4.5 Quantum physics

Question 1

What is the quantum model?

Answer 1

In 1900, Max planck’s work on the electromagnetic radiation emitted by hot objects, such as stars or a hot-red glowing lump of metal made him realise that if he radiated energy could not have continuous values. Instead planck hypothesis that the total radiated energy had to be emitted in packets, with each packet or quantum having a definite, fixed amount of energy.

Question 2

What is the photon model?

Answer 2

Einstein’s explanation opened up the paradox that light sometimes acts in a way that can only describes as acting as particles. Louis De Broiglie showed that light could behave as both a particle and a wave.

Quanta of light or other electromagnetic energy became know as photons.

Question 3

What equation relates Planck’s constant, energy of a photon and frequency?

Answer 3

Energy of a photon= Planck’s constant x frequency

E= hf

Question 4

What happens to the energy on a photon if the frequency increases?

Answer 4

The energy of a photon is directly proportional to it’s frequency. If the frequency increases, the energy on the photon would increase.

Question 5

Define a quantum?

Answer 5

A quantum (plural of quanta is a small discrete unit of energy)

Question 6

What is a photon?

Answer 6

A photon is a quantum (discrete unit of energy) associated with electromagnetic radiation.

Question 7

What is Planck’s constant?

Answer 7

The Planck’s constant, h, has a value of 6.626 x 10-34 J - Photon energies are always emitted in multiples of this.

Question 8

What equation relates wavelength, wave speed and frequency?

Answer 8

Wave speed = frequency x wavelength

Question 9

What is the equation to find the energy on a photon including wavelength?

Answer 9

Energy on a photon = Planck’s constant x wave speed / wavelength

Question 10

What is an electronvolt?

Answer 10

An electronvolt is defined as the kinetic energy gained by an electron when it is accelerated thorough a potential different of 1 volt.

Question 11

How many joules is 1 eV equal too?

Answer 11

1 eV is equal to 1.602 x 10-19 J

Question 12

How do you go from elecronvotls (eV) to joules (J)?

Answer 12

eV to joules x 1.602 x10-19

Question 13

How do you convert joules (J) to elecrtonvolts (eV)?

Answer 13

Joules (J) to elecronvolts (eV) divide by 1.602 x10-19

Question 14

What bit is apparatus can you demonstrate the photoelectric effect?

Answer 14

A gold-leaf elecroscope.

Question 15

What is the photoelectric effect?

Answer 15

The photoelectric effect can be describes as when magnetic radiation on a particular frequency is shone on the surface the a metal, electrons are emitted from the surface.

Question 16

What are the electron emitted from a metal due to the photoelectric effect called?

Answer 16

Photoelectrons

Question 17

What is a gold-leaf electroscope composed of?

Answer 17

A gold-leaf electroscope is composed of a brass stem to which a thin gold leaf is attached. There is a metal cap attached to the top of the stem and the metal to be irradiated with electromagnetic radiation is laid on the metal cap.

Question 18

How does the gold-leaf electroscope demonstrate the photoelectric effect?

Answer 18

A metal plate (usually zinc) is placed on the metal, then charged negatively by touching it with a negatively charged polyethene rod, or by electrostatic induction. When this is done, the metal stem and the gold leaf will become negatively charged, meaning the leaf and stem will repel each other.
It is also possible to make the zinc plate, metal stem and gold leaf positively charged.
When UV light is shone on the negatively charged zinc plate, the gold leaf falls.

Question 19

What does it show after the UV light is shone on the zinc plate and the gold leaf falls?

Answer 19

This shows the metal plate has lost it’s negative charge, due to emission of electron.

Question 20

Why can’t the discharge of electron be caused by ions in the air?

Answer 20

Because the electroscope is in a sealed vacuum.

Question 21

What is threshold frequency?

Answer 21

The threshold frequency is the lowest frequency of radiation that will result in the emission of electrons from a particular metal surface.

Electrons will only be emitted from the surface of a metal if the incident radiation is above the threshold frequency.

Question 22

What happens if visible light is shone on the zinc plate on the electroscope? Why does this happen?

Answer 22

Nothing happens, no matter how bright the light is because visible light’s frequency is too low to emit the electrons.

Question 23

How was the photoelectric effect explained?

Answer 23

Albert Einstein suggested that the incident radiation was not behaving as a wave but instead as packets of energy known as photons.

Question 24

What is a photons energy proportional to?

Answer 24

It’s frequency.

Question 25

Define work function.

Answer 25

The work function of a metal is the minimum energy required to release and electron from it’s surface, overcoming the electrostatic attraction between the electron and the positive metal ion.

Question 26

How is an electron emitted from the surface of a metal?

Answer 26

Incident radiation is shone on the metal, if the radiation is below the threshold frequency, this means the photons do not have enough energy to overcome the work function of the metal.
If the incident radiation has a frequency equal to the threshold frequency the electron will be omitted from the metal surface but with no kinetic energy.
While if the incident radiation is above the threshold frequency, this will cause the electron to be emitted with kinetic energy.
Each single photon can only eject one electron from the metal surface.

Question 27

What is Einsteins photoelectric equation?

Answer 27

hf = work function + KEmax

Plank’s constant x frequency = work function + maximum kinetic energy of an electron after it has been emitted.

Question 28

What is wave-particle duality?

Answer 28

The concept that every particle or quantum entity may be described as either a particle or a wave.

Question 29

What two models can we use to explain the behaviour of radiation?

Answer 29

• Wave model- diffraction and interference
• Particle model- photoelectric effect

Question 30

In 1923, what did the scientist Louis do Broglie suggest?

Answer 30

All matter, regardless of its mass could display wave-like properties.

Question 31

How can you prove particles can also act like waves?

Answer 31

Through diffraction and interference.

Question 32

How can you show electron diffraction?

Answer 32

• The wavelength of electrons is much smaller than that of light. The slit spacing of a diffraction grating is very large in comparison with the electrons wavelength, so the spacing between atoms - which is very similar to the electrons wavelength- is used instead.
• Electrons from a electron gun are accelerated through a vacuum towards a layer of polycrystalline graphite.
• The electrons diffract as they emerge from the gaps between the atomic layers in the graphite film, and interfere constructively.

Question 33

When electrons diffract, why is does it create a circular pattern instead of parallel lines seen when light diffracts?

Answer 33

The graphite atoms are not all lined up in the same direction as in the diffraction grating.

Question 34

What is the de Broglie equation?

Answer 34

λ=h/p

lambda= planks constant/ momentum

also written as λ=h/vm

Question 35

What is the difference between a photon and a wave?

Answer 35

A photon is a particle or quantum of electromagnetic radiation of energy hf.

A wave is any longitudinal or transverse means of transferring energy through a vacuum or a medium without any net transfer of matter.

Electromagnetic radiation can be considered as either a stream of photons or showing wave properties.

Question 36

How can the wave-particle duality of an electron be used to investigate the structure of the atom and nucleus?

Answer 36

Electrons, when travelling at significant speeds, will have wavelengths that are comparable to that of an atomic spacing or nuclear diameter. This means that it is possible to observe the diffraction effects and ascertain information about the atomic or nuclear structure, which is of the order of between 10–10 m to 10–15 m in diameter.

Question 37

Why do we not notice the wave nature of objects such as cars?

Answer 37

In order to notice the wave nature, diffraction needs to occur. This happens when the wavelength of the object is equal to the gap or diameter of an object that the radiation passes through or around. For massive objects moving at low speeds, this value is simply too small for it to be noticed.

Question 38

When does diffraction occur?

Answer 38

When the wavelength of the object is equal to the gap or diameter of an object that the radiation passes through or around.

Question 39

What happens to an electron’s wavelength as it is accelerated?

Answer 39

It’s wavelength decreases.