Chapter 13 Quantum Physics Flashcards
What is quantum physics?
This is about how models are applied to describe electromagnetic radiation and electrons
What is a photon?
This is a packet of electromagnetic radiation
What is the equation for photon energy?
E = hf
E is energy
h is planks constant
f is frequency
What is the alternative for E = hf with wavelength?
E = hc/λ
What is the energy of a photon proportional to?
The frequency and 1/wavelength
What is the magnitude of planks constant?
6.63x10^-34
What is an electron volt? what is its value?
This is a very small measurement of energy. It is the energy transferred to an electron when passing through a potential difference of 1 volt
1.6x10^-19J = 1eV
Why are electron volts used when talking about photons?
Because photons have very small amounts of energy
How is planks constant determined using an LED?
LED’s emit light of a single wavelength and only turn on when a threshold voltage is exceeded
If the wavelength of the LED is known, a voltage is put through an LED and is increased until the first photons of light are emitted.
Then use the equation: Threshold Pd x Charge of an electron = Energy of emitted photon
Then take this energy and put it into the equation:
eV = hc/λ
and rearrange to find h.
To get better results use multiple LED’s and use a graph of Voltage against 1/λ and find the gradient
What is surface charging?
This is the effect when a satellite has EM radiation on its metal surfaces, it becomes charged
What is the photoelectric effect?
This is the effect that when EM radiation hits the surface of a metal, photoelectrons (normal electrons) are emmitted
What was the gold leaf electroscope experiment?
An electroscope is a glass jar with a metal plate on the top and a pole into the centre of the jar. Then a gold leaf is attached to the pole. The metal, pole and the gold leaf get a negative charge and the gold leaf gets repelled and lifts away from the metal pole. Then when UV light start to hit the zinc metal on the top of the jar, the gold leaf starts to droop.
What were the 4 observations made from the photoelectric effect?
- Photoelectrons will only be emitted when the EM radiation is above a certain frequency. This is the threshold frequency.
- If the EM radiation frequency is above the threshold frequency. The release of electrons was instantaneouse
- If the EM is greater than the threshold, increasing intensity only increases the number of photoelectrons released
- The higher the frequency above the threshold, the more kinetic energy the electrons have
What did Einstein say about the photoelectric effect?
- EM radiation is a stream of photons not a wave
- Each electron must acquire a certain amount of energy
- It is a 1 to 1 interaction between electrons and photons
- As energy is proportional to Frequency, if the frequency is too low then it will not give an electron enough energy to break free of the metal. This is the threshold frequency
- The minimum energy to free an electron from a specific metal is the work function. Symbol: Φ
- Increasing intensity increases the number of photons per second increasing the number of electrons because it is a 1 to 1 interraction
- Kinetic energy of the electrons is based off the left over energy from the frequency of the photon
How is the kinetic energy of electrons from the photoelectric effect increased?
By increasing the frequency
How are more electrons emitted from the photoelectric effect?
By increasing the intensity of light
How are no electrons emitted from the photoelectric effect?
By lowering the frequency of the light
What is the work function?
This is the minimum energy to release an electron from the surface of a metal
What is Einsteins equation for the photoelectric effect?
hf = Φ + KEmax
hf - energy of the photon
Φ - Minimum energy to release a photon
KEmax - Maximum kinetic energy of the emitted electron
Why is it maximum kinetic energy?
Because the energy of the photon is split between the work function and the kinetic energy. if the work function is minimum then the kinetic energy is maximum
What is the gradient of a graph of KEmax against Frequency?
It is planks constant
What is the y-axis intercept on the graph of KEmax against Frequency?
It is - Φ
What is the x-axis intercept on the graph of KEmax against Frequency?
It is the minimum frequency
What is the unit for the work function of a metal?
It is eV
What is Louis de Broglie propose?
That all matter can be both a particle and a wave. Wave-particle duality
How can it be proved that electrons are waves and particles?
Passing electrons through a very small diffraction grating made from polycrystalline graphite which has very small gaps, causes the electrons to diffract like light. The gap needs to be really small and be around the same as the wavelength of the electron
What is the experiment that proves electrons are waves and particles?
A cathode ray tube is used to accelerate particles and then have them pass through polycrystalline graphite. this then forms an interference pattern on a screen of phosphorescent pigment that lights up when electrons hit it.
What is wavelength proportional to? (When talking about the de Broglie equation)
Wavelength is proportional to 1/momentum
What is the de Broglie equation?
λ = h / p λ = wavelength h = planks constant p = momentum
- As a particle gets larger its ……
2. Therefore its wavelength is …
- momentum is larger
2. Smaller
