Module 4.5 - Quantum Physics

Question 1

What is h in E=hc/wavelength

Answer 1

Planck’s constant

Question 2

What is c in E=hc/wavelength

Answer 2

Speed of light in a vacuum

3 x 10^8 m/s^-1

Question 3

How to calculate the photon energy if the frequency of the radiation is known

Answer 3

E=hf

Question 4

How to calculate the photon energy if the wavelength of the radiation is known

Answer 4

E=hc/wavelength

Question 5

Define electronvolt

Answer 5

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

Question 6

Formula to calculate the energy gained by an electron

Answer 6

Energy = charge x potential difference

E=QV

Question 7

What is the photoelectric effect?

Answer 7

When EM radiation of a particular frequency is shone on the surface of a metal and electrons are emitted from its surface

Question 8

Parts of the gold-leaf electroscope

Answer 8

Thin gold leaf is attached to a brass stem

Metal cap on top of the stem

Question 9

What happens when demonstrating the photoelectric effect with the gold-leaf electroscope?

Answer 9

Metal plate (usually zinc) is placed on metal cap and then charged negatively by touching it with a negatively charged polythene rod or by electrostatic induction.
Metal stem and gold leaf will become negatively/positively charged so the stem and leaf repel each other.

Question 10

When would the gold leaf move (i.e. with visible or UV light) and what does this show?

Answer 10

When UV light is shone on a negatively charged zinc plate the leaf falls.
Shows that the metal plate loses its negative charge through emission of electrons, which are repelled by the negative charge on the electroscope. The discharge of the electroscope cannot be caused by ions in the air as it is in a sealed vacuum.

Question 11

When will electrons be emitted from the surface of a metal?

Answer 11

Only if the incident radiation is above the threshold frequency.
Emission starts the instant the surface starts to be irradiated, proving incident radiation exceeds threshold frequency.

Question 12

Explanation of the photoelectric effect

Answer 12

Einstein suggested in 1905 that incident radiation wasn’t behaving as a wave but as packets of energy with fixed energy values depending on the light frequency (photons).

Question 13

What is the work function of a metal?

Answer 13

The minimum energy required to release an electron from the surface.

Question 14

What is hf in Einstein’s photoelectric equation?

Answer 14

Planck’s constant x photon energy of an incident photon

Question 15

Why can’t the photoelectric effect be explained using a wave model?

Answer 15

In the wave model, the energy of a wave depends on its amplitude (intensity) not frequency.

Question 16

What happens at the stopping potential?

Answer 16

All emitted electrons have been brought to rest

Maximum kinetic energy of electrons equals the product of the stopping potential and electron charge

Question 17

How can electron diffraction be demonstrated?

Answer 17

Electrons from an electron gun are accelerated through a vacuum towards a layer of polycrystalline graphite
Electrons diffract as they emerge from the gaps between the atomic layers in the graphite film, and interfere constructively
A electron diffraction pattern is circular because the graphite atoms are not lined up in the same direction as the diffraction grating