Exam Lessons Physics


Radioactivity is the spontaneous decay or disintegration of  the nucleus of the atom of an element during which it emits α, β or γ rays or a combination of any or all the three and energy ( or heat).

If a small sample of radium is placed at the bottom of a small hole drilled in a block of lead.  The radiation emitted from this radium emerged from the hole in a narrow beam. if  the rays were subjected to a strong magnetic field placed at the side of a beam. A photographic plate situated at appropriate sides to receive the rays showed that the paths of some rays were bent to the right, some to the left and some went straight on, unbent.

Electrically charged plates placed at the side of the beam gave the same effect.  The radiations that was bent towards the negative electric plate or the south pole of the magnetic field are called the Alpha particles ( α –particles )

The radiations deflected towards the positive electric plate or the North pole of the magnetic field is called Beta particles (β – particles ). The radiation that was neither affected by the electric or magnetic field is called gamma rays (γ ). They are actually electromagnetic radiations.

RadiationAlpha-particlesBeta ParticlesGamma – rays
NatureHelium nuclei 42HeHigh Energy electronsElectromagnetic wave of short wavelength
Velocity 5 – 7% speed of lightTravel at approx. speed of lightTravel at speed of light
Effects of magnetic fieldSlightly deflected in a magnetic field (+ve)Strongly deflected in a magnetic field (-ve)No effects
Ionizing magnetic fieldLarge, cause heavy ionizationMediumSmall
Penetrating powerLittle penetrating power e.g thin sheetGood penetrating power e.g aluminiumHigh penetrating power e.g leads

Radioactive Decay; Half-life, Decay Constant

Radioactivity is a spontaneous process. It goes on independent of external control, it is not affected by temperature, or pressure or by chemical treatment. It is a random process as no one can predict which atom will disintegrate at a given time.

The half-life of a radioactive element is the time taken for half of the atoms initially present in the element to decay.
  The rate of decay of radioactive elements is found to be proportional to the number of atoms of the material present.  If there are N atoms of a radioactive element present at a time, ti, then the probable number of disintegration per unit time or activity.


  1. With the aid of a diagram, explain the anomalous behaviour of water
  2. Describe an experiment to determine the apparent cubic expansivity of a liquid


New School Physics pg 468-471


1.   The phenomemon of radioactivity was first discovered by (A) Marie  Curie   (B) J.J. Thompson    (C) Henri Becquerel  (D) Nent Bohr (E) Enrico Fermi

2.   A radioactive substance has a half-life of 3 days. If a mass of 1.55g of this substance is left after decaying for 15days,dertermine the original value of the mass (A) 49.6g    (B) 37.2g   (C) 24.8g   (D) 12.4g

3.  Which of the following is usually used to cause fission in an atomic reactor? (A) alpha particles    (B) beta particles  (C ) electrons  (D) neutrons

4.  A substance has a half-life 30 mins after 6mins the count rate was observed to be 400. What was its count rate at zero time. (A) 200 (B) 1200   (C) 1600 (D) 2400

5.   A nuclide  20284Y emits in succession an α-particle and β-particle. The atomic number of the resulting nuclide is (A) 198         (B) 83        (C  ) 82     (D) 80.


1    (a) Define radioactivity; half-life and decay constant.

(b) Write down the relation between half-life and decay constant

(c) In 180 minutes, the activity of a certain radioactive substance falls to one –eight of its original value. Calculate its half-life.

2    (a)A nuclide X emits β-particle to form a daughter nuclide Y. write a nuclear equation to illustrate the charge conservation.

(b) the isotope of a nuclide has a half-life of 5.4X103s.Calculate its decay constant.