Quantum Physics

Question 1

Define energy of a photon

Answer 1

The energy, E, of a photon is directly proportional to the frequency, f, of the electromagnetic radiation

Question 2

What is the equation for energy of a photon?

Answer 2

E = hf

since v = fλ, this can also be written as

E = hc/λ

Question 3

Define the electronvolt

Answer 3

1 electronvolt is defined as the energy transferred when an electron travels through a potential difference of 1 volt

Question 4

What is the value in joules of an electronvolt?

Answer 4

1.6x10^-19J

Question 5

State the photoelectric effect

Answer 5

When electromagnetic radiation of high enough frequency is shone onto a metal, electrons are released (emitted) from the surface of the metal

Question 6

Define the work function (Φ) of a metal

Answer 6

The work function is the minimum energy required to remove an electron from the surface of the metal

Question 7

Define threshold frequency of a metal

Answer 7

The threshold frequency is the minimum frequency of electromagnetic radiation required to free electrons from the surface of the metal

Question 8

What is Einstein’s photoelectric equation?

Answer 8

hf = Φ + KEmax

where Φ is the work function of the metal, and KEmax is the maximum kinetic energy of the released electron

or hf = Φ + 1/2mv^2max

Question 9

Explain what effect the intensity of radiation has on the rate of electron emission, with regard to the threshold frequency

Answer 9

If the incident radiation has frequency below the threshold frequency, then no electrons will be released, regardless of the intensity. However, if the frequency is above the threshold frequency, then increasing the intensity of radiation will increase the rate of electron emission

Question 10

Explain how we know electromagnetic waves show both wave and particle behaviour

Answer 10

Diffraction and superposition of light relies on the radiation acting as a wave, but the photoelectric effect relies on it acting as discrete photons.

Question 11

What is the de Broglie equation?

Answer 11

λ = h/p

since p = mv, we can also write this as

λ = h/mv

Question 12

Explain how electrons show wave-particle duality

Answer 12

Electrons are classified as particles, with mass and charge. They can be accelerated and deflected by magnetic and electric fields, particle behaviour. However, they can also be made to diffract, which is a property of waves

Question 13

State the characteristics of the particle model (photon model) of light

Answer 13

• light and all forms of electromagnetic radiation, is emitted in brief bursts of energy. It is quantised
• the bursts of energy, photons, travel in one direction and only in straight lines
• when an atom emits a photon, it’s energy changes by an amount equal to the photon energy
• the amount of energy, E, contained in each quantum = hf

Question 14

State the value of Planck’s constant (h)

Answer 14

6.63x10^-34

Question 15

Define the photon

Answer 15

A photon is a quantum of energy of electromagnetic radiation

Question 16

Describe a demonstration of the photoelectric effect

Answer 16

• zinc connected to a negatively charged electroscope
• gold leaf with a negative charge attached to the electroscope is repelled
• when UV light is shone onto the zinc, photoelectrons are emitted and the electroscope gradually discharges
• the gold leaf is no longer repelled and falls

Question 17

Explain how an experiment using LEDs can be used to estimate Planck’s constant

Answer 17

• LEDs require a threshold voltage to conduct and emit light
• assuming that when an LED conducts, all of the electrical energy lost by an electron becomes the energy of a single emitted photon
  eV = hc/λ, where eV is the electron energy and hc/λ is the photon energy
• we know the value of e, we can read off the value of V, we know the value of λ (depending on LED colour), and we know the value of c

Question 18

Equation for threshold frequency

Answer 18

f₀ = Φ/h

Question 19

How can Planck’s constant (h) be worked out graphically?

Answer 19

The values of wavelength and threshold p.d. for the wavelength of light can be recorded, and a graph drawn of V against 1/λ. The gradient of this graph will be equal to hc/e. As the speed of light and the electron charge are known constants, we can calculate the value of h from this.