4.5 Quantum Physics

Question 1

what is a quantum?

Answer 1

it is a small discrete unit/packet of energy (plural quanta)

Question 2

what is a photon?

Answer 2

a photon is a quantum associated with electromagnetic radiation

Question 3

what was Newton’s theory of light (1672)?

Answer 3

he believed that light was composed of a stream of tiny corpuscles (particles), moving in straight lines from a light source, he supported this theory with his own laws of motion and showed that both reflection and refraction could be explained in terms of particles either bouncing off a surface or travelling more quickly as they move from a less dense to a more dense medium

Question 4

what did Max Planck discover in 1901?

Answer 4

• he investigated black body radiation and found a link between energy and frequency of radiation and that energy is emitted in discrete packets of energy (quanta)
• E is directly proportional to f

Question 5

what is the photon model?

Answer 5

Einstein opened up the idea that light behaves like a particle in 1905 and that light exists in discrete packets of energy. It was de Broglie who proposed that light behaves both like a wave and a particle and as a result quanta of light or other electromagnetic energy became known as photons.

Question 6

what is the equation used to work out the energy of a single photon?

Answer 6

E = hf

or E = hc/λ

Question 7

what is a key property of photons?

Answer 7

as they are exchange particles and have no mass they travel at c, the speed of light, they also have NO CHARGE

Question 8

define the electronvolt

Answer 8

the electronvolt is defined as the kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt, very small unit of energy

Question 9

what is 1eV equal to in joules?

Answer 9

E = QV
E = 1.6 x 10^-19C x 1V
1eV = 1.6 x 10^-19J

Question 10

outline and explain an experiment using LEDs to determine a value for the Planck constant

Answer 10

• the light emitted by LEDs comes in a range of different colours, because the colour or wavelength of the light is related to photon energy we can use different LEDs to determine a value of the Planck constant
• connect an LED of known wavelength in an electrical circuit with a 6V power supply, a 1kΩ variable resistor, an ammeter and a voltmeter across the LED
• start off with no current flowing in the circuit, then adjust the variable resistor to increase the p.d until the LED just starts to light up (use a tube block light pollution and increase accuracy)
• record the activation voltage (Vo) across the LED and the wavelength of light the LED emits
• repeat this with at least 5 other LEDS (use different colours to get different wavelengths)
• plot a graph of v against (1/λ to obtain a straight line graph, the gradient will be equal to hc/e because E = eV and E = hc/λ
• calculate gradient, times by e and divide by c
• plancks constant = 6.63 x 10^-34

Question 11

what happens if you shine electromagnetic radiation of a particular frequency on the surface of a metal?

Answer 11

electrons are emitted from its surface, this phenomenon is known as the photoelectric effect and the electrons that are released are called photoelectrons

Question 12

how does the photoelectric effect happen?

Answer 12

• free electrons on the surface of the metal absorb energy from the light
• if an electron absorbs enough energy, the bonds holding it to the metal break and its emitted from the surface
• this is called the photoelectric effect and the electrons emitted are called photoelectrons

Question 13

what is Einstein’s photoelectric equation and what does it mean?

Answer 13

hf = Φ + KEmax
where hf = the energy transferred to an electron
Φ = work function
KEmax = maximum kinetic energy of the photoelectrons

Question 14

for an electron to be emitted from the surface of the metal what is necessary?

Answer 14

the energy gained from a photon must be greater than the work function of the metal, if it isn’t the metal will heat up but no electrons will be emitted

Question 15

define work function

Answer 15

the work function of a metal is the minimum energy required to release an electron from its surface, overcoming the electrostatic attraction between the positive metal ions

Question 16

define threshold frequency

Answer 16

threshold frequency is the lowest frequency of radiation that will results in the emission of electrons from a particular metal surface, for most metals this frequency occurs in the ultraviolet region of the electromagnetic spectrum

Question 17

what is the formula for threshold frequency and where does it come from?

Answer 17

since for electrons to be released hf = Φ, the threshold frequency must be f = Φ / h
(since KEmax becomes zero as this is the minimum freq. to provide the minimum amount of energy to just release he electrons from the surface, not to make them move)

Question 18

does the kinetic energy of the photoelectrons depend on the intensity of the radiation?

Answer 18

no
the kinetic energy of the electrons is independent of the intensity of radiation, because they can only absorb one photon at a time (there is a one-to-one interaction between a photon and a surface electron)

Question 19

does the kinetic energy depend on the frequency of radiation?

Answer 19

yes

the frequency determines the amount of energy supplied by each photon to the photoelectrons

Question 20

what does increasing the intensity do at the same frequency (above the threshold frequency)?

Answer 20

• this increases the rate of emission of photoelectrons, more photoelectrons are released but their KE does not increase
• rate of emission of photoelectrons above the threshold freq. is directly proportional to the intensity of the incident radiation

Question 21

what is Φ dependent on?

Answer 21

the type of metal

Question 22

outline an experiment to demonstrate the photoelectric effect with a gold-leaf electroscope

Answer 22

• set up a gold-leaf electroscope with a metal plate cap attached at the top of the stem
• the zinc plate is initially negatively charged
• when this is done the metal stem and the gold leaf will repeal each other
• the metal plate is then exposed to ultraviolet light and the photoelectric effect causes free electrons to be ejected
• this causes the electroscope to loose it’s negative charge (becoming slightly less negative or slightly more positive) meaning the gold leaf is no longer repelled and falls down
• when using visible light nothing happens as the frequency of visible light isn’t above the threshold frequency, no matter how bright or intense the beam of light is the photoelectric effect doesn’t occur

Question 23

do all the photoelectrons that are emitted have the same kinetic energy?

Answer 23

no, the photoelectrons are emitted with a variety of kinetic energies ranging from zero to some maximum value, this value increases with frequency of the radiation, and is unaffected by the intensity of the radiation

Question 24

what occurrences show wave properties of light?

Answer 24

interference and diffraction, this can only be explained using waves interfering constructively and destructively

Question 25

what occurrences show light behaving as a particle?

Answer 25

the photoelectric effect

• Einstein explained the results by thinking of the beam of light as a series of particle-like photons
• if a photon of light is a discrete bundle of energy, then it can interact with an electron in a one-to-one way
• all the energy in the photon is given to one electron

Question 26

what experiment highlights the wave nature of electrons?

Answer 26

electron diffraction

Question 27

what pattern does the electron diffraction demonstration produce?

Answer 27

a pattern of circular rings

Question 28

how does the electron diffraction experiment work?

Answer 28

• diffraction patterns are observed when accelerated electrons in a vacuum tube interact with the spaces between carbon atoms in the polycrystalline graphite
• since the graphite atoms are not all lined up in the same direction like in a diffraction grating, this gives a circular pattern instead of the parallel lines seen when light diffracts

Question 29

who came up with the theory of wave-particle duality?

Answer 29

de Broglie

Question 30

what was de Broglie’s wave particle duality?

Answer 30

• If wave-like light showed particle properties (photons), particles like electrons should be expected to show wave-like properties.
• waves can behave both as a wave and a particle and particles can behave as both a particle and a wave

Question 31

what is the formula for the de Broglie equation?

Answer 31

λ = h / p
OR
λ = h / mv

where λ = wavelength of particle
p = momentum of particle