SPECIFIC OBJECTIVES.

1. Define photoelectric effect, work function, electron volts, ratioed frequency.
2. Perform and describe simple experiment to show photoelectric effect.
3. Explain the factors affecting photoelectric effect.
4. Explain photoelectric effect using Einstein equation.
5. Apply equation E = hf to calculate energy of a photo.
6. Solve numerical problems involving photoelectric effect.
7. Explain the applications of photoelectric effect.

Photoelectric effect.

• Id the emission of electrons from the surface of some metals or metal oxides when they are exposed to a suitable electromagnetic radiation with high enough frequency.
• The electrons emitted are called photoelectrons.

Thresh hold frequency.

• This is the minimum frequency of radiation below which no emission of electrons occur.

Thresh hold wavelength.

• This is the maximum wavelength of a radiation above which no emission of electrons will occur.

Work function.

• This is the minimum energy required by a metal surface to release an electron.

EXP: DEMONSTRATE PHOTOE;ECTRIC EFFECT USING A CHARGED ELECTROSCOPE.

Requirements.

1. Zinc plate.
2. Gold leaf electroscope.
3. UV lamp.

Procedure.

1. Charge the electroscope together with the zinc plate positively.
2. Direct the UV radiation from mercury lamp onto the zinc plate and observe the diverging of the leaf.
3. Repeat the experiment using a negatively charged electroscope.

Observations.

1. The positively charged electroscope, the leaf divergence remains the same.
2. The negatively charged electroscope the leaf divergence decreases.

Explanation.

When the zinc plate is exposed with UV, electrons are emitted from the surface. These emitted electrons do not escape due to the attraction by the positive charges on the plate and the leaf divergence remains the same. When UV is directed to the negatively charged electroscope, the emitted electrons are repelled away from the plateand the electroscope. The already negative charges move to the charged atom. As a result, the electroscope becomes discharged and the leaf divergence decreases.

Factors affecting photoelectric emission.

3. The work function of the metal surface.

• Intensity is the rate of flow of energy of radiation per unit area normal to the surface of radiation.
• Intensity is inversely proportional t the square of the distance between the surface and the source of radiation.
• The intensity determines the number of photoelectrons emitted. High intensity high number of electrons emitted.
• When these electrons flow, they produce current called photo current.
• This is the energy carried by radiation.
• It is determined by the frequency of the radiation.
• There is minimum frequency called thresh hold frequency below which no emission occurs irrespective of the intensity of the radiation.
• There is then maximum wavelength called thresh hold wavelength above which no emission of electrons will occur.
• Increasing the frequency of incident radiation increases the kinetic energy of the photoelectrons emitted.
1. Work function or type of metal surface.
• Energy metal surface has a minimal energy called work function for it to emit electrons.
• The energy is determined by minimum frequency i.e. thresh hold frequency.

Laws of photoelectric emission.

1. Rate of emission of photoelectrons is directly proportional to the intensity of incident radiation.
2. The photoelectrons are emitted with a rate of kinetic energy from zero up to maximum which increases as the frequency increases and is independent as the intensity of radiation.
3. Each metal surface has its own work function or minimum frequency for photo emission to occur.

Einstein equation.

Quantum theory.

• It is assumed that the energy of a radiation is carried in packets called quanta. In each packet there are particles of energy called photon.
• This packet of energy carrying the total energy required by the surface to release the electrons and also give the released electrons some kinetic energy to move away from the surface or atom.
• If photoelectric effect takes place in an electric field where the surface is connected to the cathode, the kinetic energy is equal to the work done in moving the electron from the atom. This is called electron volt.

Stopping potential.

• This is the voltage at the anode which completely stop the photoelectron from reaching the anode.
• Its value is negative.
• The work done in stopping electrons from reaching the anode is equal to electron volts.
• This energy is equal to the kinetic energy of the photoelectrons.

Application of photoelectric effect.

Photocell.

• An electric device that converts the energy of radiation into electric energy.
• There are 3 types of photocell.
1. Photoconductive cell.
2. Photo emissive cell.
3. Photovoltaic cell.

1. Photoconductive cell.
• Are also called photo resistive cell.
• Are made by depositing a thin film of semi- conductor (germanium, silicon and selenium) or a suitable insulator.
• The conducting leads are attached to each end of the semi- conductor. The semi- conductor is then covered using glass to allow light to reach the semi- conductor.
• The cell is so sensitive that it can detect infra-red source a long distance away.

Uses of photoconductive cell.

1. Flame or fire detector.
2. Lighting control for street lights.
3. Burglar alarm.
4. Smoke detector.
6. X- ray measurement.
7. Photographic equipment.

1. Photo emissive devices.
• These are based on the same principal as the apparatus that was originally used to investigate the photoelectric effect.
• Light incident on the cathode causes electrons to be emitted. These move to the anode causing a current to flow in the external circuit. The current that flows is proportional to the incident light intensity.
• A photocell is used to generate a signal from the sound track recorded on a movie film. The signal is recorded as variations in the transparency of the track.
• The changes in the intensity of the light transmitted by the track are converted to an electric signal by the photocell s the film passes.
• The signal is amplified and used to drive loudspeakers. A photocell can be used to count boxes on a conveyer belt as they pass.
• As a box passes between the light source and the cell the output of the cell decreases and then increases again. An external circuit counts the pulses electronically.

1. Photovoltaic devices.
• They generate an e.m.f when light falls on them.
• They are based on the principal that a potential difference is produced at a junction between two dissimilar materials.
• When light is incident on the junction region electrons are freed from atoms on either side.
• The potential difference cases the electrons to move in one direction across the junction (from negative to positive) and a current is thus generated in the external circuit.
• The selenium photocell is used as the light censor in a photographic exposure meter.
• The solar cells are constructed from layers of P- and n- type silicon. They generate sufficient current to charge the batteries that operate the traffic lights.

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