Quantum Physics

Question 1

what did Planck’s model propose?

Answer 1

his new model proposed that electromagnetic radiation had a particulate nature

-it was tiny packets of energy, rather than a continous wave

(einstein called these packets photons)

Question 2

what is a photon?

Answer 2

a quantum of elecromagnetic energy

Question 3

what is the difference in use between the photon model and the wave model?

Answer 3

we use the photon model to explain how electronmagentic radiation interacts with matter

use wave model to explain electromagentic radiation’s propagation through space

Question 4

what charge do photons have?

Answer 4

photons have no charge

they are neutral

Question 5

what are the eqautions for photon energy?

Answer 5

E= hf

E= energy (J)

h= planck constanst 6.63 x 10^-34

f= frequency (Hz)

OR

E= hc/λ

λ= wavelength (m)

c= speed of light 3.00 x 10⁸ ms^-1

Question 6

in E = hf what is E proportional to?

Answer 6

E is directly proportional to frequency

so if frequency increases energy increases

Question 7

define the electronvolt

Answer 7

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

Question 8

what is 1 eV equal to?

Answer 8

1.60 x 10^-19 J

Question 9

what is meant by the threshold voltage?

Answer 9

the minimum potential difference at which a diode begins to conduct

( voltage needed to give the electron the same energy as a photoon emitted by the LED. all of the elctron’s kinetic energy after it is accelerated over this potential difference is transferred into a photon )

Question 10

how can you use an LED to estimate the Planck Constant?

Answer 10

-use the the calculation;

Ve = hf

which can also be

Ve = hc/λ

this straight line graph is obtained from using a variety of different-wavelength LEDs

the eqaution y = mx + c is equivalenet to V = hc/λe

so planck’s constant can be determined from the gradient of this graph hc/e

Question 11

what is meant by the photoelectric effect

Answer 11

the emission of photoelectrons from a metal surface when electronmagnetic radiation above a threshold frequency is incident on the metal

Question 12

what happens if you were to shine electromagentic waves of a high enough frequency onto the surface of a metal? explain why this happens

Answer 12

the metal will instaneoulsy eject electrons

• free electrons on the surface of the metal absorb energy from the light
• if an electron absorbes enough energy. the bond holding it to the metal break and it is emitted from the surface

-

Question 13

describe what happens in the demonstration of the photoelectric effect with a gold-leaf electroscope

Answer 13

Question 14

what were the conclusions from the experiment to show the photoelectric effect with a gold-leaf electroscope?

Answer 14

1. ) for a given metal, no photoelectrons are emitted if the radiation frequency below a ceratin value (the threshold frequency)
2. ) the photoelectrons are emitted with a variety of kinetic energies ranging from zero to some mximum value

this value of max kinetic energy increase with the frequency of the radiation, and is unaffected by the intenisity of the radiation

3.) the number of the photoelectrons emitted per second is proportions to the intensity of the radiation

Question 15

why cant the photoelectric effect be explained by the wave theory?

Answer 15

According to the wave theory:

• the higher the intensity the more energy is transferred to each electron
• so gradually each electron would gain enough energy to leave the metal because kinetic energy should increase with intensity

BUT with the wave theory:

• there is no explanation for the kinetic energy only depending on the frequency
• there is no explanation for the threshold frequency, accordint to the wave theory the electrons should be emitted eventually, no matter what the frequency is.

Question 16

what is meant by the work function Φ?

Answer 16

this is the minimum energy required to free an electron from the surface the metal

Question 17

in the photon model when lights hits a metal surface what does intensity affect?

Answer 17

intensity affects the rate of emmision of photoelectrons

an increase in intensity causes a higher rate of emision

(these emitted photoelectrns don’t move any faster tho, you do that by increasing frequency of the light)

Question 18

does the energy gained by an electron (on a surface metal) from a photon have to be higher or lower than the work function for the electron to be emitted?

Answer 18

higher thant the work function

Question 19

give an eqaution for the threshold frequency relating the work function and planck’s constant

Answer 19

f = Φ/h

frequency (hz/s^-1) = work function (J) / planck’s constant (6.6 x 10^-34)

Question 20

complete the sentence:

the maximum kinetic energy of photoelectrons is independant of the ___________ of the incident radiation

Answer 20

the maximum kinetic energy of photoelectrons is independant of the intensity of the incident radiation

Question 21

state Einstein’s photoelectric eqaution and show how it can be incorparated into a y= mx + c

Answer 21

hf = Φ + KE_max

-to put this in a graph you rearrange to:

KE_max = hf - Φ

• so plot the graph KE_max against f
• your gradient will be h

Question 22

why does the rate of photoelectron emission increase as intensity of radiation increase (provided its above the threshold frequency)?

Answer 22

because there is a higher intensity then more photons are hitting the electrons pers second which means more collisions, so more successful collisions creating photoelectrons

Question 23

what does the conservation of energy tell you in the photoelectric effect?

Answer 23

energy of each photon must be conserved, this energy does two things:

• frees electrons from the surface of the metal in a one-to-one interaction
• any remainder is transferred into the kinetic energy of the photoelectron

So the kinetic energy an electron will be carrying when it leaves the metal will be hf minus any energy its lost on the way out

Question 24

what was De Brogile PhD thesis?

how was this expressed in an eqaution?

Answer 24

if ‘wave like’ light showed particle properties (photons), ‘particles’ like electrons should be expected to show wave-like properties

λ= h/p

wavelength (m) = planck’s constant (6.63 x 10^-34 Js) / momentum (kg ms-1)

Question 25

complete the sentence:

as particles become larger their wave propertices become _______ to observe

Answer 25

as particles become larger their wave propertices become harder to observe

Question 26

what is meant by the probability wave?

Answer 26

the likelihood of finding a particle at a point is directly proportional to the sqaure of the amplitude of the wave at that point

Question 27

what is observed when accelerated electrons in a vacuum tube interact with the spaces between carbon atoms in graphite molecules?

Answer 27

diffraction patterns

which confirm that electrons shows wave like properties

Question 28

how does a smaller accelerating voltage (slower electrons) affect the size of diffraction

Answer 28

the size of diffraction is wider and more spaced out

Question 29

in the de Broglie’s eqaution

if momentum is higher then wavelength is ________ and the spread if lines is ______

(complete the sentence)

Answer 29

if momentum is higher then wavelength is shorter and the spread if lines is smaller

Question 30

in wave-particle duality when will you only get diffraction?

Answer 30

you will only get diffraction if a particle interacts with an object of about the same size as it de Brogile wavelength. (this is the wavelength, λ , associated with a object and is related to its momentum and mass)