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PHOPHOTOTOELEELECTRCTRICIC EEFFFEFECTCT

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Photoelectric EffectPhotoelectric Effect What is it : When metal surfaces are exposed to electromagnetic

radiation with sufficient energy they absorb the photons of energy and emit electrons. This process is called the photoelectric effect.

How did it all start? Henrich Hertz was the first to discover this phenomena

in 1887 when he was investigating radio waves. In 1901 Max Planck showed that energy is quantized,

E=hf. Albert Einstein explained the photoelectric effect in

1905.

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The effect of light on a metal surface The photo-electric effect can be

demonstrated by means of an ultraviolet lamp, a zinc plate, an electroscope and two ordinary light bulbs of 40 W and 200 W.

An electroscope can be charged by induction by holding a charged acetate rod near the top plate.

Mobile negative charge in the metal plate is repelled down to the leaf. The leaf and central pole piece now have the same type of charge so the leaf rises.

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With the rod still nearby, the plate is touched so more charge moves to the plate through the person. The finger is pulled away and then the charged rod is removed. This is called CHARGING by INDUCTION

You can charge the electroscope positively by using a polythene rod instead.

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The process can be repeated whilst a polished zinc plate is placed on top of the electroscope.

The same effect will be achieved and the leaf will have been left in a raised position. It will fall slowly over time but not appreciably during a short demonstration.

ZINC PLATE

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When the rod is removed, the extra negative charge redistributes itself evenly all over the leaf, central pole and zinc plate.

Why does it do this?ZINC PLATE

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Polished zinc

Start with an electroscope that is charged negatively.

The U-V light causes photoelectrons to be emitted. These are repelled by the surface and escape. Charge is lost by the electroscope so the leaf falls.

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U-V photons

Polished zinc When all the extra charge has gone, the leaf has fallen to its resting position.

No further electrons will escape because the surface will not repel any liberated electron.

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U-V photons

Polished zinc

White light - a mixture of all VISIBLE colours

The electroscope is charged negatively.

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Charge is not lost by the electroscope. The leaf does not fall no matter how bright(intense) the light is or for how long it is shone onto the zinc.

The white light does not cause photoelectrons to be emitted.

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Why?Why do ultraviolet photons liberate photoelectrons whilst visible light photons do not?

The energy of a photon is given by: E = hf where h is Planck's constant and f is the frequency. Also E = hc because c = fthe wave equation)

This means that the higher the frequency, the greater the energy. Visible light contains frequencies that are

too low for photoelectric emission.

Alternatively, the shorter the wavelength, the greater the energy of the photons.

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Answer: none of the photons in white light has enough energy to release even one photoelectron

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Observations

Ultraviolet light causes a negatively charged electroscope to discharge – the leaves of the electroscope collapse when UV light shines on it.

White light does not release e- from the zinc plate even when irradiated with light of a much higher intensity or for a longer period.

When the electroscope is positively charged nothing happens because it is much more difficult to remove e- from a positive object.

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When a glass plate is placed between the ultraviolet source and the zinc plate, the electroscope stops discharging.

CONCLUSION1. Photoelectrons are emitted for a specific metal if

the frequency of radiation exceeds a certain limit (threshold frequency, fo).

2. The rate of photoelectron emission for a single frequency radiation beam is proportional to the intensity of radiation i.e. the more intense the radiation of the same frequency the more photoelectrons are emitted.

3. The emitted photoelectrons have kinetic energy ranging from zero to a maximum.

4. Maximum kinetic energy depends on frequency.

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5. The intensity of radiation has no effect on the kinetic energy of the emitted photoelectrons.

6. Emission starts as soon as the surface is irradiated with effective radiation.

7. Photoelectric current depends on intensity.

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Threshold frequency Each specific metal has a minimum frequency

called the threshold frequency for which electrons will just be released from the metal. The frequency of the incident light must be equal to or greater than the threshold frequency before electrons can be released.

More e- are liberated from a metal if light with a higher frequency than that of the threshold frequency of the metal strikes the metal surface – an increase in intensity of this light increases the number of e- that are liberated per second.

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Planck’s Quantum Theory In 1901, Max Planck, suggested that the

radiation of energy was not a continuous process. Planck made the following assumptions:

1. Energy is radiated in ‘packages’ or quanta.2. Each quantum consists of a specific amount

of energy, E, which is directly proportional to the frequency of the radiation:

3. A fraction of a quantum can never be radiated nor absorbed, only whole numbers of quanta.

hfE

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After these investigations After these investigations there was a problem.there was a problem.

Wave theory : An electromagnetic wave produces an electric field, which

exerts force on the electrons on the surface of a metal. The force will push the electrons from the surface.

Higher intensity of electromagnetic radiation results in a high electric field which then produces a bigger electric force on the electrons. This force will push off the electrons with a higher speed.

Emission should take place at any frequency because the electrons would absorb energy from the incoming radiation until they have energy enough to escape “So why threshold frequency?”

A The Quantum Theory (particle nature of light) was the answer (Einstein, 1905)

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Einstein’s theory Einstein’s theory of the of the Photoelectric effectPhotoelectric effect

EM radiation consists of small particles or lumps/packets of energy called photons.

Each photon carries energy proportional to its frequency.

NB: There are free electrons in metals.

When light is directed onto a metal surface a photon will collide with a free electron.

The interaction between a photon and an electron is a one to one correspondence.

The photon can then be reflected without a change in its kinetic energy or it transfers all its kinetic energy to the electron.

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The electron gains all the kinetic energy from the photon.

If the energy gained is sufficient the electron will escape from the metal surface. This is the process of photoelectric effect.

Part of the energy gained by the electron is used to release it from the surface (i.e. to overcome the force of attraction between the electrons and the metal ions) and the rest of the energy is the kinetic energy of the electron as it leaves the metal.

The minimum energy required to overcome the forces is called the work function (W).

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The magnitude of this energy is a few electron volts.

The frequency that corresponds to this energy is the threshold frequency (fo).

The relation between the work function and the threshold frequency is given by

W = hfo

Electrons are only emitted if the frequency of radiation greater than the threshold frequency

(hf > W)

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Energy of photonEnergy of incident photon = work function of the metal + maximum kinetic energy

of the released electrons.

hf = W + ½ mv2

where : hf = the energy of each photon of frequency f

W = work function of the metal surface½mv2 = maximum kinetic energy of the emitted

electrons

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Graph of EGraph of Ek k of photoelectrons of photoelectrons vs frequency of em-radiationvs frequency of em-radiation

Maximum kinetic energy is measured in electron volts eV.The threshold frequency (f0) of this material is 6,4 x1014 Hz.

Graph of KE of electron and frequency of incident light on metal

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WHY IS THE PHOTOELECTRIC WHY IS THE PHOTOELECTRIC EFFECT SO IMPORTANT?EFFECT SO IMPORTANT?

It helped explain the particle nature of light.

It is the basis of the quantum theory.

It is used in photocells e.g. in solar calculators, alarms and automatic door openers

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The Dual Nature of Light What is light – a wave or a particle? The wave theory cannot explain all the known

facts in connection with light. Diffraction and interference can only be explained

by the wave theory. The quantum hypothesis offers an excellent

explanation for the photo-electric effect but use the concept of frequency to calculate the energy of a photon.

Light has both a wave- and particle nature. The wave nature predominates during the

propagation of radiation, while the particle nature predominates during the interaction with matter.

Applications of the photoelectric effect

The photoelectric effect has many practical applications which include the photocell, photoconductive devices and solar cells

A photocell A photocell is usually a vacuum tube with two electrodes. One is a photosensitive cathode which emits electrons when exposed to light and the other is an anode which is maintained at a positive voltage with respect to the cathode. Thus when light shines on the cathode, electrons are attracted to the anode and an electron current flows in the tube from cathode to anode. The current can be used to operate a relay, which might turn a motor on to open a door or ring a bell in an alarm system

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The system can be made to be responsive to light, as described above, or sensitive to the removal of light as when a beam of light incident on the cathode is interrupted, causing the current to stop. Photocells are also useful as exposure meters for cameras in which case the current in the tube would be measured directly on a sensitive meter.

The photocell is at the centre of the many applications of the photoelectric effect. It consists of a curved emitter and a rod as collector, so as not to inhibit light from reaching the emitter.

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The structure of a typical photocell is shown below:  The flash of a camera uses the photoelectric effect

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Photocells are used in garage door openers. An example is shown in the diagram below:

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Spacecraft The photoelectric effect will cause

spacecraft exposed to sunlight to develop a positive charge. This can get up to the tens of volts. This can be a major problem, as other parts of the spacecraft in shadow develop a negative charge (up to several kilovolts) from nearby plasma, and the imbalance can discharge through delicate electrical components. The static charge created by the photoelectric effect is self-limiting, though, because a more highly-charged object gives up its electrons less easily

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Closely related to the photoelectric effect is the photoconductive effect which is the increase in electrical conductivity of certain non metallic materials such as cadmium sulfide when exposed to light. This effect can be quite large so that a very small current in a device suddenly becomes quite large when exposed to light. Thus photoconductive devices have many of the same uses as photocells.

Solar cells, usually made from specially prepared silicon, act like a battery when exposed to light. Individual solar cells produce voltages of about 0.6 volts but higher voltages and large currents can be obtained by appropriately connecting many solar cells together.

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