13 - Quantum Physics

Question 1

What is a photon?

Answer 1

A tiny packet of electromagnetic energy.

Question 2

What is the energy of a photon directly proportional to?

Answer 2

The frequency of a the photon.

Question 3

What is the Planck constant?

Answer 3

6.63x10^-34 Js

Question 4

Why is it impractical to measure the energy of photons in joules?

Answer 4

Because they are so small,

Question 5

What unit do we use to measure the energy of photons?

Answer 5

The electronvolt (ev)

Question 6

Define the electronvolt.

Answer 6

The energy transferred when an electron moves through a potential difference of 1v.

Question 7

What do we multiply ev by to convert to joules?

Answer 7

The charge of an electron.

1.6x10^-19

Question 8

What is the photoelectric effect?

Answer 8

When high energy electromagnetic radiation shines on a metal, it causes electrons to be emitted from the surface of the metal.

Question 9

What are emitted electrons called?

Answer 9

Photoelectrons

Question 10

What is the threshold frequency?

Answer 10

The lowest frequency of electromagnetic radiation that will cause a certain metal to emit photo electrons.

Question 11

What is the threshold frequency unique to?

Answer 11

The metal.

Question 12

How does increasing the intensity of the light affect the photoelectric effect?

Answer 12

It increases the number of photoelectrons emitted but not their energy.

Question 13

How does increasing the frequency of the light affect the photoelectric effect?

Answer 13

It increases the energy of the photoelectrons but not the number that is released.

Question 14

What is the work function of a metal?

Answer 14

The minimum energy required to free an electron from the surface of a metal.

Question 15

How do photons interact with electrons in the metal?

Answer 15

In a one to one interaction.

Question 16

What does the kinetic energy of the photoelectron depend on?

Answer 16

How much energy is remaining once work has been done to overcome the work function.

Question 17

How fast is emission if the frequency of the light is over the threshold frequency?

Answer 17

Instantaneous.

Question 18

What happens when a photon hits a metal?

Answer 18

Its energy is transferred to the electron in a one to one interaction. If this energy is greater than the work function, the electron is emitted from the metal. Any left over energy is converted to kinetic energy.

Question 19

What will happen if the light hitting the metal is at the threshold frequency?

Answer 19

The electrons will be free from the surface of the metal however they will have no kinetic energy and so they will not move anywhere.

Question 20

What is Einstein’s photo electric effect equation?

Answer 20

Energy of a photon = work function + maximum KE

Question 21

Why does Einstein’s equation use maximum kinetic energy?

Answer 21

Because the work function is the lowest amount of energy required to free an electron.

In reality, it could be more as the force that has to be overcome depends on the relative positions to the protons.

Question 22

How can we prove that electrons can behave as a wave?

Answer 22

They diffract when fired through graphite.

spacing between atoms in graphite is equal to wavelength of electrons

Question 23

What does wave-particle duality mean?

Answer 23

Something behaves both as a wave and as a particle.

e.g. Light - reflection = wave / photoelectric effect = particle

Question 24

What does an electron diffraction pattern look like?

Answer 24

Rings