3.1 - photo-electricity

Question 1

describe the structure of metals

Answer 1

metals consist of delocalized (conductive) e- and positive metal ions

they can both collide off another

Question 2

describe the photoelectric effect

Answer 2

photoelectrons were emitted from the surface of a metal when electromagnetic waves above a certain threshold frequency were directed at the metal

Question 3

that did the observations of the photoelectric effect go against

Answer 3

wave theory

• stating light was a wave

Question 4

state the 1st main observation of the photoelectric effect

• describe how it went against wave theory

Answer 4

1) all metals have a threshold frequency, above this no e- will be emitted
- wave theory; since waves are continually delivering a beam of energy increasing the intensity should provide enough energy to remove e-, and the lower the frequency the longer it should take to produce phtote-

Question 5

state the 2nd main observation of the photoelectric effect

• describe how it went against wave theory

Answer 5

2) no. of e- released is proportional to the intensity of the electromagnetic wave
- increasing the intensity should remove e- with a varying greater Ek - max

Question 6

state the 3rd main observation of the photoelectric effect

• describe how it went against wave theory

Answer 6

photoelectrons are produced, with no time delay, at the threshold frequency

• emission should take longer using lower intensity, there should be a delay

Question 7

state Einstein’s theory on light

Answer 7

light is split up into wave packets, photons

Question 8

give the equation for energy of a photon

Answer 8

E = hf

(h = 6.63x10^-34Js)

Question 9

describe Einstein’s photon theory used to describe photoelectricity

Answer 9

• incident light on the surface of a metal, e- absorbs 1 photon and gains the full energy of the photon
• the energy supplied by a photon needs to exceed the metals work function for the e- to be emitted

(if energy < ø, the e- vibrates and dissipates the energy off as heat before being hit by another photon, thus isn’t removed)

Question 10

define work function, ø

Answer 10

the min energy needed for an e- to escape the metals surface

Question 11

state the equation used to calc Ekmax

Answer 11

Ekmax = hf - ø

(J = J - J)

Question 12

when photoelectrons are released, why does their Ek vari up to a maximum

Answer 12

since not all e- are released directly straight from the surface

• inner e- can collide with other e- and +ve metal ions before being released, resulting in a lower Ek

Question 13

Describe a demonstration to show the photoelectric effect

Answer 13

zince plate attached to an electroscope (with a gold leaf)

• all negatively charged causing the leaf to repel from the plate
• U.V. light removes e-, removing the negative charge, causing the leaf to move down
• only works for any frequencies of/above the U.V section, since the photons provide enough energy to overcome the work function