13 - Quantum Physics

Question 1

quantum physics

Answer 1

branch of physics dealing with phenomena on the very small scale, often less than the size of an atom

Question 2

photon

Answer 2

a quantum of electromagnetic energy - photon energy is given by E=hf

Question 3

planck constant

Answer 3

symbol h, important constant in quantum mechanics that relates the energy of a photon to its frequency - experimental value of 6.63e-34 Js

Question 4

electronvolt

Answer 4

derived unit of energy used for subatomic particles and photons, defined as energy transferred to or from an electron when it passes through a pd of 1 volt
1eV = 1.6e-19 J

Question 5

photoelectric effect

Answer 5

emission of photoelectrons from a metal surface when em radiation above a threshold frequency is incident on the metal

Question 6

photoelectrons

Answer 6

electrons emitted from surface of a metal by the photoelectric effect

Question 7

gold-leaf electroscope

Answer 7

device with a metallic stem and a gold leaf that can be used to identify and measure electric charge - historically used as a voltmeter for measuring large voltages

Question 8

threshold frequency (f0)

Answer 8

minimum frequency of em radiation that will cause the emission of an electron from the surface of a particular metal

Question 9

work function (Φ)

Answer 9

minimum energy needed to remove a single electron from the surface of a particular metal

Question 10

Einstein’s photoelectric effect equation

Answer 10

equation relating the energy of a photon, the work function of a metal, and the maximum kinetic energy of any emitted photoelectrons: hf=Φ+KEmax

Question 11

wave-particle duality

Answer 11

a theory that states that matter has both particle and wave properties and also electromagnetic radiation has wave and particulate (photon) nature

Question 12

polycrystalline graphite

Answer 12

thin layers of graphite with regularly arranged carbon atoms in different orientations

Question 13

De Broglie equation

Answer 13

equation relating the wavelength and the momentum of a particle: λ = h/p

Question 14

Planck’s discovery

Answer 14

em radiation can only exist in certain values - coming in quanta. this proposed a particulate nature of em radiation

Question 15

photon model vs wave model

Answer 15

photon model used to explain how em radiation interacts with matter, wave model explains propagation of em radiation through space

Question 16

explanation of the photoelectric effect

Answer 16

when light is incident on a metal surface the energy from a photon can be transferred to an electron in a one-to-one interaction - if the energy absorbed by the electron is greater than or equal to the work function, it instantaneously escapes the surface as a photoelectron

Question 17

why ejection of photoelectrons does not depend on intensity of incident light

Answer 17

this increases number of photons per unit time, not the energy per photon

Question 18

why Einstein’s photoelectric effect equation includes MAXIMUM kinetic energy

Answer 18

some surface electrons are closer to positive metal ions than others, which affects how much energy is required to free them. work function is MINIMUM energy required (most electrons need more) therefore an electron freed by the minimum required energy has the most energy left over from the incident photon, so the maximum KE possible

Question 19

de Broglie’s contributions

Answer 19

first proposed wave-particle duality, realising all particles travel through space as waves - anything with mass that is moving has wave-like properties , and that wavelength of a particle is inversely proportional to its momentum

Question 20

why wave properties are harder to observe the larger a particle is

Answer 20

momentum at the same speed is greater, so wavelength is smaller

Question 21

wave and particle properties of electrons

Answer 21

particle: accelerated and deflected by electric and magnetic fields; have mass and charge
wave: diffract under certain conditions

Question 22

determining Planck constant experimentally using LEDs

Answer 22

LEDs only emit light when
the potential difference across them exceeds the threshold p.d. required. A potential divider is
set up to vary the voltage through the LED. A small black tube is placed over the LED, to make
it obvious when the LED has lit up. By varying the p.d. across the LED, we can determine the
threshold p.d., V, required to turn it on. As the LED produces light of a specific colour, we know
the wavelength of the light. Each photon from the LED is emitted when a single electron loses
energy. By equating the energy of an individual electron in the LED with an individual photon
produced, we can use the equation eV = hc/λ to determine the Planck constant (e and c are constants)

Question 23

demonstrating photoelectric effect using gold leaf electroscope

Answer 23

Initially, the gold leaf and the stem have the same charge, so they repel each other. If UV light is
shone on to the zinc plate, free electrons will be released from the surface of the plate, and the
negative charge will slowly be lost, so the gold leaf will gradually fall back to the stem.

Question 24

features of graph of KEmax against incident frequency

Answer 24

-gradient = h
-gradient becomes non-zero at f0
-negative y-intercept if followed backwards from gradient is -ϕ

Question 25

using an electron gun to provide evidence of wave-particle duality for electrons

Answer 25

-When a beam of electrons is fired at a thin piece of polycrystalline graphite the electrons are diffracted by the gaps between atoms, evidenced by a diffraction pattern when they hit a screen -that they are accelerated by a high pd and hit the screen with discrete impacts evidences their particle nature