4.5 Quantum physics

Question 1

What is the particulate nature (photon model) of electromagnetic radiation?

Answer 1

Acts like a wave and particle

Question 2

What is a single packet of EM radiation called?

Answer 2

Quantum

Question 3

What is the equation to find the energy carried by one wave packet (Photon)?

Answer 3

E=hf=hc/λ

Question 4

What is the equation to find the energy carried by one wave packet (Photon)?

If frequency increases what happens?

Answer 4

E=hf=hc/λ

Increase in frequency, wave packet carries more energy

Question 5

When a photo interacts with another particle how much energy does it transfer?

Answer 5

All or None

Question 6

What is the charge on a photon?

Answer 6

Neutral

Question 7

What happens when you accelerate an electron between 2 electrodes?

Answer 7

It transfers some of its energy (eV) into kinetic energy

Question 8

What is the definition of an electron volt?

Answer 8

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

eV=1/2mv^2

Question 9

Define the threshold voltage (Vo)

Answer 9

The voltage needed to give electrons the same energy as a photon (emitted by an LED)

Question 10

Current will only pass through an LED after what?

Answer 10

after a minimum voltage is placed across it - the threshold voltage

Question 11

Describe an experiment to measure the Planck Constant

Answer 11

1. Connect an LED of know wavelength in the electrical circuit shown
2. Start off with no current flowing through the circuit, then adjust the variable resistor until a current just begins to flow through the circuit and the LED lights up
3. Record the voltage (Vo) across the LED, and the wavelength of light the LED emits.
4. Repeat the experiment with a number of LEDs of different colours that emit light at different wavelengths
5. Plot a graph of threshold voltages (Vo) against 1/λ
6. You should get a straight line graph with a gradient of hc/e-which you use to find h

Grad=hc/e e=charge of electron, c=speed of light

Question 12

What is 1 eV in joules?

Answer 12

e x V = 1.6x10^-19C x 1JC^-1= 1.6X10^-19 J

Question 13

What is the photoelectric effect?

Answer 13

When you shine EM waves of high frequency onto a surface of a metal, it will instantaneously eject electrons.

Question 14

Describe the Photo electric effect

Answer 14

1. Free electrons on the surface of the metal absorb energy from light
2. If an electron absorbs enough energy, the bonds holding it to the metal break and it is emitted from the surface
3. The electrons emitted are called photo electrons

Question 15

How do you demonstrate the photoelectric effect?

Answer 15

The photoelectric effect can be demonstrated using a gold leaf electroscope – a zinc plate on top
of a negatively charged stem, with a negatively charged piece of gold leaf attached to the stem.

Initially, the gold lead and the stem have the same charge, so they repel each other.

If UV light is shone on to the zinc plate, free electrons will be released from the surface of the plate, and the
negative charge will slowly be lost, so the gold leaf will gradually fall back to the stem.

Question 16

What 3 conclusions were reached from the gold leaf experiment?

Answer 16

1. For a given metal, no photo electrons are emitted if the radiation has a frequency below a certain value-Threshold frequency
2. The photo electrons are emitted with a variety of kinetic energies ranging from zero to some maximum value. This value of maximum kinetic energy increases with the frequency of the radiation, and is unaffected by the intensity of the radiation.
3. The number of photo electrons emitted per second is proportional to the intensity of the radiation

Question 17

Why can’t the photoelectric effect be explained by wave theory?

According to wave theory:

Answer 17

1. For a particular frequency of light, the energy carried is proportional to the intensity of the beam
2. The energy carried by the light would be spread evenly over the wave front
3. Each electron on the surface would gain a bit of energy from each incoming wave
4. Gradually, each electron would gain enough energy to leave the metal

So high intensity should mean more energy transferred to each electron so KE increase with intensity. There’s no explanation for KE being dependent on only frequency

No explanation for threshold frequency, electrons should be emitted eventually no matter what frequency according to wave theory

Question 18

According to the photon model

Answer 18

1. When light hits the surface, the metal is bombarded by photons
2. If one of these photons is absorbed by a free electron the electron will gain energy equal to hf

Question 19

What is the work function (⌀)?

Answer 19

minimum energy required to free an electron from the surface of the metal

Question 20

What’s the equation for threshold frequency?

Answer 20

f=⌀/h

Question 21

What is hf?

Answer 21

The energy transferred to an electron

Question 22

Is the kinetic energy of an electron effected by intensity?

Answer 22

No, because they only absorb one photon at a time.

Question 23

What is the rate of photo electron emission proportional to?

Answer 23

Intensity of radiation, provided its above the threshold frequency-more photons per second means more collisions

Question 24

What’s Einstein’s photoelectric equation?

Answer 24

hf=⌀ + KEmax

Question 25

What shows light behaving as a wave?

Answer 25

Interference and diffraction patterns (Young's double slit experiment)

Question 26

What shows light behaving as a particle?

Answer 26

Photoelectric effect, 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 given to one electron

Question 27

What is the equation for DeBroglie wave particle duality theory?

Answer 27

λ=h/p p=momentum

Question 28

What confirms electrons wave like properties

Answer 28

-Electrons behave as waves/diffract -As gaps between atoms are of similar wavelength to electrons -The ordered pattern of atoms/atoms act as a grating allowing interference to produce pattern on screen rings occur because atomic ‘crystals’ at all possible orientations to beam

Question 29

If momentum is higher, how does it effect wavelength and the spread of lines?

Answer 29

Wavelength shorter | spread of lines smaller

Question 30

Wavelength for electrons accelerated through a vacuum tube is about the same size as what?

Answer 30

EM waves in the x-ray part of the spectrum

Question 31

What wavelength would an electron need to be diffracted by the atoms in poly-crystaline structures?

Answer 31

1x10^-10m

Question 32

What fact is used in an electron microscope about diffraction?

Answer 32

Shorter wavelength gives less diffraction effect. Diffraction effects blur detail on an image. For tiny details shorter wavelength is best

Question 33

Define a photon

Answer 33

A quantum of electromagnetic energy

Question 34

Explain what energy levels are and how they can be used to explain the emission of photons from atoms. (4 marks)

Answer 34

energy levels explanation: electrons have discrete energies in atom each photon produced by electron moving between levels photon energy equal to energy difference between levels electron loses energy/making transition in correct direction

Question 35

Define threshold frequency

Answer 35

The minimum frequency of the EM waves / light / uv / | photon for the removal of (surface) electron(s)