Physics - Common Exam Q (WACE) (U3&4)

A radio antenna is able to convert electrical signals into radio signals, transmitting information to distant receivers. The antenna does this by oscillating a charge along its length. Describe the waves produced and how the signal is able to be picked up by the receiving antenna (4 marks) (WACE 2018)

- Electromagnetic waves are transverse waves made up of mutually perpendicular, oscillating perpendicular electric and magnetic fields (1 to 2 marks)
- Oscillating charges produce electromagnetic waves of the same frequency as the oscillation; electromagnetic waves cause charges to oscillate at the frequency of the wave (1 to 2 marks)

Describe the characteristics of a black body and use the black body radiation curves (it was in the question) to explain why the concept of light quanta was necessary (5 marks) (WACE 2017)

- A black body is a theoretical body that absorbs and emits radiation perfectly but never reflects the incident radiation
- as temperature increases, intensity increases overall
- and the peak intensity shifts to a shorter wavelength (or opposite for temperature decrease)
- the ultraviolet catastrophe predicted classically that there would be a high amount of UV light
- but there wasn't, which gave rise to the idea of light being emitted in specific amounts

Describe why the colour and intensity of the tungsten changes as it is heated (2 marks) (WACE 2018)

2 WAYS TO ANSWER:

#1:
- as the molecules are heated, they vibrate faster and thus produce a higher frequency (accelerate and decelerate more rapidly the greater the deceleration the greater the frequency of light produced)
- as the temperature increases there are more molecules vibrating at faster speeds and so the intensity of the higher frequencies increases

#2:
- black body radiation where the dominant wavelength decreases as the temperature increases hence the colour changes
- as the temperature increases the power increases and hence an increase in intensity

Explain how emitted photons produced by a mercury atom produces visible light as it is incident on fluorescent material (3 marks) (WACE 2018)

- the high energy photons (UV) are absorbed causing the electrons of the fluorescent material to jump to a higher state
- these electrons then fall back to the ground state in a series of steps
- emitting light some of which is in the visible spectrum

Discuss the interactions that must occur to produce 2 gamma rays travelling in opposite directions to each other (5 marks) (WACE 2018)

- a positron and an electron must meet/interact due to electromagnetic forces
- as positron is the electron anti-particle
- they will annihilate producing energy in the form of photons
- to conserve energy two high energy photons must be produced
- two photons travel in opposite directions to conserve momentum

Discuss how Hubble's Law supports the Big Bang Theory (5 marks) (WACE 2018)

(this could be a 4 mark question if you removed the very first dot point)

- Hubble's law suggests cosmological red shift is caused by the stretching/expansion of photons of light from galaxies as space/time itself expands
- Hubble's law also explains that galaxies further away are travelling faster then those nearby
- this causes photons wavelength to increase/red shift
- this provides evidence that galaxies are moving further apart and therefore the universe is expanding in all directions
- this supports the Big Bang Theory hypothesis that all galaxies must have originated from a singularity if galaxies trajectories are traced back to their origin