__Unit 16: Modern Physics__ Relativity, Quantum and Nuclear Physics

__Unit 16: Modern Physics__

__Reading__

BJU book: Ch. 27, 28, 29

__Lab__

Transistor Switching lab (our final circuit-build lab)

__Homework__

Relativity homework questions, taken from Ch. 27 in your book. These are hosted in Canvas.

__Transistor Switching lab__

- We will explore how transistors work as switches and amplifiers, by making circuits using transistors
- We will measure the current gain (IE/IB) of the 2N2222 transistor, and discuss what that means

Your lab report for this lab has 5 required 'elements', which are listed in Canvas. Your report must have all 5.

transistor_switching_lab_handout.pdf |

__Quantum Physics chapter notes__

A

*photon*is simply a packet of light (or other electromagnetic energy). The energy

*E*of a photon is given by

**where h is Planck's constant and**

*E=hf**f*is the frequency. Planck's constant is 6.6 x 10^-34 J-s, but also can be expressed in electron volts as 4.1 x 10^-15 eV-s. Therefore, you can say 1 eV = 1.6 x 10^-19 J. The frequency

*f*is in cycles/s, otherwise known as Hertz (Hz).

Velocity of a wave is

*frequency*x

*wavelength*

*. This is basic wave mechanics that we learned a long time ago. Furthermore, light moves at 3 x 10^8 m/s, so you can set*

**v = fλ***v = c = 3.0 x 10^8 m/s*. The wave equation when you are talking about light becomes

**and can be rearranged as**

*c = fλ***or**

*f = cλ***.**

*λ = cf*Bohr said that you have energy levels within a hydrogen atom (n=1, n=2, n=3), and the single electron in the H atom can only occupy these discreet energy levels or 'orbitals'. The orbital closest to the nucleus is n=1, and the numbers get bigger as you go outward. When an electron jumps down a level (from example from n=2 to n=1), it emits a photon having that much energy. Therefore, in order to jump up a level (or two, or three levels), it must absorb a photon having sufficient energy. If the incoming photon doesn't have enough energy (i.e. high enough frequency), it won't elevate the electron to a higher level. Like I said above, the energy of a photon is given by this equation

*E = hf*, and therefore the higher the frequency is, the more energy that photon will possess. The energy can be calculated in Joules (a really big unit of energy) or electron volts (a really small unit of energy). I gave you the conversion factor above, so you can convert between J and eV easily. Another thing: the total energy

*E*of each 'orbital' is stated as a negative number, just like you would expect from thermodynamics: the closer an electron is to the nucleus, the larger the negative number becomes, because it's sitting in a larger 'energy hole'. So, for example,

*n=1*might be -100 eV and

*n=3*might be -11 eV. Anyway, you need to read up on this and be familiar with it before starting the homework. If you don't understand the Bohr atom, and how to calculate the energy of a photon, you won't be able to do the homework.

For a couple of the homework questions, you need to know that the

__radius__of a given energy level

**(n=1, n=2, etc) within the Bohr atom is directly proportional to n^2. In other words,**

*n***. That means n=2 would have a radius 4 times larger than n=1, and n=3 would have a radius 9 times larger than n=1, and so on. ALSO, the total**

*radius ∝ n^2*__energy__of a given energy level

*n*is

*inversely proportional*to -

*n^*2. In other words,

**. So for example if**

*energy ∝ 1/-n^2**n=1*was -100 eV, then

*n=2*would be -25 eV, and

*n=3*would be -11 eV. If you kept going out from the nucleus, n(infinity) would have total energy = 0.

In summary, the concepts from this chapter are new, but the math is easy and straight forward.