Free KCSE Physics Topical Questions KNEC Standards

K.C.S.E Physics Q & A - MODEL 2019PP2QN19

31/7/2020

K.C.S.E Physics Q & A - MODEL 2019PP2QN19

(a) Define half life as used in radioactivity.
(b) Figure 13 shows a device used to detect radioactivity by forming tracks when air is ionised.

(i) State the name of the device shown.
(ii) State how air in the device gets ionised.
(iii) Describe how tracks are formed
(c) (i) State two factors that determine the speed of the photoelectrons emitted from a metal surface
(ii) The energy of a photon of light is 2.21 eV. (electronic charge is 1.6x10^-19C and planks constant h is 6.63x10^-34Js)
I. Express this energy in joules.
II. Determine the frequency of the light that produces the photon.

answers

(a) Half life is the time taken for half the number of nuclides initially present in a sample to decay
(b). (ii) Expansion closed chamber
(ii) Due to collisions between the molecules from the radiations source and the air molecules
(iii) When the piston is pulled down air expands hence cools.
(c)(i) The work function of the metal
Energy of incident radiation frequency/wavelength

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K.C.S.E Physics Q & A - MODEL 2019PP2QN18

31/7/2020

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K.C.S.E Physics Q & A - MODEL 2019PP2QN18

(ii) Describe how a magnet can be demagnetised using the electrical method.
(b) You are provided with two iron bars P and Q. One of the bars is magnetized while the other is not. Explain how themagnetized bar can be identified without using a magnet or magnetic material.
(c)Figure 12 shows two identical copper coils X and Y placed close to each other. Coil X is connected to a DC power supply while coil Y is connected to a galvanometer.

Explain what is observed on the galvanometer When the Switch is closed.
(ii) State What is observed on the galvanometer when the switch is opened.
(iii) State what wou1d be observed if the number of turns in coil Y is doubled.

answers

(a) Place the bar magnet inside a solenoid placed in the an East-West direction then pass an alternating current through the solenoid
(b) Suspend each of the bars through its center of gravity and set it swinging in a horizontal plane shows a vertical axis through its centre
The magnetized bar will always come to rest in a north-south direction
(c) (i) The galvanometer pointer first deflects then moves back when the switch is closed a current builds up in a coil X and due to change influx linkage an emf is induced in coil Y making an induced current to flow through the galvanometer during the time current in coil X is rising. When current through coil X is steady no induced current flows through the galvanometer since there is no change in flux linkage
Therefore pointer deflects back
(ii) Pointer deflects in the opposite direction.
(iii) Magnitude of deflection increases

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K.C.S.E Physics Q & A - MODEL 2019PP2QN17

31/7/2020

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K.C.S.E Physics Q & A - MODEL 2019PP2QN17

(a) Figure 10 shows a ray of light travelling from glass to air.

Determine the:
(i) critical angle of the glass — air interface
(ii) refractive index of glass
(b) Apiece of metal is embedded at the centre of an ice block 15cm from the surface of the ice. Given that the refractive index of ice is 1.32, determine how far from the surface of the ice block the metal appears to be.
(c) Figure 11 shows, two rays of incident on a diverging lens.

Complete the ray diagram to show the path of the rays after passing through the lens.
(d) (i) State two differences between the human eye and a camera lens.
(ii) State the name of the part of the eye that enable the lens to focus images of objects at different distances.

ANSWERS

(d)(i) Eye has a variable focal length whole the focal length of camera lens is fixed
Image distance from the lens is almost constant in the eye while in a camera it can be varied
Eye lens is muscle while glass/clear plastic
(ii) Ciliary muscles

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K.C.S.E Physics Q & A - MODEL 2019PP2QN16

31/7/2020

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K.C.S.E Physics Q & A - MODEL 2019PP2QN16

(a) Define the following terms as used in waves:
(I) Amplitude
(ii) Wavelength
(b) Figure 8 shows water waves approaching a straight reflector at a speed of 40 cm/s.

(I) Draw on the diagram the reflected waves.
(ii) Given that the distance between the source and the reflector is 15 cm determine:
I. the wavelength of the waves
II. the frequency of the waves
(c) Figure 9 Shows light rays from two coherent sources S1 and S2 falling on a screen.
Dark and bright fringes are observed between A and B

(i) State how:
I. bright fringes are formed
II. dark fringes are formed
(ii) State what is observed when light of a higher frequency is used.

ANSWERS

(a)(i)Amplitude is the maximum displacement of a particle from the mean position
(ii)Wavelength is the distance between two successive points in a wave which are in phase
OR
Distance between two successive crests or troughs in a transverse
Wave/between rarefaction or compressions in a longitudinal wave.

(c)(I) due to constructive interference
(II) due to destructive interference
(ii) The fringes get closer
OR
When the frequency is higher the wavelength reduces hence the fringe separation decreases

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K.C.S.E Physics Q & A - MODEL 2019PP2QN15

31/7/2020

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K.C.S.E Physics Q & A - MODEL 2019PP2QN15

(a) On the axes provided, sketch a graph of capacitance against the area of overlap of the plates of a parallel-plate capacitor.

(b) (I) Draw a circuit diagram that may be used to investigate the discharging process of a capacitor.
(ii) On the axes provided sketch the graph of potential difference between the plates against time for

the discharging process.
(c) Figure 7 shows three capacitors connected to a 6V battery.