Flashcards
Photoelectric Effect
Cards in this deck (28)
- 1Define an electron voltWork done on an electron when accelerated across a potential difference of 1V (1.60x10^-19 J)
- 2What is a photon?Quantum of energy of electromagnetic radiation
- 3Energy of photon is directly proportional to.....Frequency of that radiation
(constant of proportionality = Planck constant) - 4What 3 equations must we know for questions on photon energy?E = hf
E = hc/λ
c=fλ - 5What is the photoelectric effect?The photoelectric effect refers to the emission, or ejection, of electrons from the surface of, generally, a metal in response to incident light
- 6The incident light must have....A high enough frequency
- 7What is the name of the apparatus which we can demonstrate the photoelectric effect with?Golf-leaf electroscope
- 8Golf leaf electroscope diagram:
- 9The cap is usually made of what?Zinc (metal)
- 10What could we use to negatively charge the cap?A static rod or an EHT
- 11What happens to the golf leaf when the cap is negatively charged?The gold leaf rises as it's repelled by the negative charge on the metal stem
- 12What happens when we shine visible light onto the zinc plate?Nothing at all
- 13What happens when we shine UV light onto the zinc plate?As the frequency is high enough, an electron is emitted from the surface, so the negative charge on the metal stem is lost so the golf leaf falls
- 14What happens if we place glass between the UV source and the zinc plate?The leaf will stop falling
(If wave, light would pass through, but glass blocks it, so particle) - 15If we positively charge the zinc plate, then....The gold leaf stays up even when under UV bombardment
- 16Not explained by wave theory of light:1. Instantaneous effect of UV light : the intensity of the incident ray does not matter
2. Electron emission does not take place below a certain frequency (of incident radiation) - 17What the wave theory actually suggests (in correlation to previous flashcard) :Emission should take place with any frequency, because Amplitude of waves would add up due to superposition, so energy would increase over time. Therefore, electron would eventually be released by any frequency
- 18Define the work function (symbol ∅)Minimum energy required by the photon in order to release a surface electron
- 19For every material, the work function is....Specific
- 20Energy of a photon is directly proportional to frequency, meaning....If there is a minimum photon energy, there also must be a minimum frequency (called the threshold frequency f0)
- 21What is the work function equation?∅ = hf0
(Work function = Planck constant x threshold frequency) - 22When a photon hits the zinc plate surface, each individual photon.....Interacts with a single surface electron in the plate
- 23The surface electron gains the energy of the photon (hf) and the electron will be emitted if.....The energy gained is grater than the work function
- 24What is the Planck constant?6.63 x 10^-34
- 25The work function + the excess Ek gives us....The total energy of the incoming photon
(E = ∅ + Ek(max) ) - 26What is the equation for using electron volts?Energy = Electron volts x elementary charge (1.60 x 10^-19)
- 27How would we find the velocity of an electron ejected with 2.2eV ?2.2 x (1.60 x 10^-19) = 3.52 x 10^-19 J (Ek)
Now we can say:
0.5 x Me x v^2 = 3.52 x 10^-19 J
So root(3.52 x 10^-19 J / 0.5 x 9.11x10^-31) = 8.8x10^5 ms-1 - 28If a photon has the frequency 1.5 x 10^15 Hz and a work function 4.0x10^-19 , find the total energy, the kinetic energy(max) and the ejection velocityE = hf ; 9.9x10^-19 J
E - ∅ = Ek(max) ; 5.9x10^-19 J
Ejection velocity = root(E/0.5 x Me) = 1.15x10^6 ms-1