Modern Physics Internal - Explanations - Level 3

Question 1

How can we demonstrate the photoelectric effect?

- Summary

Answer 1

UV light falls on the Zinc plate on an electroscope, the gold leaves on the negatively charged plate converge, and the leaves on the positively charged plate diverge.

Question 2

How can we demonstrate the photoelectric effect?

- Steps

Answer 2

Assuming the plate starts negatively charged

1. When light strikes the metal plate, the photoelectric effect occurs (describe photoelectric effect)
2. Because of this loss of electrons, the plate becomes less negatively charged. This causes the repulsive forces between the leaves to become weaker.
3. As a result, the leaves will come closer together.

Question 3

What does the intensity of incident radiation affect?

Answer 3

The higher the intensity of the incident radiation (light), the greater the number of photons in it, which causes more photoelectrons to be ejected from the surface of the metal, resulting in a higher photoelectric current.

(Frequency and energy remains the same)

Question 4

Uses of a photoelectic cell

Answer 4

1. Measure the intensity of light
2. Burgular alarm
3. TV cameras

• Electron collector is kept at the focal point so that electrons that are ejected can be collected.
• Electron collector is connected to the positive terminal so that electrons will be attracted and collected easily.
• Microammeter is used because the current produced by the cell is very small.

Question 5

Wave theory

Answer 5

1. Light waves carry energy continuously
2. Any frequency of incident radiation should emit electrons
3. High intense light produces electrons with greater kinetic energy
4. There will be a time lag between the turning on of the light source and the emission of light

Question 6

What can wave theory explain?

Answer 6

Except for reflection, refraction and interference phenomenon, the photoelectric effect cannot be explained fully with wave theory.

Question 7

Quantum theory

Answer 7

(M. Plank & A. Einstein)
Radiation carries energy packets called quanta or photons. One photon carries a fixed amount of energy which is proportional to the frequency of the incident light.

Question 8

Photoelectric cell in a circuit

Answer 8

When light is incident on the cathode, photoelectrons are emitted. If some of the electrons strike the collector of the photoelectric cell, there is a current in the external circuit.

If an opposing voltage is applied, some of the emitting electrons can be pushed back to the metal, and if the voltage continues increasing, all the electrons can be pushed back and the current will be stopped.

Question 9

Energy and work of electron in circuit

Answer 9

Loss in kinetic energy = Gain in electrical potential energy

Change in energy = work done on electron

Question 10

What does the frequency of incident radiation affect?

Answer 10

The higher the frequency of the incident radiation (light), the greater energy it has. Since each photon can be absorbed by only one photoelectron, the energy of the photons directly affects the kinetic energy of the released photoelectrons, as their velocity is increased.

Question 11

What is the cutoff voltage related to?

Answer 11

The cutoff voltage is directly proportional to the frequency of the incident radiation.

Question 12

Graph of cutoff voltage over frequency

Answer 12

• x-intercept is the threshold frequency (fo)
• y-intercept is Φ/e
• Gradient is h/e

Question 13

Graphy of energy over frequency

Answer 13

• x-intercept is the threshold frequency (fo)
• y-intercept is Φ
• Gradient is always plank constant (h)
• More reactive metal is shifted closer to the y-axis (smaller fo and Φ)
• Less reactive metal is shifted further from the y-axis (larger fo and Φ)

Question 14

The Bohr Model of the atom assumptions

Answer 14

1. The electron in a hydrogen atom travels around the nucleus in a circular orbit.
2. The energy of the electron in orbit is proportional to its distance from the nucleus. The further the electron is from the nucleus, the more energy it has.
3. Only a limited number of orbits with certain energies are allowed for an atom (orbits are quantized).
4. Radiation/energy is absorbed when an electron jumps to a higher energy orbit and is emitted when an electron fall into a lower energy orbit
5. The energy of the light emitted or absorbed is exactly equal to the difference between the energies of the orbits.

Question 15

What does n = 1 represent?

Answer 15

N values refer to the energy levels of the atom and n = 1 represents the ground state with the lowest energy. (Higher quantum numbers have greater energy).

Question 16

How is an emission spectrum produced?

Answer 16

1. When a thin gas in which the atoms do not experience many collisions (because of low density) is heated at low pressure or subjected to a high voltage, the atoms and orbiting electrons of the gas (eg. H atoms) gain energy.
2. The electrons with high energy levels will jump to higher energy levels, being in an excited state.
3. At these higher energy levels, electrons become unstable and fall back to lower energy levels to become more stable. As they fall back, the electrons release the extra energy they possess as photons. (△E=hf)

Question 17

Are alpha particles suitable for radiation therapy?

Answer 17

Alpha particles are not suitable for radiation therapy, as it has a very short range that is less than tenth a millimeter inside the body.

Question 18

Hazards of alpha particles

Answer 18

• Alpha particles are hazardous when ingested into the body, having a large destructive power within its short-range
• Maximum damage from alpha particles occurs when they come into contact with fast-growing membranes and living cells

Question 19

Hazards of beta particles

Answer 19

• Beta particles are hazardous when ingested into the body, having a large destructive power within its greater range (compared to alpha)

Question 20

Gamma rays

Answer 20

• Electromagnetic rays from nucleus
• Largest range of penetration (than alpha and beta particles) due to higher energy
• Gamma emission can occur by itself, but often gamma rays are emitted together with alpha or beta radiation

Question 21

Are gamma rays suitable for radiation therapy?

Answer 21

Gamma rays are the most suitable radiation for radiation therapy, as it has a very large range. However, it is also the most dangerous radiation, as it can penetrate through large, thick materials.

Question 22

Laws of conservation in atomic reaction

Answer 22

In atomic reactions…

1. Mass-energy remains unchanged
2. Mass number remains unchanged
3. Atomic number remains unchanged

Question 23

What determines the frequency/energy of emitted radiation/electrons?

Answer 23

The frequency/energy of the emitted radiation/electrons depends on the energy difference between the two energy levels.

Question 24

How does the sun generate energy?

Answer 24

The sun is a main-sequence star, generating energy by the nuclear fusion of hydrogen nuclei into helium.

Question 25

How is an absorption spectrum produced?

Answer 25

Atoms can absorb the energy of a photon of light. When white light (containing photons with a broad range of frequencies/energies) are incident on atoms, those photons that have the exact amount of energy to excite the atom from one of its energy levels to another, will be absorbed and therefore photons of that
frequency will removed.

This will produce a series of dark lines, which are observed at each of the wavelengths that correspond to the absorbed photons.

Question 26

How is a continuous spectrum produced?

Answer 26

(Occur when pressure of gas is higher)
When solids, liquids or dense gases are heated, the atoms emit light of all frequencies and produce a continuous spectrum where, the lines in the spectrum fall very close to each other without any line spacing (no missing frequencies).

Question 27

1. Lyman series

Answer 27

(UV region)

• High frequency, short wavelength
• Line series for H atom when excited electrons jump from higher energy levels to the first energy level

Question 28

2. Balmer series

Answer 28

Visible light spectrum (line series) that is produced by excited Hydrogen electrons, that jump from higher energy levels to the second energy level

Question 29

3. Paschen series

Answer 29

(Infrared region)

• Low frequency, long wavelength
• Line series for H atom when excited electron jump from higher energy levels to the third energy level

Question 30

Lines of energy levels

Answer 30

• When moving out of an atom, the energy difference (line spacing/length of line) between each level is getting smaller, while the energy on each level in getting larger.
• As the energy/frequency increases, wavelength decreases

Question 31

Why is the Bohr theory successful?

Answer 31

• Attempted to correct the deficiencies of the Rutherford model
• Provides a model to explain why atoms emit energy and produce a line spectra. For the lines in the H atoms, it accurately predicts wavelengths
• Explains the existence of the absorption spectra
• Explains the existence of energy levels in atoms and accurately predicted the ionisation energy of the H atom

Question 32

How can you identify the lines in a series?

Answer 32

The first line in a given series, far from the rest, represents the electron jump to the lowest level from the next upper level.
- This line always has the lowest energy and frequency

The second and onward lines represent higher energies with higher frequencies and a shorter wavelength.

Question 33

Measure of nuclear binding energy

Answer 33

1. Binding energy is usually positive because net energy is required to separate nuclei into individual protons and neutrons.
2. Thus, the mass of an atom’s nucleus is usually less than the sum of the individual masses of the constituent protons and neutrons when separated. This difference in mass is the mass defect, a measure of the nuclear binding energy of the nucleus, which results from the forces that hold the nucleus together.
3. During the splitting of the nucleus, some nucleons are converted into large amounts of energy (E = mc^2), and thus this mass is removed from the total mass of the original particles and is missing in the resulting nucleus. This missing mass is the mass defect, which represents the energy released when the nucleus is formed.

Question 34

At fission, what are the effects on mass and energy

Answer 34

The mass of the reactants is greater than the mass of the products. This mass difference is seen as an energy increase in the reactants. At fission, there is a mass decrease and energy increase, as some of the nucleons are converted to energy.

Question 35

Alpha particle

Answer 35

• Nucleus of a helium atom, (2 protons and 2 neutrons) that is emitted as radiation from a decaying heavy nucleus
• Nucleus of highest stability
• Very short range of penetration due to large mass and +2 charge

Question 36

Beta particle

Answer 36

• High energy electron emitted as ionizing radiation from a decaying nucleus
• Larger range of penetration (than alpha particles) due to higher energy
• Beta emission is accompanied by the emission of an electron

Question 37

Why do different colours of light either allow or prevent a current from flowing?

Answer 37

1. The orange filter only allows through photons
   of orange light, which does not give the electrons sufficient energy to exceed the work function of the metal and escape it, so no current flows, and thus the doorbell rings continuously.
2. However, the green filter allows through higher energy photons (having a higher frequency than the photons of orange light) which have more energy than the
   work function of the metal. This means electrons struck by these photons can escape the metal
   and be collected, allowing a current to be
   detected, and thus the doorbell is stopped from ringing.

Question 38

What is the difference between the Bohr and Rutherford model of the atom?

Answer 38

1. The Rutherford model does not specify
   discrete orbits for the electrons.
2. The Bohr model requires an electron to be at specific
   radii only, dependent on their angular
   momentum being an exact multiple of a set
   value, h/2π.

Question 39

What causes the discrete spectrum?

Diffraction grating

Answer 39

1. When emitted radiation is passed through a diffraction grating, the light appears as a discrete set of lines, of different colours (VIBGOR) with different frequencies.
   (Violet - higher frequency, Red - lower frequency)
2. Electrons can only exist in certain discrete energy levels.
3. The emission lines correspond to photons of discrete energies, that are emitted when the excited electrons in the gas transition from higher energy levels to lower levels. The energy of a photon equals E2 – E1 (where, E2 – E1 is the energy change between upper and lower energy level).
4. As the energy of the photon is discrete, the colour and frequency of the emission lines are also discrete (f = E/h)

Question 40

What is the spacing and intensity of emission line spectra determined by?

Answer 40

The spacing and intensities of the lines depend on the type of element (emission line spectra are unique to the different gases of each element).

Question 41

Why does increasing the intensity of light not affect the current flowing (at the cutoff voltage)?

Answer 41

At the cutoff voltage, electrons do not have enough energy to escape. Increasing the intensity of light (brightness) will only increase the number of photons, not their energy. Therefore, current will not flow.

Question 42

How will the work function be affected by a change in the threshold frequency (fo) of a metal?

Answer 42

The threshold frequency is directly proportional to the work function. Therefore, an increase in threshold frequency will be due to an increase in work function.

Question 43

How will the gradient be affected by a change in the threshold frequency (fo) of a metal?

Answer 43

The gradient is Planck’s constant (6.63 x 10^-34), which is constant for all metals. Therefore, the gradient will remain the same.

Question 44

How will the cutoff voltage be affected by a change in the threshold frequency (fo) of a metal?

Answer 44

The cut-off voltage directly depends on the kinetic energy of the photoelectrons. This kinetic energy is the difference between the energy of the incident photon and the energy of the work function (Ek max = hf - hfo)

The work function is directly related to the threshold frequency. For a given frequency, if the threshold frequency is increased, the kinetic energy and hence, cutoff voltage, will be decreased.

Question 45

How do energy levels change as they increase from n = 1?

Answer 45

The energy values of the quantum states get closer and closer together as the states go up to n = ∞ (equivalent to 0 for calculations), and so the energy transition values (and hence frequencies, as photon energy is directly proportional to frequency), will get closer and closer together.

In any series, the first transition from the energy state immediately above the common destination state is always the lowest energy transition and hence emits the lowest frequency photon.

Question 46

Energy transition of UV photon versus infared photon

Answer 46

Energy transition that emits a UV (High frequency) photon - Highest
Energy transition that emits an Infrared (Low frequency) photon - Lowest

Question 47

Relationship between transition resulting in photon a with the smallest wavelength

Answer 47

The smallest wavelength is associated with
the largest frequency, which is related to
the greatest energy change.

Question 48

How can you apply the cutoff potential?

Answer 48

Reverse the battery terminals (and adjust
the voltage.)

Cut off potential is the voltage required to
stop the electrons leaving the emitter. The
collector needs to be negative to repel the
electrons back to the emitter to provide the
cut off potential.