Quantum

Question 1

Explain how the photoelectric effect cannot be explained in terms of the wave model for em waves.

Answer 1

The wave model cannot explain the threshold frequency

Nor why the kinetic energy of the electrons is dependent on frequency.

Question 2

Why does the metal plate of the electroscope have to be clean for the experiment to work?

Answer 2

The metal has to be in direct contact with the air for electrons to be freed.

Question 3

Why does shining a uv light on a gold leaf electroscope cause the gold leaf to fall, when visible light doesn’t?

Answer 3

• The electroscope is negatively charged and so initially the gold leaf is raised as it is repealed by the rest of the system.
• When the uv light is shone on the zinc plate it causes electrons to be freed from the surface and repealed away from the negatively charged system.
• As this continues the whole system begins to lose charge and become neutrally charged. This causes the gold leaf to fall back to its neutral position when all the elctrons have been freed.
• Shining visible light on the zinc plate would not work because visable light has a lower frequency than UV waves. The frequency of visible light is not above the threshold frequency of zinc and so cannot free electrons as the photon size of visible light is not big enough. The photon size of uv light is big enough to be absorbed by the zinc and cause electrons to be released.

Question 4

Give one difference and one similarity between a quantum and a photon.

Answer 4

Both are bundles of energy but a photon is a bundle of energy for electromagnetic waves only.

Question 5

Why does the photoelectric effect not work if the system is positively charged?

Answer 5

The electrons freed by the wave of high enough frequency will simply be restricted to the system. Therefore there will be no overall change in charge.

Question 6

Define threshold frequency.

Answer 6

The minimum frequency required to cause electrons to be freed from the surface of a metal.

Question 7

Give one example of a wave behaving like a particle.

Answer 7

The photoelectric effect

Question 8

Give one example of a particle behaving like a wave.

Answer 8

Electron diffraction

Question 9

Describe and explain how the frequency of the wave affects the gold leaf electroscope experiment.

Answer 9

The higher the frequency of a wave the greater the size and energy of each emitted photon. Increasing the frequency beyond the threshold frequency causes electrons to be emitted faster as the excess energy is converted to kinetic energy of the electrons. This means the gold leaf would fall more quickly.

If the frequency is too low and is not greater than the threshold frequency then the experiment will not work and the gold leaf will not fall.

Question 10

Describe and explain how intensity affects the gold leaf electroscope experiment.

Answer 10

Intensity affects the number of photons released. If the wave has a frequency greater than or equal to the threshold frequency then the greater the intensity the more photons released and absorbed and therefore the more electrons released. This causes the gold leaf to fall more quickly.

Question 11

Frequency affects the __________ of the electrons

Answer 11

Speed

Question 12

Intensity affects the ____________ of the electrons

Answer 12

Number

Question 13

How can you tell if something is acting like a particle or like a wave?

Answer 13

The thing is acting like a wave of it can be diffracted; diffraction only occurs when the effective wavelength is of the same order of magnitude as the gap or obstacle.

Question 14

Define the electronvolt. State its value in joule.

Answer 14

An eV is the energy acquired by an electron accelerated through a p.d. Of 1v.
1.6x10-19

Question 15

Define the work function of a metal

Answer 15

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