Lesson Notes By Weeks and Term v4 - SHS 3
NUCLEAR PHYSICS
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
NUCLEAR PHYSICS
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Physics
Class: SHS 3
Term: 1st Term
Week: 1
Grade code: 2.4.2.LI.2
Strand code: 4
Sub-strand code: 2
Content standard code: 2.4.2.CS.1
Indicator code: 2.4.2.LI.2
Theme: ATOMIC AND NUCLEAR PHYSICS
Subtheme: NUCLEAR PHYSICS
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
This lesson delves into the fascinating world of radioactive decay, focusing on three key concepts: Activity, Decay Constant, and Half-life. Understanding these concepts is not just about passing WASSCE; it is crucial for appreciating modern technology and its applications right here in Ghana. From medical treatments at Korle Bu Teaching Hospital that use radiation to kill cancer cells, to the work done by the Ghana Atomic Energy Commission (GAEC) in preserving food like yams and tomatoes, the principles of nuclear decay are at work. Today, we will learn how to quantify this decay process, focusing on the concept of half-life, which acts as a "clock" for radioactive materials.
A. The Nature of Radioactive Decay
Before we discuss half-life, let's remember what radioactive decay is. It is the process by which an unstable atomic nucleus loses energy by emitting radiation (alpha particles, beta particles, or gamma rays). This process is both spontaneous (it happens by itself, without any external influence like temperature or pressure) and random (it is impossible to predict which specific nucleus will decay next, but we can predict the behaviour of a large number of nuclei). B. Key Terms Activity (A) Definition: The activity of a radioactive sample is the rate at which its nuclei decay. It tells us how many nuclei are decaying per second. Formula: `A = λN`, where `λ` is the decay constant and `N` is the number of undecayed nuclei. Unit: The SI unit of activity is the Becquerel (Bq). `1 Bq = 1 decay per second`. Example: If a sample of radioactive material has an activity of 500 Bq, it means 500 of its nuclei are disintegrating every second. Decay Constant (λ) Definition: The decay constant is the probability that a single nucleus will decay per unit time. It is a measure of how quickly a radioactive isotope will decay. Characteristics: A large decay constant (λ) means a high probability of decay, so the substance decays quickly. A small decay constant (λ) means a low probability of decay, so the substance decays slowly. Unit: The unit of the decay constant is `per second (s⁻¹)`, `per minute (min⁻¹)`, `per year (yr⁻¹)`, etc., depending on the time scale. Half-Life (T½) Definition: The half-life of a radioactive isotope is the average time taken for half of the initial number of undecayed nuclei in a sample to decay. Explanation: Imagine you have 1,000,000 radioactive atoms. The half-life is the time it will take for 500,000 of them to decay, leaving 500,000. After another half-life, half of the remaining atoms will decay, leaving 250,000. This process continues.
| Number of Half-lives | Time Elapsed | Fraction Remaining | Percentage Remaining | | :------------------: | :----------: | :----------------: | :------------------: | | 0 | 0 | 1 | 100% | | 1 | 1 x T½ | 1/2 | 50% | | 2 | 2 x T½ | 1/4 | 25% | | 3 | 3 x T½ | 1/8 | 12.5% | | n | n x T½ | (1/2)ⁿ | 100 / (2ⁿ) % |
This shows an exponential decay relationship, which can be represented by the graph below: C. The Relationship Between Half-Life (T½) and Decay Constant (λ)