Chapter 13 - Quantum Physics

Question 1

What are photons?

Answer 1

Photons are packets of discrete EM energy

Question 2

Why were photons developed?

Answer 2

Developed as wave theory of light could not explain photoelectric effect

Question 3

What is the energy of a photon dependant on?

Answer 3

Energy of a photon is dependant on its frequency

Question 4

What is the equation for the energy of a photon?

Answer 4

E = hf or E = hc / y

Energy = Planck Constant x Frequency

Question 5

What is the value of Planck Constant?

Answer 5

6.63 x 10^-34

Question 6

What is the relationship between energy of photon and wavelength?

Answer 6

Energy of a photon is inversely proportional to its wavelength

Question 7

Why do X-rays damage the cells of bodies?

Answer 7

X-rays have more energy than radio waves which Is why X-rays damage the cells of bodies

Question 8

Where are electron volts used?

Answer 8

Electron volts are used to measure energy at the quantum scale

Question 9

What is an electron volt?

Answer 9

1 electron volt is the energy transferred to or from an electron when it moves through a PD of 1V

Question 10

What is the value of an electron volt?

Answer 10

1.60 x 10^-19

Question 11

How do you calculate the number of electron volts?

Answer 11

Energy in J / 1.60 x 10^-19

Question 12

What happens when a diode has its threshold voltage across it?

Answer 12

LED allows currents to flow and transfer energy to EM radiation/photos of a specific wavelength

Question 13

What is the property of a laser beam?

Answer 13

Laser beams consist of photons of the same frequency

Question 14

What is the formula for the power of a beam?

Answer 14

P = nhf

Power of beam = Number of photons per second x Planck Constant x Frequency

Question 15

What can be assumed about the transfer of energy from electrons to photons?

Answer 15

E = Ve = hf

This is a very unlikely assumption

Question 16

How can Planck Constant be estimated?

Answer 16

By using a circuit and LEDs of different threshold frequencies
A single LED can be used but gives a less accurate value

Question 17

What is the circuit used to approximate Planck Constant?

Answer 17

Potential divider circuit with LED in parallel with voltmeter across it
Safety resistor

Question 18

Why is a potential divider circuit used?

Answer 18

Potential dividers are better than a series circuit as it provides an adjustable PD

Question 19

How much energy is transferred in an LED at the threshold PD?

Answer 19

The energy transferred in the LED is approximately equal to the energy of a single photon it emits

Question 20

What is the formula for the energy of emitted photons in the LED experiment?

Answer 20

Threshold PD x Charge on Electron = Energy of Emitted Photon

Ve = hc / y

Question 21

How is a graph formed from the LED investigation to estimate Planck Constant?

Answer 21

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

Question 22

What is the photoelectric effect?

Answer 22

The photoelectric effect is the emission of electrons from the surface of the metal when it is illuminated by EM radiation above threshold frequency

Question 23

What is the photoelectric effect dependant on?

Answer 23

It is dependant on being over the threshold frequency and not on the intensity

Question 24

What is the result of the photoelectric effect?

Answer 24

Instantaneous emission of electrons

Question 25

How does increasing intensity affect the photoelectric effect?

Answer 25

Increasing intensity of incident radiation increases the number of electrons emitted per second, not their KE

Question 26

How is the kinetic energy of electrons increased in the photoelectric effect?

Answer 26

Kinetic energy is only increased by increasing the frequency of incident radiation

Question 27

What are the steps for the photoelectric effect to take place?

Answer 27

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

Question 28

How does increasing intensity affect number of emitted electrons?

Answer 28

Increasing intensity Increases photons per unit of time Increases photon-electron interactions Increasing number of emitted electrons

Question 29

What is the work function?

Answer 29

The work function is the minimum amount of energy needed by an electron to escape a metal

Question 30

What does photoelectric effect depend on in terms of energy?

Answer 30

Emissions can only take place if energy of photons exceeds work function (hf > O)

Question 31

Why do few electrons achieve max KE?

Answer 31

Most electrons require more energy than work function as they need to overcome attractive forces deeper leaving less energy for KE

Question 32

What is a vacuum photocell?

Answer 32

A vacuum photocell is a device which makes use of the photoelectric effect to generate a small current

Question 33

What does a vacuum photocell consist of?

Answer 33

It consists of an evacuated glass tube with a photoemissive plate at one end, and another metal plate at the opposite end

Question 34

What is a photoemissive plate?

Answer 34

A photoemissive plate is a metal that releases electrons via the photoelectric effect

Question 35

What is the formula for current using the photoelectric effect?

Answer 35

Current = Number of photoelectrons emitted per second x Elementary Charge

Question 36

What does it mean if the applied PD runs forward or backward in photoelectric circuit?

Answer 36

Forward: Photoemissive plate is cathode and opposite plate is anode Backward: Photoemissive plate is anode and opposite is cathode

Question 37

What is the difference between applying the PD forward or backwards in photoelectric circuit?

Answer 37

Forward: Electrons gain energy and most will cross Backwards: Electrons lose KE and only most energetic electrons can cross

Question 38

How does increasing bias potential impact the photoelectric current?

Answer 38

Electrostatic forces decelerate more electrons so fewer have enough KE. The current registered gets smaller and smaller

Question 39

How is maximum kinetic energy estimated?

Answer 39

Photocell Adjusting bias potential between the cathode and anode until no current is registered KE_MAX = eV

Question 40

What is the graph using Einstein's photoelectric equation used to estimate Planck's Constant?

Answer 40

Graph of KE_MAX against f KE_MAX = hf₁ - O Gradient = Planck Constant Y intercept = Work Function X intercept = Threshold Frequency

Question 41

What properties of light show that it has wave-particle duality?

Answer 41

Diffraction shows light has wave-like nature Photoelectric effect shows light has particle-like nature

Question 42

What is de Broglie's formula?

Answer 42

Wavelength = Planck Constant / Momentum y = h / p

Question 43

When are electrons diffracted?

Answer 43

Electrons can only be diffracted by metal lattices where the atomic spacing is comparable to the size of electron's de Broglie wavelength

Question 44

Why do electrons form ring interference patterns?

Answer 44

The structure of the metal it is fired through creates a circular hole for the electrons to transmit This creates a ring-shaped pattern

Question 45

How can an electron's velocity be increased in an electron gun?

Answer 45

By increasing the accelerating potential difference between the filament and node

Question 46

What is Einstein's photoelectric effect equation?

Answer 46

hf = KE_MAX + O

Question 47

Why do certain materials diffract electrons?

Answer 47

Polycrystalline graphite has carbon atoms spaced so close (10^-10m) the gap between is similar to the wavelength of electrons and so electrons diffract

Question 48

What are the two ways to estimate Planck Constant? Which is better?

Answer 48

LED Potential Divider Circuit Vacuum Photocell (Better)