Module 4.3 - Quantum Physics ✓

Question 1

State plank’s equation

Answer 1

E = hf
E = hc/l

Question 2

What were Einstein’s ideas about EM Waves?

Answer 2

He believed EM waves can only exist in discrete packets called photons. Photons act as a particle and will either transfer ALL or NONE of their energy in an interaction

Question 3

Define the electron-volt?

Answer 3

An electron-volt is the kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt

Question 4

How many joules in an eV?

Answer 4

1 eV = 1.6*10^-19 J

Question 5

Why can threshold voltage be used to find plank’s constant?

Answer 5

1) Current will only pass through an LED if a threshold voltage is placed across it.
2) This is the voltage needed to give the electrons the same energy as the photon emitted by the LED. All the electron’s KE after it is accelerated through the p.d. is transferred into a photon
3) You can use h = ((eV)*l)/c to estimate planck’s constant (comes from E = hc/l)

Question 6

How does a gold leaf electroscope behave before and after light is shone on it’s plate?

Answer 6

The electroscope plate is initially negatively charged so the gold leaf is repelled. When the plate is exposed to UV light, the photoelectric effect causes it’s free electrons to be ejected, making it loose negative charge and the gold leaf to be repelled less.

Question 7

What are the three conclusions gained form the gold leaf electroscope experiment?

Answer 7

1 - For a given metal no electrons are emitted if the radiation has a frequency below a certain value
2 - The photoelectrons are emitted with a range of kinetic energies from 0 to some maximum value. Increasing frequency increases this maximum value but intensity does not.
3 - The number of photoelectrons emitted per second is proportional to the intensity of radiation

Question 8

what does wave theory predict should happen when UV light is shone on a charged plate (photoelectric effect)?

Answer 8

1) For a particular f of light, the energy carried should be proportional to the intensity of the beam
2) The energy carried would be spread evenly over the wavefront
3) Each free electron on the metal surface would gain a bit of energy from each incoming wave
4) Gradually, each electron would gain enough energy to leave the metal

Question 9

Why does wave theory not explain threshold frequency and the behaviour of increasing intensity?

Answer 9

• The electrons should be emitted eventually no matter the frequency so threshold f does not make sense
• Higher intensity, more energy transferred to each electron, so KE should increase with intensity not just frequency

Question 10

How does the photon model of light explain the photoelectric effect?

Answer 10

1) When light hits the surface of the metal, it is bombarded by photons
2) If a free electron absorbs one of these photons, it will gain energy equal to hf. If this is enough energy to break the bonds holding it to the metal it will be emitted form the surface

Question 11

How does the photon model explain threshold frequency?

Answer 11

If the energy gained by an electron from a photon is greater than the work function, the electron is emitted. But if it isn’t the metal will heat up but no electrons be emitted. Intensity does not matter as each photon has a one on one interaction with an electron, more photons with not enough energy won’t help.

Question 12

What is the photoelectric effect equation?

Answer 12

hf = work function + KEmax

Question 13

How does the photon model explain maximum kinetic energy?

Answer 13

1) The energy transferred to an electron is hf
2) The KE it will have when leaving the metal will be hf - the energy lost to breaking the bonds holding it there (multiple ways to do this so range of KEs)
3) The minimum energy an electron can loose escaping is the work function so the maximum KE is hf - wf
4) Each electron can only absorb one photon at a time, so increasing the intensity just means more electrons emitted per second rather than each electron having more energy

Question 14

What was Louis De Broglie’s suggestion?

Answer 14

‘if wave-like light showed particle properties then particles must show wave like properties’

Question 15

State the De Broglie equation

Answer 15

l = h/p
l > wavelength
h > plank’s constant
p > momentum

Question 16

What does the de broglie equaion do?

Answer 16

It relates a wave property (l) to a moving particle property (p).

Question 17

What does the de broglie wavelength mean for a particle?

Answer 17

The de broglie wave of a particle is a ‘probability wave’. The likelihood of finding the particle at a point is directly proportional to the square of the amplitude at that point.

Question 18

How did electron diffraction show the wave like properties of electrons and prove de broglie’s equation?

Answer 18

• Diffraction patterns are observed when accelerated electrons interact with small spaces between carbon atoms
  1) A smaller accelerating voltage gives more widely spaced rings
  2) The debroglie equation states that with decreasing momentum wavelength increases
  3) wave equation shows that the ring spacing increases with increasing wavelength.

Question 19

Why can an electron microscope see with more detail than a light microscope?

Answer 19

Diffraction effects blur detail on an image, so to resolve smaller detail in an image a smaller wavelength is needed. Light blurs detail more than ‘electron waves’.

Question 20

What are the units for plank’s constant?

Answer 20

m^2 kg s^-1

Question 21

What is the work function of a metal?

Answer 21

energy φ required for an electron to escape from metal surface
the MINIMUM energy…….. (needed for a mark)

Question 22

Explain what is meant by the de Broglie wavelength of an electron.

Answer 22

electron wavelength depends on its speed/momentum

Question 23

What did the plank guy have to do with his equation?

Answer 23

When plank was investigating black body radiation he suggested EM waves can only be released in discrete packets called quanta. (A quantum is a single packet)

Question 24

What does the equation E=hf show about photons?

Answer 24

The higher the frequency of EM radiation, the more energy a photon carries

Question 25

What evidence is there for light being a wave?

Answer 25

The double slit experiment shows light produces interference and diffraction patterns, which can only be explained by waves interfering

Question 26

[Exam] State what is meant by the photoelectric effect?

Answer 26

The emission of electrons from the surface of a metal when electromagnetic waves (of frequency greater than the threshold frequency) are incident on the metal.