Yr 12 - Quantum Physics

Question 1

What is a photon?

Answer 1

Discrete packet of energy

Question 2

What is the rest mass for a photon at the speed of light?

Answer 2

0

Question 3

What is a LED?

Answer 3

Diode

Question 4

What is the threshold voltage?

Answer 4

Max PD of a circuit before any current flows

Question 5

What is an electron volt?

Answer 5

Amount of KE gained by electron that is accelerated through a PD of 1V

Question 6

How do you convert from Joules to electon-volts and vice versa?

Answer 6

1ev = 1.6x10^-19 J

Question 7

Define each letter of the equation

hf = ɸ + KEmax

Answer 7

h = planks constant
f = frequency
ɸ = work function (J)
KEmax = KE of photoelectrons

hf = energy (J)

Question 8

Describe the Photoelectric effect (experiment)

6 Marks

Answer 8

• EM waves exist as quantas of energy (discrete packets) called photons
• When an EM wave hits metal surface photons are absorbed why electrons 1:1
• Photoelectrons emitted if energy > ɸ
• KE of Photoelectrons depends of energy of photon
• KEmax = Energy of photon - work function
• Number of electrons emitted depends on intensity of EM waves as higher intensity means more photons of light and therefore more photoelectrons emitted.

(Gold leaf, Zinc plate, negative charges)

Question 9

Describe the setup & effect of the gold leaf experiment

Answer 9

Zinc plate with gold leaf attached

• Zinc plate negatively charged
• Gold leaf falls when EM radiation exceeds (threshold frequency/work function) as electron charges are ejected
• Leaf is upright because it is negatively charged and is repelling the negative charges of the zinc plate.

Question 10

What does ‘Irradiated’ mean?

Answer 10

Exposed to radiation

Question 11

In terms of the work function, when does:

• Ek = 0
• Ek > 0

Answer 11

• When E = ɸ
• When E > ɸ

Question 12

Do photoelectrons have a constant speed?

Answer 12

No, it varies and is not constant

Question 13

How does Intensity affect frequency/energy?

Answer 13

Intensity has no effect on frequency or energy.

Question 14

What is the threshold frequency?

Answer 14

Minimum frequency of em radiation causing the emission of an electron from surface of a metal.

Question 15

What is the work function?

Answer 15

Minimum energy to remove a single electron from the surface of a metal.

Question 16

Define each letter of the equation:

hf = ɸ + KEmax

Answer 16

h = planks constant

f = frequency of incident wave

ɸ = work function of metal

KEmax = Make KE an emitted photoelectron can have (as KE varies but does have a maximum which is a result of the amount of energy above the work function)

Question 17

What is the photoelectric effect?

Answer 17

When em radiation >ɸ is incident on the surface of a metal
- One election absorbs One photon 1:1 ratio
- Emitted electron is a photoelectron (Photon and electron)

Question 18

Why is graphic suitable for DE Broglie’s electron diffraction experiment?

Answer 18

The spacing between atoms is similar to De Broglie wavelength

Question 19

What is meant by ‘wave-particle duality’?

Answer 19

Em radiation / matter can display both wave & particle properties

Question 20

How does the photoelectric effect explain that waves behave like particles?

Answer 20

It shows that waves behave like particles as there is no way to explain the results unless waves split into distinct particles.

Question 21

Explain:

• Independent
• Dependant
• Control

variables

Answer 21

Independent - You change

Dependent - You then measure effect

Control - You keep the same to ensure a fair test

Question 22

How does electron diffraction explain that particles behave like waves?

Answer 22

Diffraction pattern gives evidence for wave nature of electrons (or particles)