Quantum Physics Flashcards
When did scientists believe light was only a wave?
In the late nineteenth century.
What must light be to explain the photoelectric effect?
A particle, photon. (as well as being a wave).
What is a single packet of EM radiation called?
A quantum.
What would happen if a photon interacts with another particle?
Transfer either all or none of its energy.
What charge do photons carry?
None (neutral).
What is the definition of an electron volt?
The kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt.
How do you measure the Planck Constant with an LED?
1) Connect an LED of known wavelength in the electrical circuit shown.
2) Start off with no current flowing through the circuit, then adjust the variable resistor until a current just begins to flow through the circuit and the LED lights up.
3) Record the voltage across the LED, and the wavelength of light the LED emits.
4) Repeat for different colours of LEDs (different wavelengths).
5) Plot grapf of voltages against 1/(Wavelength).
6) Straight line graph, find gradient.
7) eV = hc/(wavelength)
What happens when you shine electromagnetic waves of a high enough frequency onto the surface of a metal?
Electrons will instantaneously eject.
How are the electrons ejected?
1) Free electrons on the surface of the metal absorb energy from the light.
2) If an electron absorbs enough energy, the bonds holding it to the metal break and it is emitted from the surface.
3) This is called the photoelectric effect and the electrons emitted are called photoelectrons.
How can the photoelectric effect be demonstrated?
By using a gold-leaf electroscope.
1) The electroscope plate is initially negatively charged, so the gold leaf is repelled.
2) The zinc plate is then exposed to ultraviolet light and the photoelectric effect causes its free electrons to be ejected. This causes it to lose its negative charge - the gold leaf is no longer repelled and so drops down.
What conclusions can be drawn from the photoelectric effect?
For a given metal, no photoelectrons are emitted if the radiation has a frequency below a certain value - called the threshold frequency.
The photoelectrons are emitted with a variety of kinetic energies ranging from zero to some maximum value. This maximum kinetic energy increases with the frequency of the radiation, and is unaffected by the intensity of the radiation.
The number of photoelectrons emitted per second is proportional to the intensity of the radiation.
What would happen with the photoelectric effect according to wave theory?
1) For a particular frequency of light, the energy carried is proportional to the intensity of the beam.
2) The energy carried by the light would be spread evenly over the wave-front.
3) Each free electron on the surface of the metal would gain a bit of energy from each incoming wave.
4) Gradually, each electron would gain enough energy to leave the metal.
Why can’t wave theory explain the photoelectric effect?
The higher the intensity of the wave, the more energy it should transfer to each electron - the kinetic energy should increase with intensity. There’s no explanation for the kinetic energy depending only on the frequency.
There is also no explanation for the threshold frequency. According to wave theory, the electrons should be emitted eventually, no matter what the frequency is.
What would happen with the photoelectric effect according to the photon model?
1) When light hits its surface, the metal is bombarded by photons.
2) If one of these photons is absorbed by a free electron, the electron will gain energy equal to hf.
What is the work function energy?
Before an electron can leave the surface of the metal, it needs enough energy to break the bonds holding it there. This energy is called the work function energy and its value depends on the metal.
