Question
When 10B is bombarded by neutrons, a neutron can be absorbed and an alpha particle (4He) emitted. If the 10B target is stationary, the kinetic energy of the reaction products is equal to the.
A) kinetic energy of the incident neutron
B) total energy of the incident neutron
C) energy equivalent of the mass decrease in the reaction
D) energy equivalent of the mass decrease in the reaction plus the kinetic energy of the incident neutron
▶️Answer/Explanation
Ans:D
Solution: This is a mass defect question. The energy released in the reaction is equal the equivalence of the missing mass comparing the products and reactants.
Question
Correct statements about the binding energy of a nucleus include which of the following?
I. It is the energy needed to separate the nucleus into its individual protons and neutrons.
II. It is the energy liberated when the nucleus is formed from the original nucleons.
III. It is the energy equivalent of the apparent loss of mass of its nucleon constituents.
A) I only
B) III only
C) I and II only
D) I, II, and III
▶️Answer/Explanation
Ans:D
Solution: These are all true statements about binding energy.
Question
An astronaut in a rocket is passing by a space station at a velocity of 0.33 c. Looking out the window, the astronaut sees a scientist on the space station fire a laser at a target. The laser is pointed in the same direction that the astronaut is traveling. On which of the following observations will the astronaut and scientist agree?
(A) The length of the rocket
(B) The time it takes the laser to hit the target
(C) The speed of the laser beam
(D) The astronaut and scientist will not agree on any of these measurements.
▶️Answer/Explanation
Ans:C
Special relativity tells us that when two observers are moving relative to each other, they will not necessarily agree on length and time. This becomes evident when we get up near the speed of light. We start to easily notice the effect around 0.1 c and faster. The only constant that all observers will agree on is the speed of light.
Question
A very slow-moving positron interacts with a stationary electron. Which of the following statements correctly describes a possible outcome of this reaction and why it would occur?
(A) Conservation of mass indicates that if a single new particle were created in the reaction, it must have a total mass equal to the combined masses of the electron and positron.
(B) Conservation of charge indicates that all new particles created in the reaction would have no electric charge.
(C) Conservation of momentum indicates that two identical gamma rays moving off in opposite directions could be created.
(D) Conservation of energy indicates that the antimatter positron could annihilate into energy, leaving the stationary electron behind.
▶️Answer/Explanation
Ans:C
A and D are not correct. The matter and antimatter particles would annihilate into energy (EM waves). In addition, neither conservation of mass nor conservation of energy properly model the interactions of the nano-world. We must use the new physics model of conservation of mass/energy. Conservation of charge dictates only that the final products must add to the same net charge as the original products. The initial momentum of the particles is approximately zero. Two identical gamma rays traveling off in opposite directions would satisfy conservation of momentum.
Question
Tritium is an isotope of hydrogen consisting of one proton and two neutrons. The isotope has a mass of 5.008 x 10-27 kg. The mass of a proton is 1.673 x 10-27 kg and a neutron has a mass of 1.675 x 10-27 kg. What is the binding energy of tritium?
(A) 1.500 X 10-29 J
(B) 1.350 X 10-12 J
(C) 4.507 X 10-10 J
(D) 4.521 X 10-10 J
Answer/Explanation
Ans:B
The binding energy is the mass defect, the difference in the mass of the constituent components and the mass of the
nucleus, multiplied by c2 • That mass defect is
mp + 2mn – mTr = 1.673 x 10-27 + 2 * 1.675 x 10-27 – 5.008 x 10-27 = 1.500 x 10-29 kg
E = Δmc2 = 1.500 x 10-29 * (3 x 108) 2