Unit notes and practice questions
Everything here is for unit 7 of the Grade 11 Physics textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.
The cards filed under this chapter on the Grade 11 Physics page. Open one at a time.
| Decay | Emitted particle | Change in A | Change in Z |
|---|---|---|---|
When you use it
Use when you must write a decay equation or find the daughter nucleus from a parent nucleus.
Watch out
Beta minus raises Z by one and leaves A unchanged, because a neutron becomes a proton and an electron. Beta plus lowers Z by one. Do not change A in any beta decay.
Drafted from Grade 11 Physics, page 287 onwards, then checked twice before it went up
When you use it
Use when a question asks for the binding energy per nucleon, after you have found the total binding energy of the nucleus.
Watch out
Divide by the mass number A, the number of nucleons, not by the atomic number Z. For oxygen-16 divide by 16, not 8. Example 7.2 divides 118.808 MeV by 16.
Drafted from Grade 11 Physics, page 287 onwards, then checked twice before it went up
When you use it
Use when a question gives the initial number of undecayed nuclei and a decay constant or half-life, and asks how many remain after a time t.
Watch out
Lambda is in s^-1, so t must be in seconds; convert years to seconds first. The half-life is t_{1/2} = 0.693/lambda. It is the time for half the nuclei to decay, not the lifetime of the nucleus.
Drafted from Grade 11 Physics, page 287 onwards, then checked twice before it went up
10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.
Question 1
According to the textbook, which subatomic particles make up the atomic nucleus?
Answer: A Protons and neutrons
Why
The atomic nucleus consists of positively charged protons and electrically neutral neutrons held together by the strong nuclear force.
Question 2
What is the empirical formula used to calculate the radius R of a nucleus with mass number A?
Answer: C R = R0 A^(1/3)
Why
The nuclear radius is given by R = R0 A1/3, where R0 is approximately 1.2 fm (1.2 × 10-15 m).
Question 3
Which statement correctly defines isotopes of an element?
Answer: B Atoms with the same number of protons but different numbers of neutrons
Why
Isotopes are members of a family of an element that have the same number of protons (same atomic number) but different numbers of neutrons.
Question 4
According to the textbook, how many megaelectron volts (MeV) are equivalent to 1 atomic energy unit (aeu)?
Answer: D 931.1 MeV
Why
The text states directly that 1 aeu = c2 × 1 amu = 931.1 MeV.
Question 5
At a range of 10-15 m, approximately how many times stronger is the strong nuclear force compared to electromagnetism?
Answer: B 100 times
Why
At a range of 10-15 m, the strong force is roughly 100 times as strong as electromagnetism, 106 times as strong as the weak force, and 1038 times as strong as gravitation.
Question 6
Which teaching hospital in Addis Ababa is highlighted in the textbook for using radioisotopes to treat cancer?
Answer: A Black Lion Specialized Hospital
Why
The textbook specifically notes that the Black Lion Specialized Hospital under Addis Ababa University uses radioisotopes for cancer treatment.
Question 7
When uranium-238 undergoes alpha decay, it emits an alpha particle (42He). What are the mass number A and atomic number Z of the daughter thorium (Th) nucleus?
Answer: C A = 234, Z = 90
Why
In alpha decay, the mass number decreases by 4 (238 - 4 = 234) and the atomic number decreases by 2 (92 - 2 = 90), giving 23490Th.
Question 8
A sample contains 400 mg of strontium-90 (half-life = 38.1 years). How many milligrams remain after 190.5 years (5 half-lives)?
Answer: D 12.5 mg
Why
After 5 half-lives (190.5 / 38.1 = 5), the remaining mass is 400 mg / (25) = 400 mg / 32 = 12.5 mg.
Question 9
Two nuclei have mass numbers in the ratio 27 : 125. What is the ratio of their nuclear radii?
Answer: B 3 : 5
Why
Because R is proportional to A1/3, the ratio of the radii is (27)1/3 : (125)1/3 = 3 : 5.
Question 10
In the fission reaction 235U + 10n → XY + 139Cs + 2 10n + energy, what is the mass number X?
Answer: A 95
Why
Balancing mass numbers: 235 + 1 = X + 139 + (2 × 1), which gives 236 = X + 141, so X = 95 (corresponding to Rb-95).
The app holds 12 questions on this unit, marked as you go. These are the first 10.
The whole Grade 11 Physics textbook, free as a PDF.
Ministry of Education edition, English, 338 pages
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