19 Quantum Physics

Question 1

Definition of a Photon

Answer 1

A quantum of electromagnetic energy

Question 2

Energy of a Photon formula

Answer 2

E = hf
Where h is Planck’s constant, f is frequency

Question 3

Electronvolt eV to Joules J conversion

Answer 3

1 eV = 1.60 x10^-19 J

Question 4

What is the photoelectric effect?

Answer 4

The emission of electrons from a metal surface when the surface is irradiated with electromagnetic radiation of a high enough frequency

Question 5

What is stopping potential (Vs)?

Answer 5

The value of potential difference where detected photocurrent just becomes zero
E(k, max) = e(Vs)

Question 6

Evidence of the particulate nature of light from the photoelectric effect

Answer 6

Instantaneous emission of photoelectrons -> photon-electron interaction is one-to-one

Question 7

Einstein’s photoelectric effect relation

Answer 7

hf = ϕ + E(k, max)
Energy of photon = work function + max E(k) of electron

Question 8

Effect on changing frequency on photocurrent

Answer 8

As frequency increases, the stopping potential increases in magnitude (more negative)
Photocurrent remains constant

Question 9

Effect of changing intensity on photocurrent

Answer 9

As intensity increases, maximum photocurrent increases
Stopping potential remains constant

Question 10

de Broglie Wavelength equation

Answer 10

λ = h/p
Where p is momentum

Question 11

Heisenberg Uncertainty Principle

Answer 11

ΔpΔx ≈ h
ΔpΔx must be in the same direction (usually at the slit)

Question 12

Ground state of an atom

Answer 12

The lowest state an atom can be at where all electrons in the atom occupy the lowest possible energy states

Question 13

Excited state of an atom

Answer 13

When one or more electrons of an atom are not occupying the lowest possible energy state

Question 14

Ionisation

Answer 14

The removal on an electron from an atom to create an ion

Question 15

What is an emission line spectrum?

Answer 15

A series of distinct coloured lines against a dark background

Question 16

What is an absorption line spectrum?

Answer 16

A series of distinct dark lines against a continuous background

Question 17

Number of emission lines

Answer 17

n choose 2, where n is number of energy states

Question 18

Number of absorption lines

Answer 18

n -1, where n is number of energy states

Question 19

Characteristics of an X-ray spectrum

Answer 19

• Broad continuous spectrum (Braking radiation)
• Sharp peaks (Characteristic lines)
• Sharp cut-off (Minimum wavelength)

Question 20

How is Braking Radiation produced?

Answer 20

• External electrons does not collide with internal electrons
• The electron instead experiences an acceleration, deceleration or deflection, which emits braking radiation

Question 21

How is characteristic X-ray lines formed?

Answer 21

1. Accelerated incident electron collides with an electron from the innermost shell.
2. Inner electron is kicked out
3. An electron from a higher energy shell transitions to the vacant spot, which emits a photon

Question 22

Why is there a sharp cut-off at the end of the x-ray spectrum?

Answer 22

• X-ray photon with the maximum energy is emitted when all E(k) of electron is transferred
• Thus, there is a maximum frequency of photons, which means there is a minimum wavelength of the photon

Question 23

Why is the energy levels of an atom negative?

Answer 23

Electrical potential energy is defined to be 0J when it is at infinity. Since electrons are attracted to nucleus, work is done against the electron in order to bring the electron from infinity to the point