Electrons from Atoms

Question 1

what is the only way (we need to know) that delocalised electrons in metals can escape their from the metals surface

Answer 1

by absorbing a certain well-defined amount of energy

Question 2

referring to the photoelectric effect, what is a practical method for supplying this energy

Answer 2

by shooting a beam of photons at the metals surface

Question 3

what is the new method of energy adsorption we need to know

Answer 3

• thermionic emission

- done by heating the metal to a certain temp

Question 4

if the escaped electrons happen to be in an electric field produced by a pd, what would be the equation for the kinetic energy they have

Answer 4

• E(k) = QV

- you already know the Q for an electron

Question 5

how would you solve the question “how fast would an electron be moving if it was accelerated from rest through a pd of 2500V”

Answer 5

• use E(k) = QV to work out its kinetic energy
• use KE = mv^2 / 2
• you know the mass and KE so rearrange for v

Question 6

what is a cathode ray

Answer 6

a beam of fast moving electrons

Question 7

how can you use thermionic emission to generate a cathode ray

Answer 7

• use thermionic emission to release electrons from a metal

- then accelerate them through an electric field

Question 8

how are the paths of the electrons optically visualised (nothing to do with bubble chambers)

Answer 8

• the fast moving electrons would hit a screen painted with a certain chemical
• when hit the screen would glow at that point
• aka it will emit light at the points the electrons hit it

Question 9

what does shooting electron beams at a crystal allow us to explore

Answer 9

the structure of the crystal

Question 10

what are the two main principles used in this method of crystal structure exploration

Answer 10

• electron diffraction

- wave-particle duality

Question 11

what is the equation for the wavelength of a particle

Answer 11

Y = h / p

Question 12

what are the variables in that equation

Answer 12

• Y = wavelength (m)
• p = momentum / mv (kgms-1)
• h = plancks constant (6.63x10-34)

Question 13

how would you solve “what is the wavelength of an electron in a beam which has been accelerated through 2000V”

Answer 13

• use E(k) = QV to work out kinetic energy
• use KE = mv^2 / 2 to work out velocity
• use Y = h / mv to work out wavelength

Question 14

what main condition needs to be met in order for the electron diffraction to occur

Answer 14

the gap needs to be the same size as the wavelength of the electron

Question 15

how are electrons diffracted when shot at the crystal

Answer 15

• the gaps between the atoms in the crystals act as diffraction gratings
• so the electrons produce a diffraction pattern when they pass through them

Question 16

how are the spacings between the atoms actually worked out

Answer 16

• theres a screen behind the crystal
• the diffraction patterns shows up on the screen
• measuring the pattern allows you to calculate the gap between the atoms