chapter 13 - quantum physics

Question 1

what is Planck’s constant?

Answer 1

6.63X10^-34 J h=plancks constant

Question 2

what is the formula for the energy of a photon?

Answer 2

E= Planck’s constant X frequency of electromagnetic radiation
or
E= (Planck’s constant X speed of light) / wave length

Question 3

what is the relationship between energy of photon and wavelength? and how is this important

Answer 3

Energy is inversely proportional to its wavelength.

short wavelength photons such as x rays have lot of energy so can damage cells.

Question 4

what is the unit of energy on a quantum scale and what is the value in joules?

Answer 4

the electronvolt and it is 1.6x10^-19J

Question 5

experiment to find Planck’s constant ?

Answer 5

LEDS convert electrical energy into light energy. when the threshold PD is reached visible photons are emitted of specific wavelength. at this p.d. work done = V x Q. record P.D. with voltmeter. use equation eV= hc/ wavelength and rearrange to get V= hc/e X 1/ wavelength . plot on graph V against 1/wavelength. rearrange the gradient to find h.

Question 6

what is the photoelectric effect?

Answer 6

it is when electrons are freed from the surface of a metal after electomagnetic radiation hits.

Question 7

what are the three key observations from photoelectric effect

Answer 7

1) photoelectrons only released if incident radiation is
above threshold frequency. no matter intensity
2) if incident radiation is above threshold frequency then emission of electrons are instantaneous
3) if above threshold frequency, increasing intensity increases rate of emission, not increase ke , only increases ke when frequency increases.

Question 8

what practical can be show the photoelectric effect?

Answer 8

gold leaf electroscope- touch top of plate with negative electrode from high voltage power supply. charges electroscope. excess electrons are deposited on plate and stem. as stem and leaf have same charge they repel. place clean piece of zinc on plate. shine UV light on plate, then gold leaf starts to fall down to stem as negatively charged electrons are freed from surface.

Question 9

how many photons are needed to free an electron?

Answer 9

one, it is a one on one interaction where if photon has enough energy then electron is freed.

Question 10

what is the work function

Answer 10

the energy required to free an electron from surface of metal

Question 11

what is the relationship between energy and frequency?

Answer 11

E is directly proportional to f

Question 12

what is einsteins photoelectric equation?

Answer 12

hf= work function + KE max

Question 13

what does a graph of ke against frequency show?

Answer 13

x axis intercept= threshold frequency
y axis intercept= work function
gradient = plancks constant
no photons emitted to the left of x axis intercept

Question 14

how do electrons act like particles?

Answer 14

they have a mass and a charge, can be accelerated and deflected by electric and magnetic fields

Question 15

how do electrons act like waves

Answer 15

they can be diffracted when they pass through gaps similar to wavelength size such as graphite

Question 16

state the de Broglie equation?

Answer 16

wavelength= plancks constant / momentum

or wavelength = Planck’s constant / (mass x velocity)

Question 17

what is the relationship between wavelength and kinetic energy?

Answer 17

wavelength is inversely proportional to the square root of kinetic energy

Question 18

why is harder to observe protons then electrons?

Answer 18

mass of protons are larger then electrons so at same speed the momentum is significantly greater . as a result wavelength is much smaller so harder to observe.
wavelength is inversely proportional to momentum

Question 19

how does a graph of ke against frequency differ for different metals?

Answer 19

they will always have the same gradient however the work function will be different.

Question 20

what is the wave particle duality?

Answer 20

describes how all matter has both wave and particle properties.