• 9702 June 2013 Paper 22 Worked Solutions | A-Level Physics

Question 1

(a)

SI unit of power:

Power = energy/time = (force x distance/time) = kgm²s^-2 / s

Power = kgm²s^-3

(b)

Turbine that is used to generate electrical power from wind

Power, P = CL²ρv³

Where L is the length of each blade (3 blades) of the turbine, ρ is the density of air, v is the wind speed, C is a constant

(i)

C has no units:

C = P / L²ρv³

Units: L² : m²              ρ = kgm^-3        v³ : m³s^-3

C = kgm²s^-3 / (m² kgm^-3 m³s^-3) = kgm²s^-3 / kgm²s^-3

Thus, the units of the components cancel each other, leading to C having no units.

(ii)

L = 25.0m,      ρ of air = 1.30,            C = 0.931.

Efficiency = 55%        Electric Power output, P = 3.50x10⁵W

Wind speed = v

55% = 3.50x10⁵W

Power available from wind, 100% = 3.50x10⁵ x100/55 = 6.36x10⁵W

P = CL²ρv³

So, v³ = P/ CL²ρ = 6.36x10⁵ / (0.931 x (25)² x 1.3)

v = 9.4ms^-1

(iii)

Reasons why the electrical power output of the turbine is less than the power available from the wind:

Not all kinetic energy of the wind is converted to kinetic energy of the blades / generator. The conversion to electrical energy is also not 100% efficient since heat is produced in the generator / bearing, etc.

Question 2

{Detailed explanations for this question is available as Solution 1089 at Physics 9702 Doubts | Help Page 231 - http://physics-ref.blogspot.com/2016/01/physics-9702-doubts-help-page-231.html}





Question 3

{Detailed explanations for this question is available as Solution 864 at Physics 9702 Doubts | Help Page 173 - http://physics-ref.blogspot.com/2015/06/physics-9702-doubts-help-page-173.html}

Question 4

(a)

Apparatus that demonstrates Brownian motion. Include diagram:

Apparatus: cell with particles e.g. smoke (container must be closed)

Diagram showing suitable arrangements with light illumination and microscope

(b)

Observations made using the apparatus:

Specks/flashes of light are seen to go in random motion.

(c)

2 conclusions about properties of molecules of a gas that follow from the observations:

choose any 2:

We cannot see what is causing the smoke to move hence, the molecules are smaller than the smoke particles.

The continuous motion of the smoke particles implies that motion of the molecules is continuous.

The random motion of the particles implies the random motion of molecules.

Question 5

(a)

Frequency, f =50Hz                transverse wave           speed, v=40ms^-1

(i)

Wavelength of transverse wave on string:

v = fλ

λ = 40/50 = 0.80m

(ii)

How this arrangement may produce a stationary wave on the string:

The waves travel along the string and are reflected at Q / wall / fixed end. The incident and reflected waves interfere / superpose.

(b)

(i)

Label all nodes N and all antinodes A:

Nodes labeled at P, Q and the 2 points at zero displacement

Antinodes labeled at the 3 points of maximum displacement

(ii)

Length of string PQ:

PQ represents 1.5λ. Hence PQ = 0.8 x 1.5 = 1.2m

(iii)

Draw stationary wave at time (t+5.0ms) + explanation:

Period, T = 1/f = 1/50 = 20ms.

5ms is ¼ of cycle

Horizontal line through PQ drawn on Fig 5.2
{The reasoning as to why a horizontal line is obtained has been explained as Solution 41 (c)(iv) at Physics 9702 Doubts | Help Page 7 - http://physics-ref.blogspot.com/2014/11/physics-9702-doubts-help-page-7.html to a question similar to this part. Read it there}

Question 6

(a)

Charge is defined as the product of current with time.

(b)

Resistance, R = 18.0Ω            Power supply of 240V            switched on for 2.60Ms

(i)

Power transformed in heater, P:

P =V²/R = (240)² / 18 = 3200W

(ii)

Current in heater, I:

I = V/R = 240 /18 = 13.3A

(iii)

Charge passing through heater in this time, Q:

Charge, Q = It = 13.3 x 2.6x10⁶ = 3.47 x 10⁷C

(iv)

Number of electrons per second passing a given point in heater:

Number of electrons = 3.47x10⁷ / 1.6x10^-19 = 2.17x10²⁶

Number of electrons per second = 2.17x10²⁶ / 2.6x10⁶ = 8.35x10¹⁹ s^-1

  

Question 7

(a)

(i)

W = 206          X = 82

Y = 4               Z = 2

(ii)

Why mass seems not to be conserved in the reaction:

Mass – energy is conserved. Mass on the right hand side is less because energy is released.

(iii)

Meaning of spontaneous:

The reaction is spontaneous means that the reaction is not affected by external conditions / factors / environment OR 2 examples temperature and pressure.