#g482

[brandnewdress]

So.... what the actual fuck was that physics exam?!

[lazy-vulcan]

Last exam today yesssssssss

[physicsandchemistryrevision]

AS Physics G482

Quick notes Malus' law See "Malus' law" (old post, which adds more context to Malus' law)

Malus' law states that: Intensity of a wave is directly proportional to the square of the amplitude of the wave.

So Malus' law tells us that I ∝ A², where I is wave intensity (power per unit area) and A is wave amplitude.

 Why I ∝ A²?

It can be shown that

wave intensity ∝ energy contained in a wave so I ∝ E

and

energy contained in a wave ∝ amplitude² so E ∝ A² 

This implies that

I ∝ E ∝ A²

∴ I ∝ A²

—

This means that if we increase the wave amplitude by 4 times the original wave amplitude, we get twice the wave intensity.

And if we measure 3 times the intensity, the wave has 9 times the amplitude of the original wave.

[mynameisamylol]

The exam itself went pretty well but about half way through I felt really ill.

At first I just felt kind of sick, which was annoying but bearable. Then I started to feel really light-headed and my hands and lips tingled, I thought I was going to pass out. It was so awful.

I couldn't concentrate and spent like 20 minutes answering the easy questions until eventually I decided to leave. An invigilator took me outside and I got some fresh air and after about 5 minutes I felt much better. I went back in and still didn't feel great but I managed to do the more challenging questions on the paper (and luckily I got 5 minutes extra time).

All day I've felt slightly light-headed and I'm really hoping that this is just down to being overworked (as I was working until like 11pm last night). And tomorrow I have my final exam and I need to be on form because its going to be another challenging exam.

I would give so much to not have a repeat of today, it was awful. I just hope that it doesn't affect my overall grade too much.

[liveslow-dienumb]

I've now reached the 'hysterical denial' stage of revision.

If I can't see any notes/past papers/ text books then my exam does not exist

[physicsandchemistryrevision]

AS Physics G482

Electricity I-V characteristics

Ohmic conductor

Obeys Ohm's law because I and V remain constant.

Forward-biased diode

Has a turn on voltage required to give electrons energy = hf to create a photon of light.

Thin, metal wire

Temperature increases - I will decrease due to increase in resistance.

Thermistor

Semiconductor so I = Anev Temperature increases - n increases due to v decrease - less resistance.

[physicsandchemistryrevision]

AS Physics G482

Quantum physics Emission spectra

Emission spectra for an atomic gas

The angle at which the first order maxima n = 1 appears depends upon the wavelength of the light - the larger the wavelength the further distance away n = 1 will be from n = 0.

d sinθ = λn

The excited atoms only emit light at very particular wavelengths. Each element has its own particular wavelength.

Why do 'excited' atoms only emit particular wavelengths of light?

The electrons in atoms can only exist in particular energy levels. For example, in hydrogen:

  0 eV means the electron is free to move.

Ground state is the lowest energy level of the electron. In hydrogen, a ground state electron requires 13.6 eV of energy to completely 'ionise' the atom (ie, to free the electron).

The electron energy levels in atoms are quantised in that they only have particular values.

Because the electron jumps to a higher energy level it must have gained energy, and due to the principle of conservation of energy it must then lose it again. It does this by jumping back down to a lower energy and releasing its energy in the form of a photon.

Energy lost ΔE (E1-E2) by the electron equals the energy E = hf of the photon emitted.

ΔE = hf

If the electron moves to a third or higher energy level, all variables of energy change must be taken into account (E1-E2, E3-E2, E3-E1) and this will produce multiple wavelengths of light due to the different photon energies.

[physicsandchemistryrevision]

AS Physics G482

Quantum physics The photoelectric effect

Einstein's photoelectric equation Einstein further developed Planck's theory of quantum energy. He suggested that the ultraviolet quantum of energy (hf) hitting the metal plate was used to release an electron from the surface atom, and any energy remaining became kinetic energy of the electron.

Using the law of conservation of energy he introduced the equation:

hf = ϕ + (½mv2)max

photon energy = energy to release electron + kinetic energy of electron                        =               (work function)     +         KEmax

Proving Einstein's equation

KEmax = hf - ϕ     y      = mx + c

The threshold frequency is the lowest frequency of radiation that will result in the emission of an electron from a particular metal surface. For most metals, this frequency occurs in the ultraviolet region of the EM spectrum.

Metals that hold their surface electrons less tightly (such as sodium) will have a lower threshold frequency f = ϕ / h.