Question

3. 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 an arrangement of atoms that makes up a solid. Unit cell is the smallest arrangement of the crystal that acquires its properties.

b) De Broglie wave is a wave that would a body acquire if it were an elementary particle. The photoelectric effect is the emission of electrons when the light is shone on metals.

c) See the solution below:

$$\epsilon=1.78\%/100\%=0.0178.\\\space\\ \sigma=\frac{F}{A}=\frac {mg}{\pi r^2}=6.87\text{ MPa}.\\\space\\ E=\frac{\sigma}{\epsilon}=386\text{ MPa}.$$