Define photoelectric effect .

The photoelectric effect is the phenomenon in which electrons are emitted from the surface of a metal upon the absorption of electromagnetic radiation .

Define photoelectron .

A photoelectron is an electron emitted from the surface of a metal during the photoelectric effect .

What does the one-photon-per-electron interaction in the photoelectric effect provide evidence for?

It provides evidence that light is quantised , carried in discrete packets called photons .

Only frequencies of light above the .......... will cause the emission of a photoelectron.

Only frequencies of light above the threshold frequency will cause the emission of a photoelectron.

True or False? Diffraction and interference can be explained using the particle (photon) model of light.

False. Diffraction and interference are explained by the wave model of light; the photoelectric effect and atomic line spectra require the particle (photon) model.

Name the three scientists regarded as the pioneers of quantum theory.

Max Planck , Niels Bohr and Albert Einstein .

What happens to the gold leaf of a negatively charged, zinc-plated electroscope when UV light is shone onto the plate?

Photoelectrons are emitted from the zinc plate, reducing the negative charge on the plate, rod and leaf, so the gold leaf falls back towards the central rod.

Why does moving the UV source closer to the metal plate make the gold leaf fall more quickly?

Decreasing the distance increases the intensity of radiation incident on the plate, increasing the number of photoelectrons emitted per second .

Why does using a higher-frequency light source not change how quickly the gold leaf falls?

The rate of photoelectron emission depends on intensity , not frequency; frequency instead affects the maximum kinetic energy of the photoelectrons.

A filament light source causes no change in the gold leaf's position because its frequency is .......... the metal's threshold frequency.

A filament light source causes no change in the gold leaf's position because its frequency is below the metal's threshold frequency.

Why does a positively charged plate show no change in the gold leaf's position when irradiated with UV light?

Any electrons emitted are attracted back by the excess positive charge on the surface of the metal, so no net charge is lost.

True or False? Photoelectrons are emitted only after the metal has absorbed UV light for a measurable amount of time.

False. Emission is instantaneous : a single photon interacts with a single electron, so photoelectrons are released as soon as radiation of sufficient energy is incident on the surface.

How does increasing the intensity of incident radiation affect the number of photoelectrons emitted?

It increases the number proportionally , since intensity increases the number of photons per second striking the surface, and each photon can release at most one photoelectron.

Photoelectric emission only just occurs when hf 0 = .......... , where f 0 is the threshold frequency.

Photoelectric emission only just occurs when hf 0 = Φ (the work function) , where f 0 is the threshold frequency.

On a graph of maximum kinetic energy KE max against frequency f , what do the gradient, y-intercept and x-intercept represent?

Gradient = Planck's constant h ; y-intercept = -Φ (work function); x-intercept = threshold frequency f 0 .

True or False? The maximum kinetic energy of photoelectrons increases with the intensity of the incident radiation.

False. KE max depends only on the frequency of the incident radiation; intensity affects only the number of photoelectrons emitted.

In the photoelectric equation, what units must hf , Φ and KE max all be expressed in, and how do you convert a value given in eV?

All terms must be in joules (J) . Convert eV to J by multiplying by 1.6 × 10 -19 .

Define threshold frequency .

The threshold frequency is the minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal.

Define threshold wavelength .

The threshold wavelength is the longest wavelength of incident electromagnetic radiation that would remove a photoelectron from the surface of a metal.

Define work function .

The work function (Φ) is the minimum energy required to release a photoelectron from the surface of a metal.

Using the energy well analogy, why do more tightly bound electrons require more energy to escape the metal's surface?

They sit deeper in the energy well, so more energy is needed to reach the top of the well and be released.

Alkali metals, such as sodium and potassium, have threshold frequencies in the .......... region, because the attractive forces between surface electrons and positive ions are relatively weak.

Alkali metals, such as sodium and potassium, have threshold frequencies in the visible light region, because the attractive forces between surface electrons and positive ions are relatively weak.

Why do transition metals, such as zinc and iron, have threshold frequencies in the ultraviolet region?

The attractive forces between surface electrons and positive metal ions are much stronger , so higher-energy photons are needed to release an electron.

True or False? A metal will emit photoelectrons if bombarded with enough low-frequency photons, provided their total energy exceeds the work function.

False. Each electron interacts with only one photon . If a single photon's energy is below the work function, no electron is emitted, however many low-frequency photons arrive.

Why is the maximum kinetic energy of photoelectrons independent of the intensity of the incident radiation?

Because each electron can absorb only one photon , so kinetic energy depends only on the photon's frequency , not on how many photons arrive.

Define photoelectric current .

Photoelectric current is the rate of emission of photoelectrons per second.

How does photoelectric current vary with the intensity of the incident radiation?

It is directly proportional to intensity, since more photons per second striking the metal release more photoelectrons per second.

Increasing the intensity of incident radiation increases the .......... of photoelectrons emitted, not their maximum kinetic energy.

Increasing the intensity of incident radiation increases the number of photoelectrons emitted, not their maximum kinetic energy.

True or False? Increasing the frequency of the incident radiation increases the photoelectric current.

False. Photoelectric current depends on intensity (photons per second), not frequency; frequency instead affects the maximum kinetic energy of the photoelectrons.

On graphs against frequency f , how do maximum kinetic energy and photoelectric current each vary, above the threshold frequency?

KE max increases linearly with frequency; photoelectric current stays constant , independent of frequency (provided intensity is constant).

A LevelPhysicsOCRFlashcards4. Electrons, Waves & PhotonsThe Photoelectric Effect

The Photoelectric Effect (OCR A Level Physics): Flashcards

Exam code: H556

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Define photoelectric effect.
The photoelectric effect is the phenomenon in which electrons are emitted from the surface of a metal upon the absorption of electromagnetic radiation.
Define photoelectron.
A photoelectron is an electron emitted from the surface of a metal during the photoelectric effect.
What does the one-photon-per-electron interaction in the photoelectric effect provide evidence for?
It provides evidence that light is quantised, carried in discrete packets called photons.
Only frequencies of light above the .......... will cause the emission of a photoelectron.
Only frequencies of light above the threshold frequency will cause the emission of a photoelectron.
True or False?
Diffraction and interference can be explained using the particle (photon) model of light.
False.
Diffraction and interference are explained by the wave model of light; the photoelectric effect and atomic line spectra require the particle (photon) model.
Name the three scientists regarded as the pioneers of quantum theory.
Max Planck, Niels Bohr and Albert Einstein.
What happens to the gold leaf of a negatively charged, zinc-plated electroscope when UV light is shone onto the plate?
Photoelectrons are emitted from the zinc plate, reducing the negative charge on the plate, rod and leaf, so the gold leaf falls back towards the central rod.
Why does moving the UV source closer to the metal plate make the gold leaf fall more quickly?
Decreasing the distance increases the intensity of radiation incident on the plate, increasing the number of photoelectrons emitted per second.
Why does using a higher-frequency light source not change how quickly the gold leaf falls?
The rate of photoelectron emission depends on intensity, not frequency; frequency instead affects the maximum kinetic energy of the photoelectrons.
A filament light source causes no change in the gold leaf's position because its frequency is .......... the metal's threshold frequency.
A filament light source causes no change in the gold leaf's position because its frequency is below the metal's threshold frequency.
Why does a positively charged plate show no change in the gold leaf's position when irradiated with UV light?
Any electrons emitted are attracted back by the excess positive charge on the surface of the metal, so no net charge is lost.
True or False?
Photoelectrons are emitted only after the metal has absorbed UV light for a measurable amount of time.
False.
Emission is instantaneous: a single photon interacts with a single electron, so photoelectrons are released as soon as radiation of sufficient energy is incident on the surface.
State the photoelectric equation.
$h f = Φ + \frac{1}{2} m v_{m a x}^{2}$
How many photons can a single electron interact with in the photoelectric effect?
Only one photon.
How does increasing the intensity of incident radiation affect the number of photoelectrons emitted?
It increases the number proportionally, since intensity increases the number of photons per second striking the surface, and each photon can release at most one photoelectron.
Photoelectric emission only just occurs when hf₀ = .........., where f₀ is the threshold frequency.
Photoelectric emission only just occurs when hf₀ = Φ (the work function), where f₀ is the threshold frequency.
On a graph of maximum kinetic energy KE_max against frequency f, what do the gradient, y-intercept and x-intercept represent?
Gradient = Planck's constant h; y-intercept = -Φ (work function); x-intercept = threshold frequency f₀.
True or False?
The maximum kinetic energy of photoelectrons increases with the intensity of the incident radiation.
False.
KE_max depends only on the frequency of the incident radiation; intensity affects only the number of photoelectrons emitted.
In the photoelectric equation, what units must hf, Φ and KE_max all be expressed in, and how do you convert a value given in eV?
All terms must be in joules (J). Convert eV to J by multiplying by 1.6 × 10^-19.
Define threshold frequency.
The threshold frequency is the minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal.
Define threshold wavelength.
The threshold wavelength is the longest wavelength of incident electromagnetic radiation that would remove a photoelectron from the surface of a metal.
Define work function.
The work function (Φ) is the minimum energy required to release a photoelectron from the surface of a metal.
Using the energy well analogy, why do more tightly bound electrons require more energy to escape the metal's surface?
They sit deeper in the energy well, so more energy is needed to reach the top of the well and be released.
Alkali metals, such as sodium and potassium, have threshold frequencies in the .......... region, because the attractive forces between surface electrons and positive ions are relatively weak.
Alkali metals, such as sodium and potassium, have threshold frequencies in the visible light region, because the attractive forces between surface electrons and positive ions are relatively weak.
Why do transition metals, such as zinc and iron, have threshold frequencies in the ultraviolet region?
The attractive forces between surface electrons and positive metal ions are much stronger, so higher-energy photons are needed to release an electron.
True or False?
A metal will emit photoelectrons if bombarded with enough low-frequency photons, provided their total energy exceeds the work function.
False.
Each electron interacts with only one photon. If a single photon's energy is below the work function, no electron is emitted, however many low-frequency photons arrive.
Why is the maximum kinetic energy of photoelectrons independent of the intensity of the incident radiation?
Because each electron can absorb only one photon, so kinetic energy depends only on the photon's frequency, not on how many photons arrive.
Define photoelectric current.
Photoelectric current is the rate of emission of photoelectrons per second.
How does photoelectric current vary with the intensity of the incident radiation?
It is directly proportional to intensity, since more photons per second striking the metal release more photoelectrons per second.
Increasing the intensity of incident radiation increases the .......... of photoelectrons emitted, not their maximum kinetic energy.
Increasing the intensity of incident radiation increases the number of photoelectrons emitted, not their maximum kinetic energy.
True or False?
Increasing the frequency of the incident radiation increases the photoelectric current.
False.
Photoelectric current depends on intensity (photons per second), not frequency; frequency instead affects the maximum kinetic energy of the photoelectrons.
On graphs against frequency f, how do maximum kinetic energy and photoelectric current each vary, above the threshold frequency?
KE_max increases linearly with frequency; photoelectric current stays constant, independent of frequency (provided intensity is constant).

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