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Figure 5 shows a vacuum photocell in which a metal surface is illuminated by electromagnetic radiation of a single wavelength - AQA - A-Level Physics - Question 3 - 2018 - Paper 7
Question 3
Figure 5 shows a vacuum photocell in which a metal surface is illuminated by electromagnetic radiation of a single wavelength. Electrons emitted from the metal surfa... show full transcript
Worked Solution & Example Answer:Figure 5 shows a vacuum photocell in which a metal surface is illuminated by electromagnetic radiation of a single wavelength - AQA - A-Level Physics - Question 3 - 2018 - Paper 7
Step 1
Calculate the wavelength, in nm, of the electromagnetic radiation incident on the metal surface.
Answer
To find the wavelength, we start with the photoelectric equation: E = hf = rac{hc}{\lambda} Where:
- is the energy of the photon (work function in this case, converted to Joules),
- is Planck's constant (),
- is the speed of light (),
- is the wavelength in meters.
Convert work function from eV to Joules:
Using the rearranged equation for wavelength: Substituting values:
Step 2
Explain the effect on the photocurrent that each model predicts and how experimental observations confirm the photon model.
Answer
The classical wave model predicts an increase in the photocurrent with increased intensity. This is because, as the energy transferred into each electron increases (over time), the number of emitted electrons should increase, leading to a higher photocurrent.
On the other hand, the photon model predicts that the photocurrent will remain at zero until the threshold frequency is met. After this, increasing intensity, associated with more photons rather than energy per photon, will not affect the photocurrent since each photon contributes to the same energy.
Experimental observations confirm the photon model because increasing the intensity without adjusting the potential divider does not lead to an increase in photocurrent; it remains constant until the threshold frequency is attained.
Step 3
Explain why the photocurrent decreases when this adjustment to the potential divider is made.
Answer
When the potential difference is increased, it may create a larger electric field which requires more energy for the electrons to escape from the metal surface. If fewer electrons have sufficient energy to reach terminal T, the photocurrent will decrease as fewer electrons contribute to the flow of current. The increased potential effectively raises the energy barrier that emitted electrons must overcome.
Step 4
Which conclusion about the relationship between the work functions of A, B and C is correct?
Answer
From the data in Table 2, we observe that for Metal Surface A, the voltmeter reading is lower than Surface B but the same as Surface C. This suggests that Metal Surface A has the lowest work function. Therefore, the correct conclusion about the relationship between the work functions of A, B, and C is:
- A < B < C