# 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 E

_{K}of the ball with distance h fallen. The ball reaches the ground after falling through a distance h

_{0}.

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 E

_{P}of the ball. At h = h_{0}, 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.

**(b)**

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

(ii)

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
E

_{k}.
The gradient should be reasonable (same magnitude as that for E

_{k}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.}

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

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

Delete