• 9702 November 2008 Paper 2 Worked Solutions | A-Level Physics

Paper 2

Question 1

(a)

Current in wire is I. Charge Q passes 1 point in wire in time t.

(i)

Relationship between I, Q and t:

Q = It

(ii)

Which are base quantities:

I, t

(b)

Current in wire is due to electrons, each with charge q that move with speed v along wire. There are n of these electrons per unit volume. For wire having cross-sectional area S, current I given by           I = nSqv^k where k is a constant.

(i)

Units of I, n, S, q and v in terms of base units:

Base units:

I: A

n: m^-3

S: m²

q: As

v: ms^-1

(iii)

Considering homogeneity of equation, value of k:

I = nSqv^k

[A] = [m^-3][m²][As][ms^-1]^k

e.g. For m: 0 = -3 + 2 + k

So, k = 1

Question 2

{Detailed explanations for this question is available as Solution 1092 at Physics 9702 Doubts | Help Page 232 - http://physics-ref.blogspot.com/2016/01/physics-9702-doubts-help-page-232.html}

Question 3

(a)

Distinguish between moment of a force and torque of a couple:

The moment of a force is the product of the force to the perpendicular distance of the force from the pivot / axis / point.

The torque of a couple is the product of (the magnitude of) one force to the perpendicular distance between the two forces.

(b)

One type of weighing machine, known as steelyard, is illustrated. Two sliding weights can be moved independently along rod. With no load on hook and sliding weights at zero mark on metal rod, metal rod is horizontal. Hook is 4.8 cm from pivot. A sack of flour is suspended from hook. In order to return metal rod to horizontal position, 12 N sliding weight is moved 84 cm along rod and 2.5 N weight is moved 72 cm.

(i)

Weight of sack of flour:

(Anti-clockwise moment = Clockwise moment)

W(4.8) = 12(84) + 2.5(72)

Weight, W = 250N     (248N)

(ii)

Why steelyard would be imprecise when weighing objects with weight of about 25N:

EITHER There is friction at the pivot OR It would require only a small movement of the weights

Question 4

{Detailed explanations for this question is available as Solution 416 at Physics 9702 Doubts | Help Page 78 - http://physics-ref.blogspot.com/2015/03/physics-9702-doubts-help-page-78.html}

Question 5

Some smoke particles are viewed through microscope, as illustrated. Brownian motion is observed.

(a)

Brownian motion is haphazard / random / erratic / zig-zag movement of (smoke) particles

(b)

Explain why Brownian motion provides evidence for movement of molecules as assumed in kinetic theory of gases:

Brownian motion is due to unequal / unbalanced collision rates (on different faces). (This unequal collision rate is due to the) random motion of (gas) molecules / atoms

(c)

Smoke from poorly maintained engine contains large particles of soot. Why Brownian motion of such large particles is undetected:

EITHER

The collisions with the air molecules average out. This prevents the haphazard motion.

OR

The particle is more massive / heavier / has large inertia. So, the collisions cause only small movements / accelerations.

Question 6

{Detailed explanations for this question is available as Solution 241 at Physics 9702 Doubts | Help Page 39 - http://physics-ref.blogspot.com/2014/12/physics-9702-doubts-help-page-39.html}
(a)

Diffraction of a wave is the bending / spreading of the wave (into geometrical shadow) when the wave is incident at an edge / aperture / slit / (edge of) obstacle

(b)

(i)

Experiment that may be used to demonstrate diffraction of a transverse wave:

Examples for the apparatus:

Laser & slit / a point source & slit / lamp and slit & slit

A microwave source & slit

Water / ripple tank, source & barrier

Example for the detector:

A screen

Aerial / microwave probe

A strobe / lamp

What is observed

(ii)

How experiment in (i) may be changed to demonstrate diffraction of a longitudinal wave:

For apparatus, e.g. loudspeaker, and slit / edge

For detector, e.g. microphone & c.r.o / ear

What is observed

Question 7

{Detailed explanations for this question is available as Solution 409 at Physics 9702 Doubts | Help Page 76 - http://physics-ref.blogspot.com/2015/03/physics-9702-doubts-help-page-76.html}

Question 8

Thoron is radioactive gas. Variation with time t of detected count rate C from sample of the gas shown. Radioactive decay said to be random and spontaneous process.

(a)

By reference to radioactive decay, random process:

In a random process, a nucleus has a constant probability of decay per unit time / in a given time

(b)

Feature of Fig which indicates that process is

(i)

Decay process:

The count rate / activity decreases

(ii)

Random:

The count rate fluctuates / is not smooth

(c)

Second similar sample of thoron prepared but it is at much higher temperature. Variation with time of count rate for this second sample is determined. Feature of decay curves for the 2 samples that suggests that radioactive decay is a spontaneous process:

EITHER The (decay) curves are similar / same OR The curves indicate the same half-life.