OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(ii) demonstration of the photoelectric effect using, e.g. gold-leaf…
Full specification wording
The photoelectric effect: (ii) demonstration of the photoelectric effect using, e.g. gold-leaf electroscope and zinc plate
A gold-leaf electroscope with a clean zinc plate on its cap can demonstrate the photoelectric effect.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 2 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A gold-leaf electroscope with a clean zinc plate on its cap can demonstrate the photoelectric effect. The zinc plate is cleaned with emery paper to remove the oxide layer, then charged negatively; the gold leaf diverges because like charges repel. When ultraviolet radiation is shone on the zinc plate, electrons are emitted from the surface, the negative charge is reduced and the leaf falls. If the plate is charged positively, the leaf remains diverged because emitted electrons are attracted back to the positive plate. If visible light is used instead of ultraviolet, no emission occurs because its frequency is below the threshold frequency for zinc. This shows that emission depends on frequency, not on intensity alone.
What earns the marks
- The zinc plate must be clean, for example rubbed with emery paper, so that the oxide layer does not prevent electron emission.
- The electroscope is charged negatively and the gold leaf diverges because the leaf and stem carry like charges.
- Ultraviolet radiation incident on the zinc plate causes electrons to be emitted, reducing the negative charge so the leaf falls.
- A positively charged electroscope does not discharge because emitted electrons are attracted back to the positive plate.
- Visible light does not discharge the negatively charged electroscope because its frequency is below the threshold frequency for zinc.
- The demonstration shows that emission depends on the frequency of the radiation, supporting the photon model.
Common mistakes and how to put them right
Forgetting to clean the zinc plate.
Put it right: An oxide layer can prevent emission and make the demonstration fail.
Charging the electroscope positively and expecting the leaf to fall.
Put it right: A positive charge attracts emitted electrons back.
Using a bright visible lamp and expecting discharge.
Put it right: Intensity does not compensate for frequency below the threshold.
Saying the leaf falls because the plate is heated.
Put it right: The effect is due to electron emission, not temperature.
Exam tips
- Describe the charging step, the cleaning step and the observation in a logical sequence.
- Name the radiation used, ultraviolet, and state that its frequency exceeds the threshold frequency for zinc.
- Explain the role of the electroscope: it detects the loss of charge when electrons leave the plate.
- Use the positive-charge control observation to strengthen the explanation of electron emission.
Try the lesson
The start of the The photoelectric effect: (ii) demonstration of the photoelectric effect using, e.g. gold-leaf electroscope and zinc plate lesson, made from checked teaching notes for this specification point.
Listen
A gold-leaf electroscope with a clean zinc plate on its cap can demonstrate the photoelectric effect. The zinc plate is cleaned with emery paper to remove the oxide layer, then charged negatively; the gold leaf diverges because like charges repel. When ultraviolet radiation is shone on the zinc plate, electrons are emitted from the surface, the negative charge is reduced and the leaf falls. If the plate is charged positively, the leaf remains diverged because emitted electrons are attracted back to the positive plate. If visible light is used instead of ultraviolet, no emission occurs because its frequency is below the threshold frequency for zinc. This shows that emission depends on frequency, not on intensity alone. A strong answer earns marks for these points. The zinc plate must be clean, for example rubbed with emery paper, so that the oxide layer does not prevent electron emission. The electroscope is charged negatively and the gold leaf diverges because the leaf and stem carry like charges. Ultraviolet radiation incident on the zinc plate causes electrons to be emitted, reducing the negative charge so the leaf falls. A positively charged electroscope does not discharge because emitted electrons are attracted back to the positive plate. Exam tip: Describe the charging step, the cleaning step and the observation in a logical sequence.
Key words
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Gold-leaf electroscope demonstration
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Zinc plate and ultraviolet radiation
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Threshold frequency
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Charge and electron emission
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See every definition
- Gold-leaf electroscope demonstration
- A clean zinc plate on the cap of a gold-leaf electroscope can demonstrate the photoelectric effect.
- Zinc plate and ultraviolet radiation
- When ultraviolet radiation is shone on the zinc plate, electrons are emitted from the surface, the negative charge is reduced and the leaf falls.
- Threshold frequency
- Visible light does not discharge the negatively charged electroscope because its frequency is below the threshold frequency for zinc.
- Charge and electron emission
- Emitted electrons are attracted back to a positive plate, so a positively charged electroscope does not discharge.
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