Quantum & Nuclear Physics

Question 1

State what is meant by a photon.

Answer 1

It’s a quantum of energy of electromagnetic radiation.

Question 2

Describe the photoelectric effect

Answer 2

It’s the emission of electrons from a metal surface when electromagnetic radiation is incident on electrons

Question 3

With reference to the photoelectric effect, state what is meant by the threshold frequency.

Answer 3

The minimum frequency of electromagnetic radiation for electrons to be emitted from the surface.

Question 4

State what is meant by the threshold wavelength

Answer 4

Longest wavelength of an electromagnetic radiation for electrons to be emitted from the surface of a metal

Question 5

State what is meant by the work function energy of a metal.

Answer 5

The minimum amount of photon energy needed to remove an electron from the surface of a metal (with zero kinetic energy).

Question 6

State three pieces of evidence provided by the photoelectric effect for a particulate nature of electromagnetic radiation.

Answer 6

1. Energy of emitted electrons depends on frequency
2. Energy of emitted electrons does not depend on intensity
3. Rate of emission of electrons depends on intensity at constant frequency
4. Instantaneous emission of electrons
5. Increasing the frequency at constant intensity decreases the rate of emission of electrons.

Question 7

Describe two phenomena associated with the photoelectric effect that cannot be explained using a wave theory of light.

Answer 7

1. Maximum energy of electrons depends on frequency.
2. Maximum energy of electrons does not depend on intensity.

Question 8

Electromagnetic radiation of a single constant frequency is incident on a metal surface. This causes an electron to be emitted.

Explain why the maximum kinetic energy of the electron is independent of the intensity of the incident radiation, whereas the photoelectric current is proportional to intensity.

Answer 8

The kinetic energy of the electron depends on the energy of one photon, that’s determined by the frequency.

While Intensity does not determine energy of a photon but determines number of photons per unit time.

Therefore, Intensity of radiation is independent from the kinetic energy of the electron.

Question 9

Describe the appearance of a visible line emission spectrum, as seen using a diffraction grating.

Answer 9

Colored lines with a mostly dark background.

Question 10

Explain why a spectrum of light contains a number of dark lines when viewed using a diffraction grating.

Question 11

Explain how the emission spectrum provides evidence for the existence of discrete energy levels for the electron in a hydrogen atom.

Answer 11

1. Energy of a photon has a corresponding frequency
2. Discrete energy changes imply discrete energy levels.
3. Changes in electron energy levels emits a single photon.
4. Photon energy = Difference in energy levels
5. Discrete frequencies must have come from discrete energy gaps.

Question 12

State what is meant by the de broglie wavelength.

Answer 12

The wavelength associated with a moving particle.

Question 13

State what is meant by radioactive decay.

Answer 13

Spontaneous emission of particles from an unstable nucleus.

Question 14

“Radioactive decay is spontaneous and random”

What do the terms spontaneous and random mean in that context.

Answer 14

Random: cannot predict when a particular nucleus will decay next,

Spontaneous: not affected by external factors

Question 15

Explain what is meant by the binding energy of a nucleus.

Answer 15

It’s the minimum energy required to seperate the nucleons to infinity.

Question 16

Define the radioactive decay constant.

Answer 16

The probability of decay of a nucleus per unit time.

Question 17

Define radioactive half-life.

Answer 17

It’s the time for the number of atoms to be reduced to half of the original value.

Question 18

State what is meant by nucleus fusion and nuclear fission.

Answer 18

Fusion: two nuclei combine to form a single nucleus, this process releases energy.

Fission: one large nucleus divides to form smaller nuclei of similar size, where total binding energy increases

Question 19

Explain what is meant by gamma radiation (γ - radiation).

Answer 19

Photons of electromagnetic radiation emitted from nuclei.

Question 20

Recall information from the graph of binding energies for nuclei of different masses.

Answer 20

1. Binding energy per nucleon is maximum at A=56 (Iron)
2. Products of splitting Fe-56 would have lower total binding energies

Question 21

State what is meant by a mass defect.

Answer 21

Difference between mass of nucleus and the total mass of seperate nucleons which are seperated to infinity.