Question

a) Define crystal and unit cell

b) Explain de Broglie wave and the photoelectric effect.

c) A wire of length 7 𝑚 and radius 2.37 𝑚𝑚 is stretched by a load of 12370 𝑔𝑚 and increased by 1.78% of the original length. Find the stress, strain, and Young’s modulus of the material of the wire. 

Answer 1

a) Crystal is a complex system that obtains all mechanical properties of a solid. A unit cell is the smallest element of the crystal that has such properties.

b) De Broglie wavelength is a wavelength a body would be if it were a particle. The photoelectric effect: metals emit electrons when light of the corresponding wavelength is shone on them.

c) The strain: 1.78/100=0.0178.

The stress:

$$\sigma=F/A=mg/(\pi r^2)=6.87\text{ MPa}.$$

The young's modulus:

$$E=\frac\sigma\epsilon=386\text{ MPa}.$$