quanta Flashcards
What did Planck work on?
He looked at black body radiation. He theorised that radiation was emitted in discrete packets of energy. He found there was a link between energy and frequency.
What is a Quanta?
Discrete packets of energy.
E = hf
The equation which Planck made, he had to make the constant h = 6.63 x10^-34 for it to work.
What did Einstein theorise?
That concentrated packets of energy had particle-like properties and were called photons.
Photon
Concentrated discrete packets of energy which have particle-like properties.
What is the EM spectrum from a particle point of view?
Many photons with different levels of energy.
How much do photons weigh?
Weightless.
How can photons travel at the speed of light?
Because they’re weightless.
What letter represents the speed of light?
c.
How is the equation E = hc/λ formed?
Combining E = hf and c = fλ.
Electron volt
One electronvolt is the energy gained by an electron when it is accelerated through a p.d. of 1v (W = QV).
How to convert joules to eV
Divide by 1.6x10^-19.
How to convert eV to Joules
Multiply by 1.6x10^-19.
How to find Planck’s constant?
Set up a potential divider circuit with a parallel section with different coloured LEDs, an ammeter, and a voltmeter. Measure the voltage and record the wavelength (read from the packet). Plot a graph of V against 1/λ. The gradient equals Vλ. Substitute E = eV into E = hc/λ, input values and rearrange to get h.
Who worked out the photoelectric effect?
Einstein.
What is the photoelectric effect?
The emission of electrons from the surface of, generally, a metal in response to incident light.
What shows the photoelectric effect?
When a charge is given to an electroscope, they repel each other, so the gold leaf will lift and move away from the metal pole.
How can the charge of an electroscope be found?
The angle the gold leaf lifts to.
Why does the wave model not back up the photoelectric effect?
All the frequencies should combine energy to liberate the electrons.
How many photons can liberate a single electron?
ONE.
If wavelength increases…
Frequency decreases, therefore electrons have less kinetic energy, and eventually, none are liberated.
If wavelength decreases…
Frequency increases, therefore electrons have more kinetic energy.
If intensity increases…
More electrons are emitted but with the same kinetic energy. If it is below the threshold frequency, intensity has NO effect.
Electrons are trapped inside __________ and in order to escape it has to _________
Energy wells, absorb enough energy.
How does the material affect the energy well?
Different sizes, therefore different amounts of energy are needed to liberate the electrons.
Work function
The amount of energy needed for the electrons to escape their energy well.
Which formula works out the work function
hf = Φ + E.K. max.
If the electron is given just enough energy to release from the energy well, its kinetic energy equals 0, therefore…
Threshold frequency can be found by Φ/h.
It doesn’t matter how many IR photons land on the metal… if
All of them are below the threshold frequency, no single electron will be liberated.
Photoelectron
A liberated electron.
Intensity is proportional to
Rate of emission of photoelectrons.
Broglie
Said that for any particle that had momentum, it also has wavelength λ = h/p.
Relativistic mass
As a particle gets closer to the speed of light, the mass tends to increase due to relativistic effects.
The intensity of a wave at a point represents
The probability of a wave being there.
The electrons have _____ different energy levels, but its energy is _______
Infinite, finite.
How do you work out the energy changes of an atom?
Calculate the frequency and wavelength needed to give the energy to move up levels and equally how much is emitted when it falls back down levels.
Emission spectra
Shows the certain wavelengths of photons which are given off by an element after it is excited and the electrons drop back down to their original energy levels and emit energy.
Absorption spectra
Where certain frequencies of light are missing because they’re being absorbed by that element.
Threshold frequency
The lowest frequency of light at which electrons are still released from a surface.
Wave equation
v = fλ.
What experiment determines the work function of different materials and the value of h?
Stopping voltage experiment.
What does the graph from the stopping voltage experiment show?
Gradient = h, F0 (x-intercept) = threshold frequency, y-intercept = work function.
What does the y-intercept from the stopping voltage experiment show?
The voltage needed to stop an electron being liberated by light of 0 frequency and so 0 energy (the work function).
What axes are plotted from the stopping voltage experiment?
y = stopping voltage, x = frequency.
What direction does current flow?
Positive to negative (out the small end).
What direction do electrons form?
Negative to positive.
If the p.d. in a stopping voltage experiment is increased, what happens?
Electrons are accelerated faster as they move in the same direction as the current.
If the p.d. in a stopping voltage experiment is decreased, what happens?
The battery is more effective than the photoelectric effect; therefore, the electrons are slowed and start to move backwards.
What is stopping voltage?
The voltage at which the battery becomes more powerful than the photoelectric effect and the electrons are slowed.
Electric field
The electric force per unit charge.
How does an electron gun create a beam of electrons?
A hot filament causes thermionic emission of electrons. The electrons are repelled away from the negative electrode and towards the positive electrode, causing an accelerating force. Collimation takes place as the electrons are forced through a small gap. There must be a vacuum.
How are ions accelerated?
Electric fields exist between positive and negative electrodes, implying a force on the ions causing acceleration.
Force = rate of change of
Momentum.
Why are electrons only emitted above a threshold frequency?
Photon energy is proportional to frequency; therefore, photon energy must be greater than the work function to liberate an electron. All the energy must come from a single photon.
Line spectra
Specific frequencies/wavelengths show the absorption/emission lines within a narrow line of wavelengths.
How do line spectra provide evidence for the existence of energy levels in atoms?
Photons associated with particular energies show electron transitions up and down the discrete energy levels to provide the line spectra.
Kinetic energy gained by accelerating electron through a potential difference =
eV.
Why are only certain frequencies absorbed by atoms?
Electrons can only exist in discrete energy levels.
Ground state
The lowest energy level where electrons are usually found.
Energy delivered by photon (hf) =
Difference between the energy levels.
How can electrons be excited?
If a photon is absorbed or if electrons are hit by other electrons.
How is a photon emitted?
Electrons don’t remain in an excited state, so they de-excite and drop down to the ground state, emitting energy in the form of a photon.
High frequency photon means…
The more energy levels it jumps up.
Long wavelength photon means…
Less energy levels moved up.
