M4 Quantum Physics

Question 1

what is the photoelectric effect

Answer 1

If you shine radiation of a high enough frequency onto the surface of a metal, it will eject electrons.
The free electrons near the surface will absorb energy from the radiation.
Before an electron can leave the surface of the metal it needs enough energy to break the bonds holding it there (work function energy)

Question 2

what is threshold frequency

Answer 2

the minimum frequency of light needed to knock electrons off the plate

Question 3

what does it mean if frequency is above the threshold frequency

Answer 3

the amount of electrons being knocked off is proportional to the intensity of the light

Question 4

in the gold leaf electroscope experiment, what happens when red light is used

Answer 4

nothing because the red light doesn’t have a high enough frequency

Question 5

what happens if glass is placed in the path of UV light

Answer 5

it gets absorbed

Question 6

what is a photon

Answer 6

one quantum of energy of electromagnetic radiation

Question 7

what is the difference between intensity and frequency

Answer 7

intensity = the amount of photons in a given area

frequency is proportional to energy so frequency is the energy of photons

Question 8

what does this equation mean:
E = hf = φ + Ek

so what does it mean if frequency increases

Answer 8

energy of one photon = Planck’s constant X frequency = ‘energy used to dislodge electron’ + ‘energy left over which makes electron move’

so if frequency increases, the photon is carrying more energy

Question 9

what does c = fλ mean

Answer 9

speed of light = frequency X wavelength

Question 10

describe the process by which electrons are released from the cathode

Answer 10

The process is the photoelectric effect.
The EM radiation hits the cathode and the energy of the photons is used the dislodge the electrons and the excess energy which transfers to kinetic energy makes the electrons move. The energy of the photons comes from the frequency of the wave which are proportional to each other.

Question 11

define planck’s constant

Answer 11

the amount of energy a photon has at a specific frequency

Question 12

define volt

define electronvolt

Answer 12

volt - the work done to move one coulomb of charge

electronvolt - the kinetic energy gained by an electron when it’s accelerated through a pd of 1 volt

Question 13

describe the quantum well

Answer 13

represents the energy needed to release a photoelectron from the surface of a metal

the photon has some energy and the electron that the photon hits absorbs this energy
It takes a certain amount of energy to release the electron and this energy is called the work function

Question 14

what is 1 Joule in electronvolts

Answer 14

1 / 1.6 X 10^-19

= 6.25 X 10^18 eV

Question 15

what is the photon energy in electronvolts of red light of wavelength 685nm

Answer 15

685nm = 685 x 10^-9 m
f = v / λ
3 X 10^8 / 685 X 10^-9 = 4.4 X 10^14 Hz

E = hf
= 6.63 X 10^-34 X 4.38 X 10^14
=2.9 X 10^-19 J

2.9 X 10^-19 X 6.25 X 10^18
=1.8 eV

Question 16

what has greater energy, red or blue light

Answer 16

blue light because it has a higher frequency

Question 17

what is wave-particle duality

Answer 17

All particles have both particle and wave properties

Waves can also show particle properties

Question 18

when two crests meet what is it called

when a trough and a crest meet what is it called

Answer 18

two crests is constructive interference

crest and a trough is destructive interference

Question 19

what is meant by the de Broglie wavelength of an electron

Answer 19

he associated waves and particles

wavelength associated with a particle = Planck’s constant / momentum

also written as wavelength = h / p = h / mv

Question 20

what can you prove from electron diffraction

Answer 20

particle matter behaves as a wave

when you pass a beam of electrons through a thin graphite screen, the particles diffract so rings of light is formed because constructive and destructive interference is occurring

you only get diffraction if a particle interacts with an object of about the same size as its Broglie wavelength

Question 21

in electron diffraction, what happens when the voltage increases

Answer 21

the speed of electrons increases so the rings become closer together

Question 22

what is the kinetic energy of electrons with a voltage of 2000V?

Answer 22

2000 X 1.6 X10^-19

=3.2 X10^-16

Question 23

what two proposals did Bohr make about how light interacts with matter

Answer 23

• like the photoelectric effect, light interacts with matter and light consists of photons
• when light interacts with matter, it’s the electrons that absorb the energy from the incoming photons. when electrons lose energy, light is emitted by matter in the form of photons

Question 24

what happens if the frequency is below the threshold frequency

Answer 24

a single photon can still give up its energy to a single electron but this electron can’t escape from the attractive forces of the positive metal ions
the energy absorbed from the photons appears as Ek of the electrons
these electrons lose their Ek to the metal ions when they collide with them
this warms up the metal , why it gets hot

Question 25

a material emits photoelectrons when the wavelength is 380nm

calculate its work function in eV

Answer 25

E = hc / λ
= 6.63 X 10^-34 X 3 X 10^8 / 380 X 10^-9
answer X 1 / 1.6 X 10^-19 which gives it in electronvolts
= 3.3 eV

Question 26

what happens when you accelerate an electron in a potential difference?
hence, what equation can you use?

Answer 26

it transfers some electrical potential energy into kinetic energy

eV = 0.5mv^2

Question 27

what experiment can you do to determine Planck’s constant

Answer 27

use the equation h = eVλ / c
set up circuit
adjust variable resistor until LED lights up
record the voltage across the LED (this is threshold voltage)
record the wavelength of the LED
Repeat the experiment to get an average voltage
Then repeat with other LEDs with different wavelengths

plot the graph of threshold voltage (y) against 1/wavelength (x) and Planck’s constant will be e times the gradient all over the speed of light

Question 28

define work function

Answer 28

the minimum energy required to dislodge an electron from the surface of a metal

Question 29

what is the gold-leaf electroscope

what does it prove

Answer 29

demonstrates the photoelectric effect

a zinc plate is attached to the top of an electroscope and it has a negative charge
the gold leaf also has a negative charge meaning they repel each other so the gold leaf rises up
UV light is then shone onto the zinc plate and the energy causes electrons to be lost from the zinc plate meaning the leaf is no longer repelled as the negative charge is lost from the plate so the leaf falls back down

Question 30

what 3 conclusions can be made from the photoelectric effect

Answer 30

1) for a given metal, no photoelectrons are emitted if the threshold frequency isn’t met
2) photoelectrons are emitted with a variety of kinetic energies and the value of max kinetic energy increases with the frequency of the radiation (but it’s independent from intensity)
3) number of photoelectrons emitted per second is directly proportional to the intensity of the radiation

Question 31

what equation could you use to calculate threshold frequency

Answer 31

work function / planck’s constant

Question 32

what equation could you use to calculate the maximum kinetic energy of a photoelectron

Answer 32

0.5mv(max)^2

Question 33

what is the intensity of the radiation

Answer 33

power transferred by the radiation over a given area

in the photon model it is the number of photons that hit a given area every second

Question 34

what does it mean by one-to-one interaction in terms of photons

Answer 34

A photon either transfers all or none of its energy when interacting with another particle