Physics Nov / Dec 2010

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Question 1 Part I: Candidates were required to answer any five questions from this part.
A ball is projected horizontally from a height with a velocity of 40 ms^-1. Calculate the drop in height after travelling a horizontal distance of 30m. [Neglect air resistance; g = 10 ms^-1]

ANSWER

          V =d/t
            OR       t= d/V
           = 30/40
           = 0.75s
           h = ½gt²
           = ½ x gt²
           =½ x 10x(O.75)²
           =2.81m

Question 2
List three methods for the polarization of light This question was well answered by many candidates. Performance was above average.

ANSWER

Methods of polarization of light

– Double refraction
– Selective absorption
– Reflection
– Scattering

Question 4
(a) State two differences between solid friction and viscosity.

ANSWER

Question 4

(a) State two differences between solid friction and viscosity.

_____________________________________________________________________________________________________

Observation

Responses revealed that solid friction and viscosity topics are poorly treated in schools. Many candidates
had good ideas but could not assemble them together. Some candidates even mismatched points listed.

The expected answers are:

Differences between solid friction and viscosity

Solid friction	Viscosity
– Independent of surface area in contact	Depends on surface area in contact
Depends on normal reaction (of one solid surface on the other)	Independent of normal reaction
Occurs between solids surfaces in contact	Occurs between fluid layers/solid and fluid in contact/in fluids
Independent of relative velocity (between solid surfaces in contact)	Depends on relative velocity (between fluid layers)

Question 5
Explain how the addition of detergents of water reduces surface tension

ANSWER

Surface tension is due to the intermolecular forces among the surface molecules of a liquid. Addition of detergents to water reduces/weakens theses forces; hence the reduction in its surface tension


Question 6
(a) Define tensile strain. (b) Name one measuring instrument which operates on the principle of Hooke’s law. (c) Which property of an elastic material does the constant of proportionality K represent in the equation F = Ke?

ANSWER

(a) Tensile strain is the ratio of extension to original length. (mathematical definition is acceptable jf symbols are defmed).
(b)Spring balance
(c) Elastic constant/stiffness

Question 7
Use a labelled diagram to illustrate how cathode rays are deflected by a magnet.

ANSWER

Question 8
In electroplating an iron spoon with gold, state which material is made the (a) Anode; (b) Cathode; (c) Electrolyte

Question 9
List the three components of the electron gun’ in a cathode ray tube

Question 10
List three phenomena that illustrate the particle nature of light.

ANSWER

Phenomena that illustrates particle nature of light

– Photoelectric effect
– Radiation of energy from heated bodies
– (general) emission/absorption of light
– Compton effect.

Question 11
Part II : Candidates are expected to answer any three form this part

(a) Define:
        (i) work;
        (ii) power
(b) Distinguish between static friction and dynamic friction
(c)Why do tyres have treads?
(d) A car of mass M starts from rest and moves with a constant acceleration, a. it attains a
        velocity, v after time t, and maintains this velocity until time t2. It is then brought to rest with
        uniform retardation, d at time t3.
                        (i)          Draw and label a velocity-time graph for the motion.
                        (ii)         Deduce an expression for the force on the car during the period of retardation.

ANSWER

(a)(i) Work is the product ofa force (applied) and the distance moved in the direction of the force
                        OR
The product of displacement and the component of the force in the direction of the displacement.
(ii) Power is the time rate at which energy is expended.
                        OR
Power is the time rate of doing work.

NOTE: Mathematical definitions are acceptable provided symbols are correctly defined.

(b)          Any correct difference
e.g.

Static Friction	Dynamic Friction
It exists between surfaces of stationary bodies	It exists between surfaces in relative motion

– For the same system the static frictional force is greater than dynamic frictional force

(c) The tyres have treads in them to provide better grip of roads due to increased friction thus preventing vehicles from skidding

(ii) Retardation d = slope (oflast segment of graph)

= v – 0
T3 – t2
= v
t3 – t2

F = md

= Mv/t3 – t2

Question 12 Part II : Candidates are expected to answer any three form this part
(a) Explain: (i) Linear momentum. (ii) Impulse of a force (b) State three instances where Newton’s third law of motion is applicable. (c) A ball of mass 4.0 kg. hits a smooth platform vertically with a speed of 3 ms^-1 and rebounds with a speed of 2 ms^-1. Calculate the impulse experienced by the ball. (d) A rocket of mass 500 kg carrying 4000 kg of fuel is to be launched vertically. The fuel is consumed at a steady rate of 50 kgs^-1, calculate the least velocity of the exhaust gases if the rocked will just lift off the launching pad immediately after firing [g = 10 ms^-1]

Question 12
Part II : Candidates are expected to answer any three form this part

(a) Explain:
(i) Linear momentum.
(ii) Impulse of a force
(b) State three instances where Newton’s third law of motion is applicable.

(c) A ball of mass 4.0 kg. hits a smooth platform vertically with a speed of 3 ms^-1 and rebounds with a speed of 2 ms^-1. Calculate the impulse experienced by the ball.
(d) A rocket of mass 500 kg carrying 4000 kg of fuel is to be launched vertically. The fuel is consumed at a steady rate of 50 kgs^-1, calculate the least velocity of the exhaust gases if the rocked will just lift off the launching pad immediately after firing [g = 10 ms^-1]

ANSWER

(a)(i)              Linear momentum of a body moving in a straight line is the product of its mass and its velocity.
Accept product of mass and linear velocity.
Any valid additional information
e.g.
– Its unit is kgms” – Do not accept Ns
– it is a vector quantity.

             (ii)            Impulse of a force is the product of the (large) force acting on a body and the (short)
time during which it acts. Any valid additional information e.g.
Any valid additional information
e.g.
– Its unit is Ns – Do not accept kgms”
– it is a vector quantity.

(b) Instances where Newton’s third law of motion is applicable
– recoil of a gun
– jet propulsion/rocket propulsion
– action of sprinkler/lawn spray
– Accept any other valid instance(s)

(c)          Impulse = Ft = m(v- u)
          = 4[2-(-3)]
        =   20Ns

(d)            Weight, W=mg
          = (5000 + 4000) x 10
          = 90,000N.
Downward action = upward reaction
          90,000 = 50(v-0)
          v = 90,000
                    50
          = 1.8 X 10³ ms^-1.

Question 13 Part II : Candidates are expected to answer any three form this part
(a) With the aid of a labelled diagram, explain. (i) node; (ii) antinode. (b) Explain: (i) standing wave; (ii) persistence of vision. (c) How does the iris in the human eye respond to: (i) Dim light; (ii) Excessive brightness? (d) A wave is represented by the equation y = 0.20 sin 0.407t (x – 60 t) where aU distances are measured in centimeters and time in seconds. Determine the wave’s (i) wavelength; (ii) frequency.

Question 13
Part II : Candidates are expected to answer any three form this part

(a) With the aid of a labelled diagram, explain.
                       (i)           node;
                       (ii)           antinode.
(b) Explain:
(i) standing wave;
(ii) persistence of vision.

        (c)          How does the iris in the human eye respond to:
                       (i)            Dim light;
                       (ii)          Excessive brightness?

        (d)          A wave is represented by the equation y = 0.20 sin 0.407t (x – 60 t) where aU distances                       are measured in centimeters and time in seconds.
                      Determine the wave’s
                      (i)            wavelength;
                      (ii)           frequency.

ANSWER

(a) (i)

(Reflected wave)

N’
Incident wave).

Correct diagram

N=Node
A = Antinode
(Do not penalize for barriers not shown)

A node is a point on a stationary wave where there is no movement ofthe medium/points
of zero displacement.
OR
A Node is a point of destructive interference

An antinode is a point on a stationary wave where there is maximum displacement of
the medium.
OR

An antinode is a point of constructive interference.

(b)(i) Standing wave occurs when a progressive wave is reflected from a hard surface and
superimposed on the incident wave.
OR
Standing wave occurs when waves of the same frequency and amplitude traveling in opposite direction are superimposed.
(ii) Persistence of vision is a property of the eye whereby the sensation of vision lasts for a short but definite time after the object has been removed.
(c)(i) In dim light, the iris contracts increasing the size of the pupil to let in sufficient light.
(ii) In excessive brightness the iris quickly expands in size thus reducing the size of the pupil which reduces the amount of light entering the eye .

(d)(i) Either y = A sin2Π/λ (x – vt) ………………………………… (α)
OR y = A sin[2Πx/λ – 2Πvt/λ ]……………………………………… (β)
= 0.20 sin (0.40Πx – 0.40Π x 60t) …………… (γ)

Comparing equations (β) and (γ)

2Πx/λ= 0.40Πx
λ = 5 cm

(ii) 2Πvt/λ = 0.40Πx x 60t
v = fλ
2Πvt = 0.40Π x 60t
f = 12Hz

Question 14
Part II : Candidates are expected to answer any three form this part

(a) (i) What is a capacitor?
(ii) Explain the term short-circuit.
(b) Why is a 200 watt bulb brighter than a 60 watt bulb when connected to a 220 volt mains outlet?

(c)

Five identical capacitors each of capacitance 1 0 ~ are illustrated in the diagram above. Calculate the equivalent capacitance between X and Y.

(d) A 15 µF capacitor is connected in parallel across three capacitors of capacitances 30, 40 and 120 µF in series. The combination is connected across a battery of e.m.f. 220 V.
(i) Draw a circuit diagram for the arrangement
(ii) Calculate the charge in the circuit.

ANSWER

(a)(i)A capacitor is a device for storing charges/electrical energy.
OR
A capacitor is a device consisting of two parallel metal plates, carrying equal but opposite charges separated by a dielectric/an insulator.

(ii) A short circuit
An electrical connection of relatively very low resistance made between two points (at different potentials) in a circuit
OR
When two wires touch and the resistor in the circuit is by-passed/there is not enough resistance in the circuit.

(b) More current passes through the 200 watt bulb than the 60 watt bulb and the more current that passes through the bulb the brighter and more powerful the bulb.

(c)

Ca = 5 µF

1/Cb = 1/C3 + 1/C4 =1/10 + 1/10 + 1/5
Cb = 5 µF

Cc = Ca + Cb = 5+ 5 =10 µF

1/Cxy = 1/10 + 1/10 = 2/10
Cxy = 5 µF

(d)(i)

Question 15 Part II : Candidates are expected to answer any three form this part
(a) Explain: (i) stopping potential; (ii) work function. (b) State two factors that determine the kinetic energy of ejected electrons in photoelectric (c) State two uses of a photocell. (d) When the surface of a certain metal is illuminated with yellow light of wavelength 5.893 x 10^-7m, electrons of stopping potential 0.36 V are liberated. When the surface is illuminated with violet light of wavelength 3.969 x 10^-7 m the stopping potential is 1.38 V. Calculate the (i) Planck’s constant, h; (ii) Work function, w; (iii) Threshold frequency f₀,

Question 15
Part II : Candidates are expected to answer any three form this part

(a) Explain:
(i) stopping potential;
(ii) work function.

(b)          State two factors that determine the kinetic energy of ejected electrons in photoelectric

(c) State two uses of a photocell.

(d) When the surface of a certain metal is illuminated with yellow light of wavelength 5.893 x 10^-7m, electrons of stopping potential 0.36 V are liberated. When the surface is illuminated with violet light of wavelength 3.969 x 10^-7 m the stopping potential is 1.38 V.
Calculate the
              (i)            Planck’s constant, h;
              (ii)           Work function, w;
                (iii)          Threshold frequency f₀,

ANSWER

(a) (i) Stopping potential is the (negative) minimum potential difference of the anode which is required to stop the most energetic electron from reaching the anode. Any valid additional information e.g Unit is volt.
(ii) Work function is the minimum amount of energy required to liberate/eject an electron from the surface of a metal when irradiated by electromagnetic radiation. Any valid additional information e.g. joule.

(b) Factors affecting kinetic energy of ejected electrons
– work function of surface/metal irradiated
– wavelength/frequency of incident radiation

(c) Uses of photocell
– burglar alarms
– switches for street lights
– automatic doors
– television cameras
– solar calculators
– counters
Accept any other valid example

(d) Work function = Wo = E – eVs or = hf/λ – eVs

                  = hc/ – eVs
                  = hc/5.893 x 10 ^-7 – 0.36e …………..(α)
                  = hc/5.969 x 10^-7 -1.38e………….(β)

Equation (β) – Equation (α) gives

1.38e – 0.36e = hc/5.969 x 10^-7 – hc/5.893 x 10 ^-7
1.02e = hc (0.2520 – 0.1697) x 10⁷
Taking 1.6 x 10^-19 as e and 3.0 x 10⁸ as c,
1.02 x 1.6 x 10^-19 = h x 3 X 10^-8 x 0.0823 X 10⁷
h = 6.61 X 10^-34 Js

From equation (α)
Wo = 6.61 X 10^-34x 3.0 X 108
5.893 X 10-7 – 0.36 x 1.6 x 10^-19
= 3.365 X 10^-19 – 0.576 x 10^-19
= 2.79 X 10^-19 J

= 2.79 X 10^-19
6.61 X 10^-34
= 4.22 x 10^-14 Hz

Physics Nov / Dec 2010

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