P3: Quantum Phenomena

Question 1

Stopping potential and max. kinetic energy

(What is their relationship?)

Answer 1

eV = Ek (max)
[where e is charge on an electron in C, and V is stopping potential in V]

Question 2

What does “Wave-Particle Duality” mean?

Answer 2

The fact that electromagnetic radiation displays the characteristics of both a wave and a particle

Question 3

When does light behave as a wave?

Answer 3

When it interferes and diffracts and produces patterns - this can only be explained by the wave property of interference, constructively or destructively

Question 4

What is the de Broglie equation?

Answer 4

(Lamda) = h/mv
Wavelength = planck’s constant / momentum

Question 5

When does light behave as a particle?

Answer 5

Through the photoelectric effect, when photons act like discrete bundles of energies, which interact with electrons in a one-to-one way

Question 6

What is an electronvolt?

Answer 6

The kinetic energy carried by an electron after it has been accelerated from rest through a potential difference of 1 volt.

Question 7

What is a line emission spectrum?

Answer 7

A series of bright lines against a black background - how is this produced?

Question 8

What is a line absorption spectrum?

Answer 8

A series of black lines on a coloured spectrum - how is this produced?

Question 9

How do fluorescent tubes work?

Answer 9

• mercury vapour ionised by a high pd, producing free electrons
• other mercury atoms excited by f electrons
• de-excite, emitting UV photons
• photons absorbed by phosphor coating, excited atoms, who’s electrons de-excite releasing visible light photons

Question 10

What is the photoelectric effect?

Answer 10

When light (usually UV) is shone upon the surface of a metal, giving the surface a positive charge. Free electrons on the surface absorb energy from the light, and can be liberated.

Question 11

Conclusions from the photoelectric effect: (3)

Answer 11

• no photoelectrons are emitted by radiation below the threshold frequency
• the maximum kinetic energy that emitted photoelectrons can have increases with the frequency of the radiation
• number of emitted photoelectrons / second is proportional to the intensity of the radiation

Question 12

Einstein’s Photon Model of Light:

Answer 12

EM waves carry energy in photons
E = hf = hc/(lamda)
If one of these photons collides with a free electron, it will gain energy equal to hf

Question 13

Threshold frequency

Answer 13

For electrons to be emitted, the energy they gain must be greater than the work function.
f = (work function) / h

Question 14

Ev into J

Answer 14

1eV = 1.60x10^-19 J

Question 15

What is the accelerating voltage (in V) of a photon?

Answer 15

= The energy gained by it from rest (eV)