A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Anode Voltage and Microscopes in Atomic Physics — mark scheme explained
The short answer
In this section, we will explore the estimation of anode voltage needed to produce wavelengths of the order of the size of an atom, as well as the principles of operation for both the transmission electron microscope (TEM) and the scanning tunnelling microscope (STM).
The question
Calculate the anode voltage needed to produce a de Broglie wavelength of 0.1 nm.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculations, show all steps clearly and include units. For explanations, provide clear and concise descriptions of the principles and mechanisms. For discussions, give specific examples to support your points.
What the command words demand
- Calculate
- Perform a numerical calculation using the given data and appropriate equations.
- Explain
- Provide a detailed description of the principles or mechanisms involved.
- Discuss
- Present information on various aspects of the topic, including applications and implications.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 10 minutes for each question involving calculations or detailed explanations.
- Use the de Broglie equation: λ = h / p1 mark
- Find the momentum: p = h / λ = (6.63 × 10 -34 ) / (1 × 10 -10 ) = 6.63 × 10 -24 kg·m/s1 mark
- Use p = √(2m e eV), rearranged to V = p 2 / (2m e e)1 mark
- Substitute values: V = (6.63 × 10 -24 ) 2 / (2 × 9.11 × 10 -31 × 1.60 × 10 -19 )1 mark
- Simplify: V = (4.40 × 10 -47 ) / (2.92 × 10 -49 )0 marks
- Calculate V: V ≈ 151 V1 mark
- This is of the order of 150 V — wavelength comparable to atomic size0 marks
- Therefore the anode voltage required is V ≈ 150 V1 mark
Final answer: V ≈ 150 V
Work through every step correctly and you earn all 6 marks.
Another worked example
Explain the principle of operation for a transmission electron microscope (TEM).
- A high-voltage source accelerates electrons, which are then focused into a fine beam using electromagnetic lenses.1 mark
- The sample is prepared as a thin film or section to allow the electrons to pass through.1 mark
- The electron beam interacts with the atoms in the sample, causing some electrons to be scattered.1 mark
- The transmitted electrons carry information about the internal structure of the material and are collected by a detector.1 mark
- The detector converts the transmitted electrons into an image, which can be magnified up to several million times.1 mark
Final answer: A high-voltage source accelerates electrons, which are focused into a fine beam using electromagnetic lenses. The sample is prepared as a thin film or section to allow the electrons to pass through. The electron beam interacts with the atoms in the sample, causing some electrons to be scattered. The transmitted electrons carry information about the internal structure of the material and are collected by a detector, which converts them into an image that can be magnified up to several million times.
Work through every step correctly and you earn all 5 marks.
Common mistakes
Using the wrong value for Planck's constant (h) or the mass of an electron (m).
Why it happens: Students may confuse the values of fundamental constants, leading to incorrect calculations.
Fix: Always double-check the values of constants and use the correct units.
Forgetting to convert units when necessary (e.g., from nm to m).
Why it happens: Students may overlook unit conversions, leading to incorrect results in calculations.
Fix: Ensure all units are consistent before performing any calculations.
Misinterpreting the role of electromagnetic lenses in TEM.
Why it happens: Students may not fully understand how electromagnetic lenses focus and control the electron beam.
Fix: Review the function of each component in the TEM, focusing on the role of electromagnetic lenses.
Confusing the principles of operation between TEM and STM.
Why it happens: Students may mix up the specific mechanisms used by TEM and STM, leading to incorrect explanations.
Fix: Clearly differentiate between the principles of TEM (transmitted electrons) and STM (tunnelling current).
Failing to explain the importance of the feedback loop in STM.
Why it happens: Students may overlook the critical role of the feedback loop in maintaining a constant tunnelling current.
Fix: Emphasize the function of the feedback loop and its importance in creating accurate topographic maps.
Not providing specific examples of applications for TEM and STM.
Why it happens: Students may provide general statements without concrete examples, leading to incomplete answers.
Fix: Practice giving specific examples of how TEM and STM are used in material science and surface science.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Calculate Anode Voltage | Find the accelerating voltage needed to give an electron a specified de Broglie wavelength. | 6 |
| Explain TEM Operation | Describe how a transmission electron microscope forms a magnified image of a thin sample. | 5 |
| Explain STM Operation | Describe how quantum tunnelling and probe distance allow an STM to image surfaces. | 5 |
| TEM Applications Discussion | Describe two specific uses of TEM in material science with supporting detail for each. | 4 |
| STM Applications | Describe two uses of scanning tunnelling microscopy in surface science with specific examples. | 4 |
| Total across these question types | 24 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.