Chapter 13 - Quantum Physics

Question 1

What is a photon?

Answer 1

A quantum of electromagnetic energy

Question 2

How do you convert from joules to electronvolts?

Answer 2

Divide the joules by 1.6 x 10 ^ -19

Question 3

What is the energy of a photon directly proportional to?

Answer 3

Its frequency

Question 4

How can you find plank’s constant using LEDs?

Answer 4

Gather the threshold voltage to turn on a LED from multiple LEDs with different wavelengths. Plot a graph with the threshold voltage on the y-axis and 1/wavelength of light emitted by the LED on the x-axis. The gradient is (hc)/e, so use (gradient * e) / c to find plank’s constant.

Question 5

What is the photoelectric effect?

Answer 5

What a metal surface is hit with an EM wave it will emit electrons.

Question 6

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

Answer 6

Increases the number of electrons emitted from the metal, assuming the frequency of the light is above the threshold frequency. This is due to the higher number of photons landing and the one-to-one interaction between photons and electrons.

Question 7

Assuming the frequency of light is already above the threshold frequency, how does increasing the frequency of light affect the photoelectric effect?

Answer 7

The kinetic energy of the electrons emitted increases. This is due to the photons having more energy.

Question 8

What is the threshold frequency?

Answer 8

The minimum frequency a photon must have in order to have enough energy to release an electron.

Question 9

What is the work function?

Answer 9

The minimum energy an electron needs to have to escape from the surface of the metal.

Question 10

Why do we refer to the maximum kinetic energy?

Answer 10

This is the energy the electron would have if it were in a position in the metal so that it needed the minimum amount of energy to escape. Most electrons are not in this position and so need more energy than the work function.

Question 11

What is wave-particle duality?

Answer 11

Matter has both wave and particle properties.

Question 12

What is evidence of wave-particle duality?

Answer 12

Electron diffraction. When electrons are fired through gaps that similar is size to the wavelength of the electron, the electrons will diffract. They are accelerated by the p.d like particles and diffract as waves.

Question 13

On a graph of maximum kinetic energy against frequency, what is the x intercept?

Answer 13

The threshold frequency of the material

Question 14

On a graph of maximum kinetic energy against frequency, what is the gradient?

Answer 14

plank’s constant

Question 15

what is the de Broglie wavelength of a particle inversely proportional to?

Answer 15

the particle’s momentum

Question 16

What does the diffraction grating of electrons look like?

Answer 16

Rings that alternate between light and dark. The centre circle is the brightest, which each ring getting dimmer after that.

Question 17

What is the remainder energy from the interaction between a photon and an electron converted to?

Answer 17

Kinetic energy in the electron

Question 18

What are the three key observations of the photoelectric effect?

Answer 18

Electrons are only emitted if the frequency of the light is above the threshold frequency of the material.

If the frequency of the light is above the threshold frequency, the emission of electrons is instantaneous.

Increasing the intensity of the light did not increase the kinetic energy of the emitted electrons, instead it increased the number of emitted electrons.

Question 19

Describe and explain an experiment that demonstrates the photoelectric effect.

Answer 19

Connect a zinc plate to the top of a gold leaf electroscope. Give the electroscope a negative charge, this will cause the gold leaf to extend as the negative charge of the leaf and the negative charge of the stem will repel each other. Shine UV light on to the zinc plate. The electroscope will lose its charge due to losing electrons from the photoelectric, and so the gold leaf will no longer be extended.

Question 20

What piece of equipment is the photoelectric effect used in?

Answer 20

Night vision googles. They use the effect to amplify light. A material of with a very low work function is used, the small amounts of light cause the metal to emit electrons, this emission of electrons is used to build a picture.

Question 21

Why can’t the wave-model of light be used to explain the photoelectric effect?

Answer 21

With the wave model the rate of emission of electrons would be dependent on the intensity of light. The more intense the light, the more energy is transferred to the metal each second. Therefore light of any frequency would be able to cause electrons to be emitted, as energy would keep on being transferred to the metal until enough has been transferred to cause an electron to be emitted.