Free KCSE Physics Topical Questions KNEC Standards

Define Radio-activity

21/10/2021

(a) Define Radio-activity (2mks)

Radioactivity is the spontaneous disintegration of a nucleus releasing
alpha, beta and gamma radiations accompanied by the release of energy

(b) When carrying out experiments with radioactive substances the source should never be held with bare hands but with forceps. Explain? (1mk)

Radioactive substances are harmful to body cells when ingested

(i) Plot on the graph paper provided the graph of the count rate (y-axis) against time

A graph of count Rate against time(s)

(ii) From the graph, determine the half-life of Radon-220 (1mk)

Half-life of radon 220=54 sec + 2 (showing on graph a must)

(e) The diagram in the figure below shows paths taken by three radiations A,B and C from a radioactive isotope through an electric field.

(i) State the charge on plate Y (1mk)

Negative

(ii) Identify the radiations A and C (2mks)

A- Beta particles
B-Alpha particles

(iii) Give a reason why C deviates more than A (1mk)

C more massive than A

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The following is a part of a radio-active series

20/10/2021

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The following is a part of a radio-active series

Identify the radiation r, find the values of c and d (3mks)

r

r is βeta particle

c

c is 206

d

d is 82

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

15/10/2020

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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 2019PP2QN12

31/7/2020

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K.C.S.E Physics Q & A - MODEL 2019PP2QN12

Uranium decays as shown below.

Name the particle X that is emitted during this decay.

answer

Apha particle

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Define radioactivity

29/7/2020

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K.C.S.E Physics Q & A - MODEL 2018PP2QN08

Define radioactivity

answer

Spontaneous disintegration of atomic nuclei by emission of radiations.

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K.C.S.E Physics Q & A - MODEL 2017PP2QN10

28/7/2020

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K.C.S.E Physics Q & A - MODEL 2017PP2QN10

Write an equation to show how an element ^A _Z X decay to element Y by emitting a beta particle.

answer

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K.C.S.E Physics Q & A - MODEL 2016PP2QN17

27/7/2020

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K.C.S.E Physics Q & A - MODEL 2016PP2QN17

When a radiation was released into a diffusion cloud chamber, short thick tracks were observed. State with a reason, the type of radiation that was detected.
(b)The half-life of an element X is 3.83 days. A sample of this element is found to have an activity rate of 1.6 x 10^3 disintegrations per second at a particular time.
Determine its activity rate after 19.15 days.
(c) State what is meant by an extrinsic semiconductor.
(d)Figure 11, shows a depletion layer in an unbiased p-n junction

State how a battery can be used to make the depletion layer narrower.
(e)Figure 12, shows an incomplete circuit of a full wave rectifier.

(i) Draw in the figure two more diodes to complete the circuit should be obtained.

answers

(a)Alpha radiation. This radiation has a short range with intense ionization hence thick tracks

(c)It is a semiconductor in which impurities have been added to change conductivity

(d) By connecting it in a forward biased mode

(ii)Across QS

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K.C.S.E Physics Q & A - MODEL 2016PP2QN08

27/7/2020

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K.C.S.E Physics Q & A - MODEL 2016PP2QN08

The following is part of radioactive decay series

Determine the values of a and b

answers

a = 234
b= 82

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K.C.S.E Physics Q & A - MODEL 2015PP2QN16

24/7/2020

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K.C.S.E Physics Q & A - MODEL 2015PP2QN16

(a) It is observed that alpha particles have a lower penetrating power than beta particles. Explain this observation.
(b) A radioactive substance has a half life of 12 years. Determine the time it would take to decay to 12.5% of its original value.

(c) A Geiger Müller (GM) tube is used for detecting radiations from a radioactive source.
State the function of:
(i) the mica window;
(ii) bromine gas in the tube.
(d) (i) In a diffusion chamber, explain why some of the tracks formed are observed to be;
(I) Short,
(II) Straight.
(ii) State two advantages of using a GM tube instead of a diffusion cloud chamber to detect radiations from radioactive substances.

answers

(a) Alpha particles are heavier and move at lower speeds hence less penetrating power than
Beta particles which are lighter and move faster

(c) (i) allows the radiations into the tube
(ii) absorbs kinetic energy of positive ions so that they do not cause secondary ionization in the tube
(d) (i) Short - They ionize heavily loosing most of the energy hence cannot travel far.
Straight - They are massive compared to air molecules hence collision with air molecules cannot change their path.
(ii) GM is easily portable than a cloud chamber.
GM is more sensitive.
GM tube detects radiation at very low intensity and cloud chamber cannot detect radiation at very low intensity.

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K.C.S.E Physics Q & A - MODEL 2014PP2QN09

23/7/2020

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K.C.S.E Physics Q & A - MODEL 2014PP2QN09

Radium undergoes radioactive decay by emitting an alpha particle to form a daughter nuclide Q as in the reaction:

Determine the values of:
(a) x
(b) y

answer

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K.C.S.E Physics Q & A - MODEL 2013PP2QN07

23/7/2020

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K.C.S.E Physics Q & A - MODEL 2013PP2QN07

The equation below represents a nuclear reaction in which two deuterium nuclei fuse to form Helium and X.

(a) Determine the values of a and b.
(b) Identify X.

answers

a=1 and b=0
x = neutron

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K.C.S.E Physics Q & A - MODEL 2012PP2QN06

21/7/2020

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K.C.S.E Physics Q & A - MODEL 2012PP2QN06

Figure 6, shows a narrow beam of radiation from a radioactive source, incident to a post card. The emergent radiation passes through a magnetic field which is perpendicular to the plane of the paper, and into the paper.

A detector moved along line AC only detects radiations B and C. State the types of radiation detected.

answer

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K.C.S.E Physics Q & A - MODEL 2011PP2QN19

20/7/2020

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K.C.S.E Physics Q & A - MODEL 2011PP2QN19

(a) When a radiation was released into a diffusion cloud chamber, short thick tracks were observed. State with a reason, the type of radiation that was detected.
(b) The half-life of an element X is 3.83 days. A sample of this element is found to have an activity rate of 1.6 x 103 disintegrations per second at a particular time.
Determine its activity rate after 19.15 days.
(c) State what is meant by an extrinsic semiconductor.
(d) Figure 12, shows a depletion layer in an unbiased p-n junction.

State how a battery can be used to make the depletion layer narrower.
(e) Figure 13, shows an incomplete circuit of a full wave rectifier

(I) Draw in the figure two more diodes to complete the circuit.
(ii) Show on the figure the points across which the output of the rectifier should obtained.

answers

(c) A semiconductor in which impurities have been added to change conductivity.
(d) By connecting it in forward biased mode (i.e. P to + and n to -)

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K.C.S.E Physics Q & A - MODEL 2010PP2QN09

19/7/2020

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K.C.S.E Physics Q & A - MODEL 2010PP2QN09

The initial mass of a radioactive substance is 20g. The substance has a half-life of 5 years.
Determine the mass remaining after 20 years.

answer

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K.C.S.E Physics Q & A - MODEL 2009PP2QN17

17/7/2020

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K.C.S.E Physics Q & A - MODEL 2009PP2QN17

(a) Figure 11 shows the path of radiation from a radioactive source. The field is perpendicular to the paper and directed out of the paper.

Identify the radiation
(b) Radiation from a radioactive source enters a G.M tube
(i) State the effect of the radiation on the gas inside the tube
(ii) Explain hoe the large discharge current is created
(c) The following is a nuclear equation for a fission process resulting from the reaction of a neutron with a Uranium nucleus

(i) Determine the values of x and y
(ii) State the source of the energy released
(iii) Explain how this reaction is made continuous in a nuclear reactor

answers

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K.C.S.E Physics Q & A - MODEL 2008PP2QN14

16/7/2020

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K.C.S.E Physics Q & A - MODEL 2008PP2QN14

A radioactive isotope of copper decays to form an isotope of Zinc as shown below

Name the radiation emitted and give a reason for your answer
Radiation
Reason

ANSWER

Beta particle
Gain of an electron OR
Mass number has not changed but atomic number has increased by 1
Atomic number has increased by one
Nature will not affect the speed

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K.C.S.E Physics Q & A - MODEL 2007PP2QN13

15/7/2020

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K.C.S.E Physics Q & A - MODEL 2007PP2QN13

The following is part of radioactive decay series

Determine the values of a and b
a = ……………………………………. b= ………………………………

answer

a = 234
b= 82

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K.C.S.E Physics Q & A - MODEL 2006PP2QN15

14/7/2020

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K.C.S.E Physics Q & A - MODEL 2006PP2QN15

A nuclear reaction is represented by the following equation

Determine the values of a and b

answer

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K.C.S.E Physics Q & A - MODEL 2005PP2QN07

13/7/2020

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K.C.S.E Physics Q & A - MODEL 2005PP2QN07

(a) What is meant by radioactivity
(b) With an aid of a labeled diagram explain the working of Geiger Muller tube as a detector of radiation
(c) In an experiment to determine the half of a certain radioactivity substance, the activity in disintegrations per minute was measured for sometime. Table 1 shows the results obtaine

On the grid plot a suitable graph and sue it to determine the half life t ½ of the substance
(d) At time t = 40 minutes, the activity of a sample of a certain radioactive isotope with a half life 12 minutes if found to be 480 disintegration per minute.
Determine the time which activity was 3840 disintegrations per minute

answers

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K.C.S.E Physics Q & A - MODEL 2005PP1QN36

13/7/2020

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K.C.S.E Physics Q & A - MODEL 2005PP1QN36

The following represents a nuclear reaction involving the nuclide polonium P

Identify m, n and X
m
n
X

ANSWERS

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K.C.S.E Physics Q & A - MODEL 2004PP2QN03

12/7/2020

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K.C.S.E Physics Q & A - MODEL 2004PP2QN03

Figure 4 shows the cross-section of a diffusion cloud chamber used to detect radiation from radioactive sources.

a) i) State one function of each of the following:
Alcohol
solid Co2
ii) When radiation from the source enters the chamber, some white traces are observed. Explain how these traces are formed and state how the radiation is identified.
iii) A leaf electroscope can also be used as a detector of radiation. State two advantages of the diffusion cloud chamber over the leaf electroscope as a detector.
b) i) Two samples of the same radioactive material have initial masses M and 2M respectively. On the axes provided, sketch the graph of activity versus time for each sample. Label the graph for each sample.

ii) A radioactive sample of half-life 130 days initially has 1.0 x 10²c radioactive atoms. Determine the number of radioactive atoms that have decayed after 390 days.

answers

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K.C.S.E Physics Q & A - MODEL 2003PP1QN18

10/7/2020

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K.C.S.E Physics Q & A - MODEL 2003PP1QN18

The following equation shows part of a radioactive decay process.

Name the radiation x.

answer

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K.C.S.E Physics Q & A - MODEL 2002PP2QN02

10/7/2020

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K.C.S.E Physics Q & A - MODEL 2002PP2QN02

Fig. 3. Shows the path of radiation form a radioactive source after entering a magnetic field. The magnetic field is directed into the paper and is perpendicular to the plane of the paper as shown in the figure

Identify the radiation
b) Below is a nuclear reactio

i) Identify radiation K
ii) Determine the values of X and Y.
(c) Fig 3 shows a device for producing metal foils of constant thickness. Any change in thickness can be detected by the Geiger tube and recorded by the Geiger. The pressure adjusted by the roller is then adjusted to keep the thickness constant.

(i) State the change in the metal foil that will lead to a decrease in the Geiger counter reading
(ii) Give a reason for your answer in c(i) above
(iii) State the change in the roller pressure that should be made as a result of this decrease in the Geiger counter reading.
(iv) Give a reason for your in (c) (iii) above
(v) Explain why a source emitting (alpha) particles only would not be suitable for this device.
(vi) Explain why a radioactive source of a half-life of 1600 years is more suitable for use in the device than one of a half-life of 8 minutes.

answers

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K.C.S.E Physics Q & A - MODEL 2002PP1QN31

10/7/2020

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K.C.S.E Physics Q & A - MODEL 2002PP1QN31

A nucleus is represents by 10742x. State the number of neutrons in the nucleus.

answer

107 – 42 = 65

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K.C.S.E Physics Q & A - MODEL 2002PP1QN27

10/7/2020

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K.C.S.E Physics Q & A - MODEL 2002PP1QN27

Cobalt 60 is a radio isotope that has a half – life of 5.25 years. What fraction of the original atoms in a sample will remain after 21 years?

$Cobalt 60 is a radio isotope that has a half – life of 5.25 years. What fraction of the original atoms in a sample will remain after 21 years?$

answer

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<<Previous

​(a) Define Radio-activity (2mks)

​(b) When carrying out experiments with radioactive substances the source should never be held with bare hands but with forceps. Explain? (1mk)

​(i) Plot on the graph paper provided the graph of the count rate (y-axis) against time

​(ii) From the graph, determine the half-life of Radon-220 (1mk)

​(e) The diagram in the figure below shows paths taken by three radiations A,B and C from a radioactive isotope through an electric field.

​(i) State the charge on plate Y (1mk)

​(ii) Identify the radiations A and C (2mks)

​(iii) Give a reason why C deviates more than A (1mk)

​The following is a part of a radio-active series

​Identify the radiation r, find the values of c and d (3mks)

r

c

d

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

K.C.S.E Physics Q & A - MODEL 2019PP2QN12

Define radioactivity

K.C.S.E Physics Q & A - MODEL 2017PP2QN10

Write an equation to show how an element A Z X decay to element Y by emitting a beta particle.

K.C.S.E Physics Q & A - MODEL 2016PP2QN17

K.C.S.E Physics Q & A - MODEL 2016PP2QN08

K.C.S.E Physics Q & A - MODEL 2015PP2QN16

K.C.S.E Physics Q & A - MODEL 2014PP2QN09

K.C.S.E Physics Q & A - MODEL 2013PP2QN07

K.C.S.E Physics Q & A - MODEL 2012PP2QN06

K.C.S.E Physics Q & A - MODEL 2011PP2QN19

K.C.S.E Physics Q & A - MODEL 2010PP2QN09

K.C.S.E Physics Q & A - MODEL 2009PP2QN17

K.C.S.E Physics Q & A - MODEL 2008PP2QN14

K.C.S.E Physics Q & A - MODEL 2007PP2QN13

K.C.S.E Physics Q & A - MODEL 2006PP2QN15

K.C.S.E Physics Q & A - MODEL 2005PP2QN07

K.C.S.E Physics Q & A - MODEL 2005PP1QN36

K.C.S.E Physics Q & A - MODEL 2004PP2QN03

ii) A radioactive sample of half-life 130 days initially has 1.0 x 102c radioactive atoms. Determine the number of radioactive atoms that have decayed after 390 days.

K.C.S.E Physics Q & A - MODEL 2003PP1QN18

K.C.S.E Physics Q & A - MODEL 2002PP2QN02

K.C.S.E Physics Q & A - MODEL 2002PP1QN31

K.C.S.E Physics Q & A - MODEL 2002PP1QN27

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FREE NOVELS

(a) Define Radio-activity (2mks)

(b) When carrying out experiments with radioactive substances the source should never be held with bare hands but with forceps. Explain? (1mk)

(i) Plot on the graph paper provided the graph of the count rate (y-axis) against time

(ii) From the graph, determine the half-life of Radon-220 (1mk)

(e) The diagram in the figure below shows paths taken by three radiations A,B and C from a radioactive isotope through an electric field.

(i) State the charge on plate Y (1mk)

(ii) Identify the radiations A and C (2mks)

(iii) Give a reason why C deviates more than A (1mk)

The following is a part of a radio-active series

Identify the radiation r, find the values of c and d (3mks)

Write an equation to show how an element ^A _Z X decay to element Y by emitting a beta particle.

ii) A radioactive sample of half-life 130 days initially has 1.0 x 10²c radioactive atoms. Determine the number of radioactive atoms that have decayed after 390 days.