuum, there is no air
resistance.
Initial speed, u = 0 since the
feather falls from rest.
The acceleration of free fall, g,
acts downwards.
Equation for uniformly accelerated
motion: s = ut + ½ at2
When distance s = L, the time taken
to fall = T
L = 0 + 0.5gT2 = 0.5gT2
When time to fall = 0.5T, let the
distance be s
s = 0 + 0.5g(0.5T)2
Distance s = gT2 / 8
From L = 0.5gT2, gT2
= 2L
Distance s = gT2 / 8 = 2L
/ 8 = L/4 = 0.25L
Question 785: [Dynamics
> Resultant force]
Diagrams show a metal cube suspended
from a spring balance before and during immersion in water.
A reduction in the balance reading
occurs as a consequence of the immersion.
Which statement is correct?
A The balance reading will be
further reduced if the cube is lowered further into the water.
B The balance reading during
immersion corresponds to the upthrust of the water on the cube.
C The forces acting on the vertical
sides of the cube contribute to the change in the balance reading.
D The gravitational pull on the cube
is unchanged by the immersion.
Reference: Past Exam Paper – November 2010 Paper 12 Q13
Solution 785:
Go toThe diagrams show a metal cube suspended from a spring balance before and during immersion in water.
Question 786: [Work,
Energy and Power]
Steel ball is falling at constant
speed in oil.
Which graph shows variation with
time of the gravitational potential energy Ep and the kinetic energy
Ek of the ball?
Reference: Past Exam Paper – June 2005 Paper 1 Q15 & June 2008 Paper 1 Q18
Solution 786:
Answer: B.
Kinetic energy Ek = ½ mv2
Since the speed is constant, the
kinetic energy is also constant.
Gravitational potential energy Ep
= mgh
Speed = Distance / Time
As the speed is constant, the rate
at which the distance (h) decreases is constant. So, the decrease in
Ep is also constant.
