Tuesday, December 29, 2015

Physics 9702 Doubts | Help Page 230

  • Physics 9702 Doubts | Help Page 230

Question 1085: [Kinetic model of Matter]
The Brownian motion of smoke particles in air may be observed using the apparatus shown in Fig. 2.1.

Fig. 2.1

(a) Describe what is seen when viewing a smoke particle through the microscope.

(b) Suggest and explain what difference, if any, would be observed in the movement of smoke particles when larger smoke particles than those observed in (a) are viewed through the microscope.

Reference: Past Exam Paper – June 2005 Paper 2 Q2

Solution 1085:
(a) A speck of light {not the smoke particles themselves} that moves haphazardly/randomly/jerkily/etc. is seen
{We are asked to describe what is OBSERVED, not ‘why’ this is happening.}

(b) The randomness of collisions would be ‘averaged out’ {the motion would still be random, but slower}. So there would be less (haphazard) movement.
{There is now a greater surface area and, because of the random distribution of velocities of air molecules, the effects of collisions of the smoke particle with air molecules would tend to average out. This greater rate of collision would lead to a smaller, rather than a larger, randomness of collision and hence motion of the smoke particle.}

Question 1086: [Measurement]
The diagrams show digital voltmeter and analogue ammeter readings from a circuit in which electrical heating is occurring.

What is the electrical power of the heater?
A 0.53 W                    B 0.58 W                     C 530 W                      D 580 W

Reference: Past Exam Paper – June 2009 Paper 1 Q4

Solution 1086:
Answer: B.
Power = VI
The values should be given in SI units.

p.d. V = 1200 mV = 1.2 V

Current I = 0.48 A

Power = VI = 1.2 × 0.48 = 0.576 W = 0.58 W

Question 1087: [Work, Energy and Power]
(a) Explain the concept of work.

(b) A table tennis ball falls vertically through air. Fig. 8.1 shows the variation of the kinetic energy EK of the ball with distance h fallen. The ball reaches the ground after falling through a distance h0.

Fig. 8.1
(i) Describe the motion of the ball.
(ii) On Fig. 8.1, draw a line to show the variation with h of the gravitational potential energy EP of the ball. At h = h0, the potential energy is zero.

Reference: Past Exam Paper – November 2005 Paper 2 Q8

Solution 1087:
(a) Work is the product of force and the distance moved in the direction of the force.

(i) The ball falls from rest with a decreasing acceleration until it reaches a constant (terminal) speed.

The graph is a straight line with a negative gradient.
It intercepts the y-axis at a point which is above the maximum value for Ek.
The gradient should be reasonable (same magnitude as that for Ek initially).
{Gravitational potential energy = mgx. That is, it depends linearly on height x (remember that the equation for a straight line is of the form y = mx + c), so graph is a straight line. But here, h is the distance fallen, not height. So, gradient is negative.
The straight line should be well above the point for maximum kinetic energy in order to reflect the energy expended in doing work to overcome the air resistance opposing the motion.}


  1. For solution 1087, shouldn't the GPE decrease at the same rate as the KE is increasing. Like a reflection of the graph.

    1. GPE = mgh. It depends linearly on h (not on h^2). So, it's a straight line


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