Chapter 13 - Quantum Physics Flashcards
What are photons?
Photons are packets of discrete EM energy
Why were photons developed?
Developed as wave theory of light could not explain photoelectric effect
What is the energy of a photon dependant on?
Energy of a photon is dependant on its frequency
What is the equation for the energy of a photon?
E = hf or E = hc / y
Energy = Planck Constant x Frequency
What is the value of Planck Constant?
6.63 x 10-34
What is the relationship between energy of photon and wavelength?
Energy of a photon is inversely proportional to its wavelength
Why do X-rays damage the cells of bodies?
X-rays have more energy than radio waves which Is why X-rays damage the cells of bodies
Where are electron volts used?
Electron volts are used to measure energy at the quantum scale
What is an electron volt?
1 electron volt is the energy transferred to or from an electron when it moves through a PD of 1V
What is the value of an electron volt?
1.60 x 10-19
How do you calculate the number of electron volts?
Energy in J / 1.60 x 10-19
What happens when a diode has its threshold voltage across it?
LED allows currents to flow and transfer energy to EM radiation/photos of a specific wavelength
What is the property of a laser beam?
Laser beams consist of photons of the same frequency
What is the formula for the power of a beam?
P = nhf
Power of beam = Number of photons per second x Planck Constant x Frequency
What can be assumed about the transfer of energy from electrons to photons?
E = Ve = hf
This is a very unlikely assumption
How can Planck Constant be estimated?
By using a circuit and LEDs of different threshold frequencies
A single LED can be used but gives a less accurate value
What is the circuit used to approximate Planck Constant?
Potential divider circuit with LED in parallel with voltmeter across it
Safety resistor
Why is a potential divider circuit used?
Potential dividers are better than a series circuit as it provides an adjustable PD
How much energy is transferred in an LED at the threshold PD?
The energy transferred in the LED is approximately equal to the energy of a single photon it emits
What is the formula for the energy of emitted photons in the LED experiment?
Threshold PD x Charge on Electron = Energy of Emitted Photon
Ve = hc / y
How is a graph formed from the LED investigation to estimate Planck Constant?
Using the data from the different wavelength LEDs, plot a graph of threshold PD against 1/wavelength
The gradient is hc / e where ‘c’ and ‘e’ are constants
X intercept is threshold frequency
What is the photoelectric effect?
The photoelectric effect is the emission of electrons from the surface of the metal when it is illuminated by EM radiation above threshold frequency
What is the photoelectric effect dependant on?
It is dependant on being over the threshold frequency and not on the intensity
What is the result of the photoelectric effect?
Instantaneous emission of electrons
How does increasing intensity affect the photoelectric effect?
Increasing intensity of incident radiation increases the number of electrons emitted per second, not their KE
How is the kinetic energy of electrons increased in the photoelectric effect?
Kinetic energy is only increased by increasing the frequency of incident radiation
What are the steps for the photoelectric effect to take place?
Single electron absorbs energy of single photon
If energy of single photon exceeds work function, electron escapes
Energy in excess of work function absorbed by electron becomes KE
How does increasing intensity affect number of emitted electrons?
Increasing intensity
Increases photons per unit of time
Increases photon-electron interactions
Increasing number of emitted electrons
What is the work function?
The work function is the minimum amount of energy needed by an electron to escape a metal
What does photoelectric effect depend on in terms of energy?
Emissions can only take place if energy of photons exceeds work function (hf > O)
Why do few electrons achieve max KE?
Most electrons require more energy than work function as they need to overcome attractive forces deeper leaving less energy for KE
What is a vacuum photocell?
A vacuum photocell is a device which makes use of the photoelectric effect to generate a small current
What does a vacuum photocell consist of?
It consists of an evacuated glass tube with a photoemissive plate at one end, and another metal plate at the opposite end
What is a photoemissive plate?
A photoemissive plate is a metal that releases electrons via the photoelectric effect
What is the formula for current using the photoelectric effect?
Current = Number of photoelectrons emitted per second x Elementary Charge
What does it mean if the applied PD runs forward or backward in photoelectric circuit?
Forward: Photoemissive plate is cathode and opposite plate is anode
Backward: Photoemissive plate is anode and opposite is cathode
What is the difference between applying the PD forward or backwards in photoelectric circuit?
Forward: Electrons gain energy and most will cross
Backwards: Electrons lose KE and only most energetic electrons can cross
How does increasing bias potential impact the photoelectric current?
Electrostatic forces decelerate more electrons so fewer have enough KE. The current registered gets smaller and smaller
How is maximum kinetic energy estimated?
Photocell
Adjusting bias potential between the cathode and anode until no current is registered
KEMAX = eV
What is the graph using Einstein’s photoelectric equation used to estimate Planck’s Constant?
Graph of KEMAX against f
KEMAX = hf1 - O
Gradient = Planck Constant
Y intercept = Work Function
X intercept = Threshold Frequency
What properties of light show that it has wave-particle duality?
Diffraction shows light has wave-like nature
Photoelectric effect shows light has particle-like nature
What is de Broglie’s formula?
Wavelength = Planck Constant / Momentum
y = h / p
When are electrons diffracted?
Electrons can only be diffracted by metal lattices where the atomic spacing is comparable to the size of electron’s de Broglie wavelength
Why do electrons form ring interference patterns?
The structure of the metal it is fired through creates a circular hole for the electrons to transmit
This creates a ring-shaped pattern
How can an electron’s velocity be increased in an electron gun?
By increasing the accelerating potential difference between the filament and node
What is Einstein’s photoelectric effect equation?
hf = KEMAX + O
Why do certain materials diffract electrons?
Polycrystalline graphite has carbon atoms spaced so close (10-10m) the gap between is similar to the wavelength of electrons and so electrons diffract
What are the two ways to estimate Planck Constant?
Which is better?
LED Potential Divider Circuit
Vacuum Photocell (Better)
