Particles 2

Question 1

Explain what the photoelectric effect is

Answer 1

The process by which photoelectrons are emitted off the surface of a metal upon the absorption of electromagnetic radiation above a threshold frequency.
This is because, with enough energy, the bonds holding the electrons in place break.

Question 2

Define the threshold frequency

Answer 2

The minimum frequency of incident electromagnetic radiation required for photoelectrons to be emitted from the surface of a metal

Question 3

\Define the threshold wavelength

Answer 3

The longest wavelength of incident em radiation required for photoelectrons to be emitted from the surface of a metal

the threshold frequency and wavelength are inversely proportional because of the wave equation

Question 4

Define work function, giving a formula between work function and threshold frequency

Answer 4

• The minimum energy of incident em radiation required for photoelectrons to be emitted from the surface of a metal
• since E = hf for a photon,
  work function = h x threshold frequency

Question 5

What is the formula to find energy transferred to the electron

Answer 5

One electron absorbs one photon and all of the photons energy is transferred to the electron
= hf

Question 6

Why is there a max kinetic energy of the emitted electrons

Answer 6

The energy required for electrons to be emitted varies with how deep within the metal the electrons lie
Electrons with Ekmax are therefore on the surface of the metal

Question 7

Explain the concept of stopping potential

Answer 7

• It is the pd required to stop photoelectron emission from occurring
• This can be shown in a photoelectric circuit, made up of 2 metal plates, a collector plate and emitter plate
• shining photons at the emitter plate causes electrons to be emitted which cross the gap and are collected at the other plate
• If you add a power supply and increase the voltage, the emitter plate becomes more positive because it is connected to the positive terminal of the power supply
• Therefore, electrons that are emitted will be more attracted back towards the emitter plate and the electrons require more energy to cross the gap
• eventually there will be a voltage where no electrons can cross and this is the stopping potential
• At the Vs, the energy required to cross the gap = the max Ek of electrons

Question 8

What is the formula for finding the stopping potential

Answer 8

V= W/Q
so Vs = Ekmax/e

where e is the charge of an electron

Question 9

How does the photoelectric effect provide evidence that light behaves as a particle

Answer 9

• If light behaves as a particle, it is carried in discrete packets (quantised)
• This is proved in the photoelectric effect as each electron can only absorb the energy of 1 discrete packet ie photon

Question 10

Define intensity

Answer 10

The total power delivered per unit area
It is related with the number of photons striking the metal plate

Question 11

How can the stopping potential change

Answer 11

• Increasing the intensity does not change the max kinetic energy of the electrons so Vs is the same
• Increasing the frequency of photons increases the Ekmax of electrons so increases Vs
• Increasing the work function decreases the Ekmax so decreases the Vs

Question 12

What is the relationship between intensity and number of photoelectrons emitted

Answer 12

As intensity increases, more photons hit the metal plate, so more electrons can absorb a photon so more electrons can be emitted

Question 13

How can the photoelectric effect disprove the wave theory of light

Answer 13

• wave theory says that intensity of light is directly proportional to energy carried
• Therefore the kinetic energy should increase with intensity, which does not happen in the photoelectric effect
• Also, with a wave, over time the energy transferred to the electrons would gradually increase and therefore eventually, the electron would be emitted no matter the frequency of the light, which does not happen in the photoelectric effect

Question 14

Why do the electrons have kinetic energy when they leave the surface of the metal and explain how this gives the photoelectric equation

Answer 14

• If the photon that it absorbed has an energy greater than the work function, the excess energy is the kinetic energy of the electron leaving the metal
• Therefore, the work function + the max kinetic energy = the energy of the photon
• E=hf=Φ ‍+Ekmax

Question 15

How are electrons located within an atom

Answer 15

In discrete energy levels

Question 16

How can electrons move up an energy level

Answer 16

If another electron collides with the orbital electron or the orbital electron absorbs a photon, they can gain enough enough to move up energy levels
This is known as excitation.

Question 17

Define ionisation

Answer 17

When an electron gains enough energy to leave the atom entirely. The required energy is called the ionisation energy

Question 18

Why does de-excitation occur

Answer 18

• It is unstable for electrons to be in a higher energy level so they return back to their original energy level, the ground state. It will release the energy it gained in the form of a photon
• E=hf = the energy difference between energy levels

Question 19

Explain how a fluorescent tube works

Answer 19

• A voltage accelerates free electrons through the tube, which collide the mercury vapour in the tube
• the electrons in the vapour excite and then de-excite, releasing UV photons
• The electrons in the phosphorus coating of the tube absorb the uv photons and excite and de-excite, releasing visible light photons

Question 20

Define the electron volt explain why we use it, and how many joules is 1 eV

Answer 20

1 electron volt is the energy gained by an electron when accelerating from rest through a potential difference of 1 volt
- It is used because the energy difference between energy levels is very small
- 1eV = 1.6x10^-19 J

Question 21

What is a line spectra

Answer 21

A diagram which shows the specific wavelengths of light that have been absorbed/emitted during excitation and de-excitation

Question 22

How do line spectra provide evidence that electrons travel between discrete energy levels

Answer 22

The wavelengths of the light, and therefore positions on the spectra, are fixed values. This means the energy absorbed/emitted is a fixed value, which means the energies of the different energy levels are fixed values, meaning they are discrete

Question 23

Describe the difference between an absorption and emission spectra

Answer 23

• An absorption spectrum is a continuous spectrum with gaps at the wavelengths where photons have been absorbed
• An emission spectrum is a black screen with colour at the wavelengths where photons have been emitted

Question 24

How can emission spectra be formed

Answer 24

Splitting the visible light from a fluorescent tube using a diffraction grating or prism

Question 25

How can continuous spectra be formed

Answer 25

Splitting white light with a prism or when visible/infrared light pass through hot gas e.g. a star

Question 26

How can absorption spectra be formed

Answer 26

When white light (a continuous spectrum) passes through a cool gas
- The electrons in the cool gas absorb photons and are excited
- The wavelengths of photons absorbed are missing from the continuous spectrum on the other side of the gas

Question 27

What is the relationship between the emission and absorption spectra

Answer 27

The black lines in the absorption spectra correspond with the coloured lines in the emission spectra because they are discrete energy levels

Question 28

What are examples of light acting as a wave and particle

Answer 28

Wave - diffraction/interference
Particle - photoelectric effect

Question 29

What is an example of a particle with wave-like properties

Answer 29

electron-diffraction

Question 30

How did de broglie relate the wave and particle-like properties into an equation

Answer 30

λ=h/mv
wavelength is a wave property and momentum is a- moving particle property

Question 31

How does the de broglie equation show the relationship between momentum and diffraction

Answer 31

As momentum increases, wavelength decreases, so diffraction decreases so the interference pattern is closer