Module 4.5 - Quantum Physics

Question 1

In 1672, what did Isaac Newton believe light to be made of? Why?

Answer 1

> Light was made of corpuscles

> Supported his laws of motion; and reflection and refraction

Question 2

In 1901, what did Max Planck hypothesise?

Answer 2

> Realised radiated energy couldn’t have continuous values

> Hypothesised that the total radiated energy were emitted in PACKETS with each having a fixed amount.

Question 3

What did Einstein and De Broglie find in 1905?

Answer 3

> Einstein showed that atoms absorb and emit light

> De Broglie showed that light behaves as a wave and a particle.

Question 4

Define Quantum (Pl - Quanta)

Answer 4

Small DISCRETE unit of ENERGY

Question 5

Define Photon

Answer 5

A QUANTUM associated with EM radiation

Question 6

What are the two equations to find the energy of a photon?

Answer 6

E = hf    
E = hc ÷ lambda

Question 7

What is E in E=hf?

Answer 7

E - energy - J

Question 8

What is h in E=hf?

Answer 8

h - Planck’s constant - J/s

Question 9

What is f in E=hf?

Answer 9

f - frequency - Hz

Question 10

What is c in E=hc÷lambda?

Answer 10

c - wave speed - m/s

Question 11

What is lambda in E=hc÷lambda?

Answer 11

lambda - wavelength - m

Question 12

How do you work out the energy of n number of photons?

Answer 12

E = nhf

Question 13

What is an Electron volt (eV)?

Answer 13

The KINETIC energy gained by ONE electron when it is accelerated through a p.d. of 1V

Question 14

How do you work out the energy gained by an electron?

Answer 14

E = charge x p.d.

Charge of electron is 1.602x10^-19

Question 15

What is 1eV equal to?

Answer 15

1.602x10^-19J

E=QV = 1.602x10^-19 x 1 = 1.602x10^-19J

Question 16

J —> eV?

Answer 16

DIVIDE by 1.602x10^-19

Question 17

eV—>J?

Answer 17

MULTIPLY by 1.602x10^-19

Question 18

How much kinetic energy (in eV) does an electron of charge e have through a p.d. of 100v?

Answer 18

100eV

1. 602x10^-19 x 100 = 1.602x10^-17J
2. 602x10^-17 ÷ 1.602x10^-19 = 100eV

Question 19

Derive eV = hc ÷ lambda

Answer 19

energy of a photon = energy of electrons that are excited in the semiconductor material of the LED.

E = hc ÷ lambda
Therefore
eV = hc ÷ lambda

Question 20

What is the photoelectric effect?

Answer 20

When EM radiation of a particular frequency is shone on the surface of a metal, electrons are emitted. The photons from the radiation causes electrons to be released as photoelectrons.

Question 21

How is the photoelectric effect demonstrated on an electroscope?

Answer 21

> Shining small amount of UV on zinc plate, causes electrons to be emitted from the negative zinc plate causing the angle between the gold leaf and the plate to decrease
But it doesn’t change when large amounts of VISIBLE light is shone

Question 22

Define Threshold frequency

Answer 22

The LOWEST frequency of radiation that will result in the emission of electrons from a particular metal surface

Question 23

Define Work Function

Answer 23

MINIMUM energy required to release an electron from the surface of a metal - energy to overcome the attraction between the electron and metal ions

Question 24

What happens if the incident radiation has a frequency below the threshold frequency?

Answer 24

The photons DO NOT have enough energy to overcome the work function of the metal

Question 25

What happens if the incident radiation has a frequency equal to the threshold frequency?

Answer 25

Causes the electrons to be OMITTED but with NO KE

Question 26

What happens if the incident radiation has a frequency above the threshold frequency?

Answer 26

Causes the electrons to OMITTED with SOME KE

Question 27

What happens if the incident radiation has a higher intensity whilst above the threshold frequency?

Answer 27

MORE electrons are omitted PER SECOND but WILL NOT affect KE

Question 28

What is Einstein’s Photoelectric equation?

Answer 28

hf = ø + KE max

Question 29

What is hf in Einstein’s Photoelectric equation?

Answer 29

hf is the energy of the incident photon where h is Planck’s constant

Question 30

What is ø (phi) in Einstein’s Photoelectric equation?

Answer 30

Ø is the work function of the metal

Question 31

What is KE max in Einstein’s Photoelectric equation?

Answer 31

KE max is the MAXIMUM kinetic energy of an electron once it has been ejected from the metal

Question 32

What were Einstein’s two main points on the Photoelectric effect?

Answer 32

> Each single photon could only eject one single electron | > Either the photon energy was LARGER than the work function; or SMALLER so the electron would remain on the metal.

Question 33

How do you determine the minimum frequency at which electrons are released?

Answer 33

hf = ø + KE max At minimum frequency, only enough energy to release electron KE max = 0 f0 = Threshold frequency hf0 = ø f0 = ø ÷ h

Question 34

What is KE max independent of in the photoelectric effect?

Answer 34

Intensity - it is not affected by the amount of photons coming in

Question 35

What is the rate of emission of photoelectrons (above threshold frequency) proportional to?

Answer 35

Intensity - the larger the number of photons coming in, the greater the rate of emission of photoelectrons.

Question 36

What portrays light to be a wave?

Answer 36

> Refraction | > Interference

Question 37

What portrays light to be a particle?

Answer 37

> The photoelectric effect

Question 38

What indicates waves acting like a particle?

Answer 38

hf = E - Frequency of a wave can give certain photons a certain energy

Question 39

What indicates particles acting as waves?

Answer 39

Electron Diffraction - an electron is a particle yet it can diffract like a wave. This is shown by firing electrons at a thin layer of graphite in a vacuum which creates a circular electron diffraction pattern

Question 40

Why must particles have a wavelength?

Answer 40

For electrons and other particles travel through space as a wave, they must have a wavelength

Question 41

How do you work out the wavelength of a particle?

Answer 41

lambda = h ÷ p = h ÷ mv

Question 42

What are the terms of the De Broglie Equation?

Answer 42

``` lambda - wavelength h - Planck’s constant p - momentum m - relativistic mass (as v—>c , m increases) v - velocity of particle ```

Question 43

What does the intensity of a wave at a point represent?

Answer 43

The probability of a particle being there

Question 44

How do electrons act like waves?

Answer 44

The way they travel through space

Question 45

How do electrons act like particles?

Answer 45

The way they interact, such as attraction.