Physics May/June 2009

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Question 1
A particle is dropped from a vertical height h and falls freely for a time t. With the aid of a sketch, explain how h varies with (a) t; (b) t2.

ANSWER

(a) Diagram

Correct shape in relation to h and t correct shape in relation to h and t2
h varies parabolically with t h varies linearly with t2

Question 2
A particle is projected horizontally at 15ms-1 from a height of 20m. Calculate the horizontal distance covered by the particle just before hitting the ground. [g = 10 ms-2]

ANSWER

Let R represent the horizontal distance covered at time, t.

                  h   = ½ gt2
                  20 = ½ x 10 x t2

                        t = 2 s

R = ut

                        = 15 x 2

                        = 30m

Question 3
Explain why mercury does not wet glass while water does.

ANSWER

Mercury does not wet glass because (force of) cohesion/attraction of mercury molecules is greater than (force of) adhesion/attraction between glass and mercury molecules.

Water wets glass because (force of) adhesion/attraction between glass and water molecules is greater than (force of) cohesion/attraction of water molecules.

Question 4
Explain what is meant by cations. Draw and label an electrolytic cell

ANSWER

(a) Cations – are positive ions that are attracted to the cathode (during electrolysis)

(b) Diagram

Correct diagram/sketch: labeling cathode, anode and electrolyte.

Question 5
State three methods of polarizing an unpolarized light.

Question 6
State Faraday’s second law of electrolysis. An electric charge of 9.6 x 104 C liberates 1 mole of substance containing 6.0 x 1023 atoms. Determine the value of the electronic charge.

ANSWER

a) If the same quantity of electricity is passed through different voltameters/electrolytes connected in series the masses of the substances liberated/deposited during electrolysis is (directly) proportional to their chemical equivalents.
OR The mass of an element deposited/liberated during electrolysis is (directly) proportional to the chemical equivalent of the element.

(b)        Let e represent the electronic charge.

                        e =   Faraday’s constant
                                 Avogadro’s number

= 9.6 x 104
6. 0. x 1023

= 1.60 x 10-19C

Question 8
a) Define diffusion. (b) State two applications of electrical conduction through gases.

ANSWER

(a) Diffusion is the process by which substances mix (intimately) with one another due to the random motion of their molecules.

(b) Applications of electrical conduction through gases include:.
– In advertising industry/Neon signs
– In lighting/fluorescent tubes
– Identification of gases
   – Cathode ray oscilloscope/T.V. tubes

Question 9
a) List two properties of cathode rays. (b) Explain how the intensity and energy of cathode rays may be increased

ANSWER

(a) Properties of cathode rays:
Cathode rays

1. are negatively charged.
2. travel in straight line in field free space.
3. are deflected by electric/magnetic field.
4. possess(kinetic) energy.
5. possess momentum.

(b)(i) The intensity of cathode rays may be increased by raising the temperature of the cathode/increasing the current through the heater.

(ii) The energy of cathode rays may be increased by raising the potential difference between

the anode and the cathode/the anode potential.

Question 11

(a) Given a retort stand and clamp, a stout pin, a simple pendulum and a pencil, describe how you would use these apparatus to determine the centre of gravity of an irregularly shaped piece of cardboard of a moderate size.

(b) Using a suitable diagram, explain how the following can be obtained from a velocity-time graph:

(i) acceleration;
(ii) total distance covered.

( ) A body at rest is given an initial uniform acceleration of 6.0 ms-2 for 20 s after which the acceleration is reduced to 4.0 ms-2 for the next 10 s.
The body maintains the speed attained for 30 s.

Draw the velocity-time graph of the motion using the information given above. From the graph, calculate the:

maximum speed attained during the motion;
total distance traveled during the first 30 s;
average speed during the same time interval as in (ii) above.

ANSWER

(a)    PROCEDURE
Make at least 3 well-spaced pin holes round the edge of the cardboard. Clamp the pin horizontally and suspend the cardboard on it through one of the pin-holes such that the cardboard can swing freely
Hang the simple pendulum on the same pin and let its string be very close to the cardboard.
When the whole system is at rest (or in equilibrium) trace the plumbline on the cardboard. Repeat the procedure for each of the two other pin holes.
         CONCLUSION
The point at which the (three) traced lines intersect is the centre of gravity of the cardboard.
         PRECAUTIONS
–           Repeat procedure
–           Pin rigidly and firmly held by retort stand and clamp
–           Allow the simple pendulum to rest before tracing the shadow of
            the plumbline on the cardboard.
–           The string to be close to the cardboard

11(b) Diagram:

Both axes correctly labelled
Any correct shape of graph showing acceleration segment

Acceleration = gradient of AB
Total distance covered = area under the graph

11(c) Diagram

– At least one axis labelled
– correct shape of graph

Let V1 = maximum velocity after 20 sec.
V2 = maximum velocity after 30 sec.

(i) Then V1 = 6
20

                       ∴ V1 = 120 ms-1

                        Also V2 – V1   =   4
         10

V2 – V1 = 40

                        V2 = 120 + 40

                        V2 = 160 ms-1
                        Maximum speed = V2 = 160ms-1

(ii) Total distance covered = Area of Δ + Area of trapezium after 1st 30 seconds
= ( ½ x 20 x 120) + ½ (120 + 160) x 10

= 1200 + 1400

                                  =          2600m

(iii)       Average speed =   Total distance
                                               Total time
                                                =   2600                                                                                                                                                                                                   30

= 86.67 ms-1

Question 12
(a) Explain why it is not advisable to sterilize a clinical thermometer in boiling water at normal atmospheric pressure. (b) State the effect of an increase in pressure on the (i) boiling point; and (ii) melting point of water. (c) Diagram: The graph shown above is that of the saturated vapour pressure (s.v.p.) of water against temperature. Pure water is known to boil at 100oC and at an atmospheric pressure of 760 mmHg. What general conclusion can be drawn from the information given above? d) A thread of mercury of length 20 cm is used to trap some air in a capillary tube with uniform cross-sectional area and closed at one end. With the tube vertical and the open end uppermost, the length of the trapped air column is 15cm. Calculate the length of the air column when the tube is held: i) horizontally; ii) vertically with the open end underneath. [Atmospheric pressure = 76 cmHg ]

Question 12

(a) Explain why it is not advisable to sterilize a clinical thermometer in boiling water at normal atmospheric pressure.

(b)        State the effect of an increase in pressure on the
            (i)   boiling point; and
            (ii) melting point of water.

The graph shown above is that of the saturated vapour pressure (s.v.p.) of water against temperature.
Pure water is known to boil at 100oC and at an atmospheric pressure of 760 mmHg. What general conclusion can be drawn from the information given above?

d) A thread of mercury of length 20 cm is used to trap some air in a capillary tube with uniform cross-sectional area and closed at one end. With the tube vertical and the open end uppermost, the length of the trapped air column is 15cm. Calculate the length of the air column when the tube is held:

i) horizontally;

ii) vertically with the open end underneath. [Atmospheric pressure = 76 cmHg ]

ANSWER

a)    A clinical thermometer has small temperature range. The glass will crack/burst due to excessive pressure created by expansion of mercury.
(b)     Increase in pressure
          (i)   increases the boiling point;
          (ii) decreases the melting point.

At the boiling point of pure water, the saturated vapour pressure (s.v.p)

of the water is equal to the external atmospheric pressure.

(d) Diagram

Volume V is proportional to length Ɩ stated or implied

(i) P1V1 = P2 V2 implies P1 Ɩ1 = P2Ɩ2
(76 + 20) 15 = 76 x Ɩ2

            Ɩ2 =  96 x 15
                        76
            Ɩ2 = 18.95 cm

(ii) P1 Ɩ1 = P3 Ɩ3
(76 + 20) 15 = (76 – 20) x Ɩ3

            Ɩ3 =  96 x 15
     56
Ɩ3 = 25.7l cm

Question 13
(a) State two differences between a sound wave and a radio wave. (b) Explain why a vibrating tuning fork sounds louder when its stem is pressed against a table top than when held in air. (c)State two conditions necessary for the: production of stationary wave in a medium; formation of interference wave patterns; occurrence of total internal reflection of a wave. (d) A ray of light is incident on one face of an equilateral glass prism. Draw a ray diagram to show the path of the ray through the prism. Calculate the refractive index of the glass if the angle of minimum deviation is 41o.

Question 13

(a) State two differences between a sound wave and a radio wave.

(b) Explain why a vibrating tuning fork sounds louder when its stem is pressed against a table top than when held in air.

(c)State two conditions necessary for the:

production of stationary wave in a medium;
formation of interference wave patterns;
occurrence of total internal reflection of a wave.

(d) A ray of light is incident on one face of an equilateral glass prism.

Draw a ray diagram to show the path of the ray through the prism.
Calculate the refractive index of the glass if the angle of minimum deviation is 41o.

ANSWER

(a) Differences between a sound wave and a radiowave

Sound wave	Radio wave
is a mechanical wave	is an electromagnetic wave
is longitudinal	is transverse
has speed less than speed of light.	has speed equal to that of light
requires a material medium for propagation/cannot travel through vacuum.	does not require a material medium for propagation/can travel through vacuum.

(i)
(b) The vibrating tuning fork forces the table top to vibrate. The table top has a much larger area and is therefore in contact with a larger volume of air undergoing vibration. Hence, sound heard is amplified.

(c)       Conditions necessary for:

            (i)   production of stationary wave
–    Two waves must be traveling in opposite directions.
–     Incident and reflected waves must have the same frequency/wavelength.
–     Incident and reflected waves must have equal amplitude.
–     There must be superimposition.
–     There must be a barrier

(ii) formation of interference wave pattern
– Two sources must be involved.
– The two sources must be coherent/have a constant phase difference as well as the same frequency and amplitude.
– The waves that are interfering must have the same amplitude
– The distance between the sources must be of the order of the wavelengths of the waves.

(iii)   total internal reflection
   – The wave must be traveling from a denser medium to a less dense medium
   – The angle of incidence of the wave in the denser medium must be greater than the critical angle for the medium.

(d)(i) Diagram

Correct diagram

            (ii)                  Sin (A + dm)
                      n = 2
                                   Sin (A/2)

                                  Sin (60 + 41)
                         =                 2
                                  Sin   (60/2)

= 1.54

Question 14
(a) State two essential differences between a moving coil galvanometer and a d.c. generator. (b) Explain the term eddy currents and state two devices in which the currents are applied. (c) State the principle on which the potentiometer is based when it is functioning. (d) A source of e.m.f. 110 V and frequency 60Hz is connected to a resistor, an inductor and a capacitor in series. When the current in the capacitor is 2A, the potential differences across the resistor is 80 V and that across the inductor is 40 V. Draw the vector diagram of the potential differences across the inductor, the capacitor and the resistor. Calculate the: (i) potential difference across the capacitor; (ii) capacitance of the capacitor; (iii) inductance of the inductor. [π = 3.14]

Question 14

(a)        State two essential differences between a moving coil galvanometer and a d.c. generator.
(b)        Explain the term eddy currents and state two devices in which the currents are applied.
(c)        State the principle on which the potentiometer is based when it is functioning.
(d)       A source of e.m.f. 110 V and frequency 60Hz is connected to a resistor, an inductor and a capacitor in series. When the current in the capacitor is 2A, the potential differences across the resistor is 80 V and that across the inductor is 40 V. Draw the vector diagram of the potential differences across the inductor, the capacitor and the resistor.
Calculate the:
(i)         potential difference across the capacitor;
(ii)        capacitance of the capacitor;
(iii)       inductance of the inductor. [π = 3.14]

ANSWER

(a) Differences between a d. c. generator and a moving coil galvanometer.

D. C. generator	moving coil galvanometer
converts mechanical energy to electrical energy	converts electrical energy to mechanical energy.
uses split rings or commutator	uses hair springs.
rotation of coil is continuous	rotation of coil is incomplete
uses carbon brushes (as terminals)	uses jeweled bearings (as terminals)

(b) Eddy currents are currents induced in a conductor when subjected to varying magnetic field.
Any valid additional information e.g.

eddy current flows in a circular path or closed loops;
eddy current generates heat;
eddy current cannot flow through gaps or slots;
The currents move in such a direction as to oppose the change producing them.

Devices in which eddy currents are applied

When a steady current is allowed to pass through a uniform wire, equal lengths of the wire will have equal potential differences.

The p.d across a length of a wire is (directly) proportional to the length provided the wire has a uniform cross section.

Vector diagram showing
   VL   with arrow
VC   with arrow
   VR   with arrow

                          V² = VR² + (VL – VC)²

                        110² = 80² + (40 – Vc)²

                        110² -80²= (40 – Vc)²

(40 – Vc ) = (110 + 80) (110 – 80)

40 – VC = + 75.5

VC = 40 + 75.5

VC = 115.5V

            (ii)        VC = I X c
                       Xc = Vc /   =   1 /
                                    I             2π¦C                      2
                        ∴ C = I/2π¦VC          =    2 x 3.14 x 60 x 115.5

                        C = 0.0000459                         C = 45.9 x 10-6 F or 46mF

Question 15

(a) Briefly explain the following terms:
(i) emission line spectra;
(ii) line absorption spectra.

(b) Draw a labeled diagram showing the structure of a simple type of photocell and explain its mode of operation.

(d) An electron jumps from an energy level of -1.6eV to one of -1.4 eV in an atom. Calculate the energy and wavelength of the emitted radiation. [ h = 6.6 x 10-34 Js; c = 3.00 x 108 ms-1; eV = 1.6 x 10-19 J ]

ANSWER

(a)(i) Emission line spectra consist of distinct and separate bright lines of definite wavelengths on a dark background.

Any valid additional information e.g
–   They are obtained when light from a luminous source undergoes dispersion and is observed directly.
–   When an electron moves from one energy level to another energy level, line spectra are observed
– The colour of the spectra is a characteristic of the source.

(ii) Line absorption spectra are obtained when light passes through a cool gas and certain wavelengths of the light are absorbed. This gives series of dark lines, each corresponding to one of the wavelengths absorbed.
(b) Diagram

When radiation of appropriate frequency falls on the cathode (with photosensitive surface) electrons are emitted. Because the anode is positive with respect to the cathode, the electrons are attracted to the anode. The electrons flow completes the circuit and current flows.

(c)(i)   Wave properties of X-rays
           –      diffraction
           –      interference
           –      reflection
           –     refraction
           –     polarization

(ii) Uses of X-rays other than in medicine

to study crystal structures
to detect presence of cracks in welded parts
in artistic works for determining the authenticity of such works
at security check points to scan
X-ray spectroscopy for identifying isotopes of elements.

(d)

ΔE = Ei – Ef

= – 1.6 – (- 10.4)

= 8.8 e V or 1.41 x 10-18 J

                           E = hf   = hc/λ
                      or

                        λ   =   hc/E

= 6. 6 x 10-34 x 3 x108

8.8 x 1.6 x 10-19
λ = 1.4 x 10-7 m

Physics May/June 2009

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