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ConcepTest PowerPoints
Chapter 30
Physics: Principles with Applications, 6th edition
Giancoli
ConcepTest 30.1 The Nucleus
There are 82 protons in a lead nucleus. Why doesn’t the lead nucleus burst apart?
1) Coulomb repulsive force doesn’t act inside the nucleus
2) gravity overpowers the Coulomb repulsive force inside the nucleus
3) the negatively charged neutrons balance the positively charged protons
4) protons lose their positive charge inside the nucleus
5) none of the above
ConcepTest 30.1 The Nucleus
There are 82 protons in a lead nucleus. Why doesn’t the lead nucleus burst apart?
The Coulomb repulsive force is overcome by the even stronger nuclear force!
1) Coulomb repulsive force doesn’t act inside the nucleus
2) gravity overpowers the Coulomb repulsive force inside the nucleus
3) the negatively charged neutrons balance the positively charged protons
4) protons lose their positive charge inside the nucleus
5) none of the above
ConcepTest 30.2a Binding Energy I
What weighs more, an electron and a proton, or a hydrogen atom?
1) electron and proton
2) hydrogen atom
3) both the same
ConcepTest 30.2a Binding Energy I
What weighs more, an electron and a proton, or a hydrogen atom?
1) electron and proton
2) hydrogen atom
3) both the same
The total energy (or mass) of a hydrogen atom must be less than the energies (or masses) of the electron plus the proton individually in order for the electron to be bound.
ConcepTest 30.2b Binding Energy II
What is the total energy (or mass) of the hydrogen atom in its ground state?
1) 13.6 eV
2) mpc2 + mec2 + 13.6 eV
3) mpc2 + mec2
4) mpc2 + mec2 – 13.6 eV
ConcepTest 30.2b Binding Energy II
What is the total energy (or mass) of the hydrogen atom in its ground state?
The total energy (or mass) of a hydrogen atom must be less than the energies (or masses) of the electron plus the proton individually in order for the electron to be bound. The mass difference is the binding energy.
1) 13.6 eV
2) mpc2 + mec2 + 13.6 eV
3) mpc2 + mec2
4) mpc2 + mec2 – 13.6 eV
ConcepTest 30.2c Binding Energy III
On a balance scale, you put 2 neutrons and 1 proton on one side and you put a tritium nucleus (3H) on the other. Which side weighs more?
1) the 2 neutrons and 1 proton
2) the tritium nucleus
3) they both weigh the same
4) it depends on the specific
isotope of tritium
ConcepTest 30.2c Binding Energy III
On a balance scale, you put 2 neutrons and 1 proton on one side and you put a tritium nucleus (3H) on the other. Which side weighs more?
The mass of the 2 neutrons and 1 proton is less when they are bound together as tritium. The mass difference is the binding energy.
need to add 8.5 MeV to balance scale
1) the 2 neutrons and 1 proton
2) the tritium nucleus
3) they both weigh the same
4) it depends on the specific
isotope of tritium
ConcepTest 30.3 Separation Energy
Does it take more energy to remove one proton or one neutron from 16O?
1) removing a proton takes more energy
2) removing a neutron takes more energy
3) both take the same amount of energy
ConcepTest 30.3 Separation Energy
Does it take more energy to remove one proton or one neutron from 16O?
Removing a proton takes less energy because the repulsive Coulomb force between positively charged protons helps to push the proton out of the nucleus. Remember that neutrons are uncharged.
1) removing a proton takes more energy
2) removing a neutron takes more energy
3) both take the same amount of energy
ConcepTest 30.4a Particle Emission I
A radioactive substance decays and the emitted particle passes through a uniform magnetic field pointing into the page as shown. In which direction are alpha particles deflected?
B field
source
ConcepTest 30.4a Particle Emission I
A radioactive substance decays and the emitted particle passes through a uniform magnetic field pointing into the page as shown. In which direction are alpha particles deflected?
Using the right-hand rule, we find that positively charged particles (alpha particles) are deflected to the left.
B field
source
ConcepTest 30.4b Particle Emission II
A radioactive substance decays and the emitted particle passes through a uniform magnetic field pointing into the page as shown. In which direction are gamma rays deflected?
B field
source
ConcepTest 30.4b Particle Emission II
A radioactive substance decays and the emitted particle passes through a uniform magnetic field pointing into the page as shown. In which direction are gamma rays deflected?
Gamma rays are uncharged, so they will not be deflected by a magnetic field.
B field
source
Follow-up: What particles are bent to the right?
ConcepTest 30.5 Radioactive Decay Energy
A radioactive nucleus undergoes gamma decay. How large would you expect the energy of the emitted photon to be?
1) less than 13.6 eV
2) 13.6 eV
3) hundreds of eV
4) millions of eV
5) billions of eV
ConcepTest 30.5 Radioactive Decay Energy
A radioactive nucleus undergoes gamma decay. How large would you expect the energy of the emitted photon to be?
The binding energy of nuclei is of the order several MeV (millions of eV). So, we would expect the energy of gamma decay to be in the same ballpark.
1) less than 13.6 eV
2) 13.6 eV
3) hundreds of eV
4) millions of eV
5) billions of eV
Follow-up: What process could release a photon with billions of eV?
ConcepTest 30.6a Alpha Decay I
A uranium nucleus 238U (initially at rest) decays into a thorium nucleus 234Th and an alpha particle. Which one has the greater momentum?
1) the 234Th nucleus
2) the alpha particle
3) both the same
ConcepTest 30.6a Alpha Decay I
A uranium nucleus 238U (initially at rest) decays into a thorium nucleus 234Th and an alpha particle. Which one has the greater momentum?
By momentum conservation, they must have the same magnitude of momentum since the initial momentum was zero.
1) the 234Th nucleus
2) the alpha particle
3) both the same
Follow-up: In what directions are the two products emitted?
ConcepTest 30.6b Alpha Decay II
A uranium nucleus 238U (initially at rest) decays into a thorium nucleus 234Th and an alpha particle. Which one has the greater velocity?
1) the 234Th nucleus
2) the alpha particle
3) both the same
ConcepTest 30.6b Alpha Decay II
A uranium nucleus 238U (initially at rest) decays into a thorium nucleus 234Th and an alpha particle. Which one has the greater velocity?
1) the 234Th nucleus
2) the alpha particle
3) both the same
The momentum is mv and is the same for both, but the alpha particle has the smaller mass, so it has the larger velocity.
ConcepTest 30.6c Alpha Decay III
A uranium nucleus 238U (initially at rest) decays into a thorium nucleus 234Th and an alpha particle. Which one has the greater kinetic energy?
1) the 234Th nucleus
2) the alpha particle
3) both the same
ConcepTest 30.6c Alpha Decay III
A uranium nucleus 238U (initially at rest) decays into a thorium nucleus 234Th and an alpha particle. Which one has the greater kinetic energy?
1) the 234Th nucleus
2) the alpha particle
3) both the same
The kinetic energy 1/2 mv2 can be written as KE = p2/2m. The momentum is the same for both, but the alpha particle has the smaller mass, so it has the larger KE.
ConcepTest 30.7 Beta Decay
What element results when 14C undergoes beta decay?
1) 15C
2) 15N
3) 14C
4) 14N
5) 15O
ConcepTest 30.7 Beta Decay
What element results when 14C undergoes beta decay?
The reaction is:
1) 15C
2) 15N
3) 14C
4) 14N
5) 15O
Inside the nucleus, the reaction n p + e- + has occurred, changing a neutron into a proton, so the atomic number Z increases by 1. However the mass number (A = 14) stays the same.
Follow-up: How would you turn 14C into 15N?
ConcepTest 30.8a Radioactive Decay Law I
You have 16 kg of a radioactive sample with a certain half-life of 30 years. How much is left after 90 years?
(1) 8 kg
(2) 4 kg
(3) 2 kg
(4) 1 kg
(5) nothing
ConcepTest 30.8a Radioactive Decay Law I
You have 16 kg of a radioactive sample with a certain half-life of 30 years. How much is left after 90 years?
The total time (90 years) is 3 half-lives. After one half-life 8 kg left. After two half-lives 4 kg left. After three half-lives 2 kg left.
(1) 8 kg
(2) 4 kg
(3) 2 kg
(4) 1 kg
(5) nothing
Follow-up: When will the sample be reduced to nothing?
ConcepTest 30.8b Radioactive Decay Law II
You have 12 kg of a radioactive substance. Ten years later, you find that you only have 3 kg left. Find the half-life of the material.
(1) 20 years
(2) 10 years
(3) 7.5 years
(4) 5 years
(5) 2.5 years
ConcepTest 30.8b Radioactive Decay Law II
You have 12 kg of a radioactive substance. Ten years later, you find that you only have 3 kg left. Find the half-life of the material.
After one half-life 6 kg left. After two half-lives 3 kg left. So if the total time is 10 years, then the half-life must be 5 years. (2 half-lives = 10 years)
(1) 20 years
(2) 10 years
(3) 7.5 years
(4) 5 years
(5) 2.5 years
Follow-up: How much of the sample is left after another 10 years?
ConcepTest 30.8c Radioactive Decay Law III
You have 400 g of a radioactive sample with a half-life of 20 years. How much is left after 50 years?
1) more than 100 g
2) 75 - 100 g
3) 75 g
4) 50 - 75 g
5) less than 50 g
ConcepTest 30.8c Radioactive Decay Law III
You have 400 g of a radioactive sample with a half-life of 20 years. How much is left after 50 years?
Total time (50 years) is 2 1/2 half-lives.
After one half-life 200 g left
After two half-lives 100 g left.
After three half-lives 50 g left.
So after 2 1/2 half-lives 75 g left ?
No!! Exponential function is not linear!
70.7 g left
N = Noe–(0.693 / T1/2)t
1) more than 100 g
2) 75 - 100 g
3) 75 g
4) 50 - 75 g
5) less than 50 g
ConcepTest 30.8d Radioactive Decay Law IV
You have two samples, A (T1/2 = 10 yr) and B (T1/2 = 20 yr) with initially different amounts. The initial amount of sample A is 64 kg, while the amount of sample B is unknown. If you observe that the 2 amounts are equal after 40 years, what is the initial amount of B?
1) 64 kg
2) 32 kg
3) 16 kg
4) 8 kg
5) 4 kg
ConcepTest 30.8d Radioactive Decay Law IV
You have two samples, A (T1/2 = 10 yr) and B (T1/2 = 20 yr) with initially different amounts. The initial amount of sample A is 64 kg, while the amount of sample B is unknown. If you observe that the 2 amounts are equal after 40 years, what is the initial amount of B?
For sample A, after 40 years (4 half-lives), there is 4 kg left. Now work backwards from there, for sample B: 40 years is 2 half-lives, so sample B initially had 16 kg.
1) 64 kg
2) 32 kg
3) 16 kg
4) 8 kg
5) 4 kg
Follow-up: When will the samples again have equal amounts?
ConcepTest 30.9a Activity and Half-Life I
You have 10 kg each of a radioactive sample A with a half-life of 100 years, and another sample B with a half-life of 1000 years. Which sample has the higher activity?
1) sample A
2) sample B
3) both the same
4) impossible to tell
ConcepTest 30.9a Activity and Half-Life I
You have 10 kg each of a radioactive sample A with a half-life of 100 years, and another sample B with a half-life of 1000 years. Which sample has the higher activity?
If a sample has a shorter half-life, this means that it decays more quickly (larger decay constant ) and therefore has a higher activity:
In this case, that is sample A.
1) sample A
2) sample B
3) both the same
4) impossible to tell
N/t = – N
Follow-up: What is the ratio of activities for the two samples?
ConcepTest 30.9b Activity and Half-Life II
The same amount of two different radioactive samples A and B is prepared. If the initial activity of sample A is 5 times larger than that of sample B, how do their half-lives compare?
1) T1/2 of A is 5 times larger than B
2) half-lives are the same
3) T1/2 of A is 5 times smaller than B
ConcepTest 30.9b Activity and Half-Life II
The same amount of two different radioactive samples A and B is prepared. If the initial activity of sample A is 5 times larger than that of sample B, how do their half-lives compare?
A larger activity means that a sample decays more quickly, and this implies a shorter half-life.
1) T1/2 of A is 5 times larger than B
2) half-lives are the same
3) T1/2 of A is 5 times smaller than B