Quantum Physics

Question 1

Equation for energy of a photon.

Answer 1

E = hc/λ

Question 2

What is an electronvolt?

Answer 2

This is the energy when an electron travels through a p.d of one volt. ( 1eV = charge of a particle. joules)

Question 3

Equation when a charge is accelerated through a p.d.

Answer 3

eV = ½mv². ( however this doesn’t apply when it is accelerated through a high voltage to speeds approaching the speed of light as the mass will increase)

Question 4

What is threshold frequency?

Answer 4

This is the minimum frequency required to release electrons from the surface of a metal.

Question 5

What is work function of a metal? (Phi)

Answer 5

The minimum amount of energy required by an electron to escape it’s surface.

Question 6

Rules of photoelectric effect.

Answer 6

• Electrons are removed from metal surface.
• A photon can only interact or exchange energy with one electron.(1 to 1 interaction).
• An electron is removed instantaneously when the energy of incident photon is > or = work function.
• Energy must be conserved during interaction.
• Intensity only affects rate at which photons arrive at the plate and not the frequency or energy of the photons.

Question 7

Equation for energy of photon interacting with an electron. ( photoelectric equation)

Answer 7

energy of photon = work function + max K.E of electron. (hf = phi + ½mv² max )

Question 8

Equation for threshold frequency.

Answer 8

f₀ = phi / h

Question 9

If the incident radiation has frequency less than threshold frequency?

Answer 9

When interaction occurs the electron can’t escape the attractive forces of + ions . The energy absorbed from the photons appears as KE of the electrons, this is lost to the metal ions, thus warming up the metal.

Question 10

What is Emission line spectra and

Absorption line spectra?

Answer 10

E.L.S shows the composition of light emitted by hot gases .

A.L.S this is when white light is passed through cool gases to observe a spectrum of light.

Question 11

What does quantised mean?

Answer 11

In discrete values. There can’t be an energy or energies between two energy levels.

Question 12

If the electron make a transition from higher energy level than before …..?

Answer 12

Then the energy loss of the electron is higher and this leads to the emission of a more energetic photon.

Question 13

Distinctive energy levels of an atom mean that….?

Answer 13

the energy of the photons emitted hence the wavelength emitted will be unique to that atom.

Question 14

Different elements have different line spectra because…?

Answer 14

They have different spacings between their energy levels.

Question 15

Photon energy is also equal to difference in energies between two levels. So the equation is..?

Answer 15

hf = E₁ - E₂

Question 16

What are isolated atoms?

Answer 16

Gas atoms that exert negligible electrical forces on each other.

Question 17

Electron energy levels in solids are very close together so…

Answer 17

The energy levels form bands with forbidden gaps between them (no energy value lies in here)

Question 18

An electron whose energy lies in the valence band is?

Answer 18

Bound to an individual atom.

Question 19

Explain using band theory what happens in a metal when it’s electrons gain energy from a cell.

Answer 19

In metals there are empty energy levels higher in the (conduction) band into which electrons can move, and when they have moved upwards they are free to move through the metal so there is current. ( in some metals the top of valence band overlaps with bottom of conduction band so there are plenty e. to carry current).

Question 20

Explain using band theory why insulators can’t conducted electricity.

Answer 20

In insulators there is a large energy gap between the top of the valence band and the bottom of the conduction band. The voltage of a cell is insufficient to lift even the most energetic electrons across the gap into the conduction band, so electrons are not free to move thus no conduction.

Question 21

Difference between insulator and semiconductor using band theory.

Answer 21

The gap between the V.B and the C.B is very small so a few electrons have enough energy to jump across the gap. If a semiconductor is heated more electrons will gain energy needed to jump the gap.

Question 22

De Brogile equation.

Answer 22

λ = h/mv

Question 23

De Brogile equation applies to?

Answer 23

All matter.

Question 24

Light and electrons have wave-particle duality . That is?

Answer 24

• They interact with matter as a particle (proven by newtonian mechanics)
• They travel through space as a wave ( proven by diffraction of electrons)