Radiation

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Cards (70)

What is the electronvolt?
The energy gained by an electron accelerated through a potential difference of 1 V
1 eV = 1.6 x 10 ^ -19 J
How do you convert from eV to J?
Multiply by 1.6 x 10^-19
How do you convert from J to eV?
Divide by 1.6 x 10^-19
What is the photoelectric effect?
The emission of electrons from a metal surface when electromagnetic radiation is incident
Why must the light be incident on a METAL for the photoelectric effect to occur?
The phenomenon is to do with delocalised electrons
What are electrons that are removed from a metal via the photoelectric effect referred to as?
Photoelectrons
What does the photoelectric effect prove?
The particle nature of light
Define threshold frequency.
The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal
How does threshold frequency prove the particle nature of light?
If light was a wave, any frequency would cause photoelectric emission as the energy absorbed by each electron would gradually increase with each incoming wave. The emission would instead depend on amplitude.
Define the work function of a metal.
The minimum energy required to release a photoelectron from the surface of a metal
Why is a photoelectron only emitted for radiation above the threshold frequency?
Each electron can absorb one photon. It is a one on one interaction.
The work function is a property of the metal
Define stopping potential
The potential difference required to stop photoelectron emission from occurring and reduce the current to 0
Why is stopping potential important?
Allows us to find maximum kinetic energy of the released photoelectrons
Ek(max) = e x Vs
Ek(max) = maximum kinetic energy
e = charge of an electron
Vs = stopping potential
What is the photoelectric equation?
hf = ϕ + Ek(max)
h = Planks constant
f = frequency
ϕ = work function
Ek(max) = maximum kinetic energu
Photoelectric equation
Energy of an incident photon = work function + maximum kinetic energy of the photoelectron
What effect does increasing brightness have on the maximum kinetic energy of photoelectrons?
No effect
What effect does increasing brightness have on the photocurrent?
It increases
What effect does increasing frequency have on the maximum kinetic energy of photoelectrons?
Increases
What effect does increasing frequency have on the photocurrent?
No effect
What does the intensity of light mean?
The number of photons arriving per second
Graph for the PE equation
A) h
B) threshold frequency
C) work function
The kinetic energy of the photoelectrons is independent of the intensity of light because each electron can only absorb one photon
What is the kinetic energy of photoelectrons dependent on?
Frequency
What is the kinetic energy of photoelectrons independent of?
Intensity
What affect will increasing the intensity of light have?
It will increase the number of photoelectrons emitted
Why is the kinetic energy a maximum?
Work function is the minimum energy. Most electrons are lower down and therefore require more energy to escape the surface
What is the photoelectric current?
The number of photoelectrons emitted per second
What is photoelectric current proportional to?
The intensity of the incident radiation
To sum up:
Intensity = more photons
Frequency = more energy
What is ionisation?
The removal, or addition, of an electron from, or to, an atom when given sufficient energy
What is the excitation of electrons?
When an electron is given enough energy to move up an energy level, but not enough to leave the atom
Two ways of excitation:
Absorb photon
Collide with free electron
In ionisation, the electron is completely removed from the atom
How are electrons excited?
Absorption of energy equal to the difference between energy levels
In ionisation, the photon can transfer any quantity of energy above the ionisation energy
When an electron de-excites, it releases the energy it gained in the form of a photon
What are fluorescent tubes filled with?
Mercury vapour