M7 Topic 3 Part 1: Black Body radiation

Question 1

what is a blackbody

Answer 1

a object that both emits and absorbs all incident radiation

Question 2

what did Maxwell predict about EM radiation

Answer 2

accelerating charges emit EM radiation

Question 3

what does maxwell’s prediction mean for objects at non zero (kelvin) temperatures

Answer 3

these objects contain charges that are in constant motion, which therefore are constantly emitting EM radiation

Question 4

What determines the amount of electromagnetic radiation emitted from the object

Answer 4

the temperature of the object

Question 5

what law relates to black body radiators

Answer 5

Wien’s Law

Question 6

what is Wien’s law

Answer 6

入ₘₐₓ= b/T

Question 7

what does each symbol in Wien’s law mean and give their unit of measurement

Answer 7

入ₘₐₓ = The maximum Wavelength (metres (m))
b = Wien's constant (2.898x10⁻³mK)
T = temperature of the blackbody (kelvin (k))

Question 8

what is the classical understanding of blackbodies (3)

Answer 8

Classical physics states:

• that each frequency of vibration should have the same energy. (equipartition theory)
• there is no limit to how great the frequency can be
• so there is no limit to the energy of the vibrating electrons at high frequencies.

Question 9

what was the issue with classical theory of light when applied to a black body curve (2)

Answer 9

• classical theory shows that the trend should continue infinitely as the temperature increases
• experimental results show there is a sudden drop in emitted intensity in the UV region (Ultraviolet catastrophe)

Question 10

Who fixed the Ultraviolet catastrophe

Answer 10

Max Planck

Question 11

how did Max Planck attempt to solve the ultraviolet catastrophe (2)

Answer 11

Posed a Mathematical model:

• suggested the vibrational states of the charges in the black body were not continuous only specific states were available
• the transition between these states released discrete amounts of energy later known as photons.

Question 12

how is the energy of each photon calculated

Answer 12

the plank relation

Question 13

what is the the plank relation

Answer 13

E = hf

Question 14

what do the symbols in the plank relation mean and what units are they measured in

Answer 14

E = energy of the photon (joules (J))
h = planck's constant (6.626x10⁻³⁴ m² kgs⁻¹)
f = frequency (hz)

Question 15

how did einstein change planck’s model to solve the ultraviolet catastrophe (2)

Answer 15

• he changed light from being a continuous wave to discrete packets of energy called photons
• explained that a blackbody can absorb and emit discrete photons rather than continuous waves (quantum theory)