4.5 Quantum

Question 1

What is a quantum?

Answer 1

a small discrete unit of energy

Question 2

What is a photon?

Answer 2

A quantum associated with electromagnetic radiation

Question 3

What is the Planck constant?

Answer 3

h, photon energies are always emitted at multiples of this

Question 4

What is an electronvolt?

Answer 4

the kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt

Question 5

What is threshold frequency?

Answer 5

the lowest frequency of radiation that will resul in the emission of electrons from a particular metal surface. For most metals = UV

Question 6

Define work function

Answer 6

the minimum energy required to release an electron from its surface, overcoming the electrostatic attraction between the electron and positive metal ions

Question 7

Describe quantum model

Answer 7

EM radiation emitted by hot objects could no have continuous values.
Had to be emitted in packets, that have fixed amounts of energy.

Question 8

What did Newton say about light?

Answer 8

tiny corpuscles, reflection and refraction

move quicker through more dense medium

Question 9

What did Huygens say about light?

Answer 9

Wave
Refraction and reflection
slow down in more dense
predict diffraction

Question 10

What did Young’s double slit prove about light?

Answer 10

Wave

Interference to produce fringes

Question 11

What did de Broglie do?

Answer 11

Both wave and particle

Question 12

How do you calculate the energy of a single photon?

Answer 12

E = hf or E=hc/wavelength

Question 13

How do we calculate the energy gained by a proton?

Answer 13

charge x potential difference

Question 14

What is 1 eV equal to?

Answer 14

1.602 x 10^-19

Question 15

Convert from eV to J?

Answer 15

multiply by 1.602x10^19

Question 16

What did Einstein say about the work function

Answer 16

For electrons to be released,, the frequency of the incident radiation must exceed the threshold frequency for that energy

Question 17

What happens when incident radiation is above threshold frequency?

Answer 17

Electrons emitted with some kinetic energy

Question 18

Why can the photoelectric effect not be explained using a wave model?

Answer 18

energy of a wave depends on its amplitude (intensity), not its frequency

Question 19

Why can we not notice the wave nature of large objects?

Answer 19

Diffraction needs to occur
Wavelength must be equal to gap of an object that radiation passes through
For big objects at slow speeds this is too small

Question 20

What condition is necessary to produce observable electron diffraction when passing through matter?

Answer 20

Wavelength of electrons must be comparable to order of magnitude of atomic spacing

Question 21

Whey in an electron gun, is the cathode connected to the negative terminal of the supply?

Answer 21

Electrons should be repelled by cathodes. Would be slowed down if cathode was positive

Question 22

How can you demonstrate the photoelectric effect?

Answer 22

Clean zinc plate gold leaf electroscope
Negatively charged 
UV light
Gold leaf collapses and charge moves away
Indicates emission of electrons

Question 23

Photoelectric effect in terms of quantum behaviour

Answer 23

Individual photons absorbed by individual electrons
Overcome work function
Instantaneous emission
Number of electrons depends on light intensity
Einsteins photoelectric equation

Question 24

Explain photoelectric effect

Answer 24

Only about threshold frequency
Einsteins equation
Work function is minimum energy
Number of electrons is higher at higher intensity
Emission is instantaneous

Question 25

Explain what the de Broglie wavelength of an electron is?

Answer 25

Electrons are observed to behave as waves where wavelength depends on speed/momentum

Question 26

What effect does doubling intensity have on the energy of a photon?

Answer 26

Photon energy does not change.
Only changes when frequency changes
Greater intensity = more photons incident