3.1 The photoelectric effect Flashcards

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1
Q

What is the photoelectric effect?

A

» Is where where photoelectrons are emitted from the surface of a metal after light above a certain frequency is shone on it

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2
Q

What is the certain frequency?

A

» It is different for every type of metal and is called threshold frequency

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3
Q

What is the number of electrons emitted per second proportional to?

A

» The intensity of the incident radiation, provided the frequency is greater than the threshold frequency

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4
Q

What will happen if the frequency of the incident radiation is less than the threshold frequency?

A

» No photo electric emission from the metal surface can take place, no matter how intense the incident radiation is

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5
Q

When does photoelectric emission occur?

A

» As soon as the incident radiation is directed at the surface, provided the frequency of the radiation exceeds the threshold frequency
» And occurs without delay

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6
Q

Why couldn’t the threshold frequency be explained by wave theory?

A

» it suggests that any frequency of light should be able to cause electron emission as the energy absorbed by each electron will gradually increase with each incoming wave

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7
Q

What did Einstein assume?

A

» Light is composed of wavepackets or photons

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8
Q

What is the energy of a photon equal to?

A

» hf

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9
Q

What is h?

A

» Planck’s constant which is roughly about 6.63 x 10^-34 Joules

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10
Q

What did Einstein say happens when light is incident on a metal surface?

A

» An electron at the surface absorbs a single photon from the incident light and therefore gains energy equal to hf, where hf is the energy of a light photon

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11
Q

When can an electron leave the metal surface?

A

» If the energy gained from a single photon exceeds the work function of the metal

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12
Q

What is the work function?

A

» It is the minimum energy required for electrons to be emitted from the surface of a metal, and is denoted by ϕ

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13
Q

What happens to the excess energy gained by the photoelectron?

A

» Becomes its kinetic energy

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14
Q

What is the maximum kinetic energy equal to then?

A

» hf - ϕ

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15
Q

What is the photoelectric effect equation?

A

» hf = E(kmax) + ϕ

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16
Q

What equations can reflect when emission can take place from a metal surface?

A

» E(kmax) > 0
» hf > ϕ

17
Q

What is the equation for the threshold frequency?

A

» f(min) = ϕ/h

18
Q

What is the stopping potential?

A

» The minimum potential needed to stop photoelectric emission

19
Q

What happens at the stopping potential?

A

» The maximum kinetic energy of the emitted electron is reduced to zero because because each emitted electron must to extra work equal to e x Vs

20
Q

What do metal contains?

A

» Conduction electrons, which move around the positive ions and collide with one another

21
Q

What does the photoelectric equation show?

A

» The photoelectric equation is E = hf = Φ + E and it shows the relationship between the work function, maximum kinetic energy, and the frequency of light.

22
Q

Explain why, when ultraviolet light is shone on a positively charged plate, no charge is lost by the plate?

A

» Photoelectric effect involves the emission of electrons which are negative
» Any electrons emitted will only make the positive charge greater

23
Q

Explain why photoelectric emission from a metal surface only takes place if the frequency of the incident ray is greater than a certain value?

A

» hf = work function + Ekmax
» Ekmax > 0 for electrons to be emitted
» Therefore hf has to be > work function
» So f > Work function / planks constant
» For electrons to be emitted

24
Q

Why is it important to test any new theory by testing its prediction experimentally?

A

» If the test does not provide supporting evidence, the prediction is incorrect
» So the theory is incorrect and must be changed

25
Q

In the photoelectric equation, what is the name of characteristic quantity of the metal that emits photoelectrons?

A

» Work function

26
Q

What is meant by an excited atom?

A

» When an electron promotes to a higher energy level from a lower energy state

27
Q

Explain the meaning of the threshold frequency?

A

» Minimum amount of energy required for a photon for the photo electric effect to happen