AQA AS Level Physics Paper 2, November 2021: Question 1
9 marks · Medium difficulty · Practical Techniques & Data Analysis
Investigate stationary waves using microwaves, including plotting a graph, determining uncertainties, calculating microwave frequency, and identifying wave polarization.
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Question text
01 A student investigates stationary waves using microwaves.
Figure 1 shows a metre ruler fixed to a bench. The student places a microwave
transmitter T at one end of the ruler and a vertical metal reflector R at the other end.
R is at a right angle to the ruler.
Figure 1
The student places a microwave detector D approximately one-third of the distance
from T to R. When T is switched off, the microammeter connected to D reads zero.
When T is switched on, stationary waves are produced between T and R, and the
microammeter registers a current. When the student moves3 D along the ruler, the
size of the current changes between maximum and minimum values.
The student measures the current at different positions of D along the ruler to identify
a position P of the minimum current.
Figure 2 is a plot of the measurements taken near P.
Figure 2
01.1 Draw a line of best fit for these data.
[2 marks]
01.2 State a value for the position of P.
[1 mark]
position of P = cm
The student moves D along the metre ruler towards R and observes a series of
maximum and minimum readings on the microammeter. He identifies Q as the
position of the 8th minimum current from P. He measures the distance PQ to
be 50.9 cm, as shown in Figure 3.
*03* Figure 3
01.3 The absolute uncertainty in identifying any minimum current is ± 0.2 cm.
Determine the percentage uncertainty in the distance PQ.
[2 marks]
percentage uncertainty in PQ = %
01.4 Deduce the frequency of the microwaves produced by T.
[3 marks]
5 frequency = Hz
01.5 Figure 4 shows D placed at a position where the current is a maximum.
*04* Figure 4
The student rotates D by 90°, without changing its distance from T, to the position
shown in Figure 5. The current is now zero.
Figure 5
State the property of microwaves that is shown by this change in current.
[1 mark]
Mark scheme
Show the mark scheme
Question Answers Additional Comments/Guidance Mark AO
01.1 smooth line drawn within half grid square of points ✓ 2 AO3-1a
minimum between 32.6 and 32.8 cm ✓
01.2 value of their minimum (cm) ✓ Within a half grid square 1 AO3-1b
01.3 doubles 0.2 OR calculates percentage uncertainty for 0.2 (half Correct answer earns both marks 2 AO3-1b
range) ✓
0.8 (%) ✓ CAO
– SICS – – JUNE 2021
01.4 recognises that node-to-node spacing = /2 ✓ 3 2
AO3-1a
recognises the need to divide by 8 ✓ Condone use of 7 or 9 1
AO2-1h
2.36 109 (Hz) ✓ 3 sf required
For example:
0.509 × 2
λ = or 0.127(25) m seen; top line earns 1✓
and bottom line earns 2✓
3×10 ×8 8
f = = 2.36 109 (Hz) earns all 3 marks
0.509× 2
3×10 × 78
f = = 2.06 109 earns 2 marks
0.509× 2
3×10 × 98
f = = 2.65 109 earns 2 marks
0.509× 2
Allow 2 marks for 4.72 × 109 (must be 3 sf)
– SICS – – JUNE 2021
01.5 (microwaves are) polarised ✓ 1 AO1-1b
Total 9
How to answer it
Stationary Waves Using Microwaves
What this question tests
This question assesses your practical and analytical skills regarding stationary waves using microwaves. You are tested on plotting experimental graphs, identifying minima/nodes, calculating percentage uncertainties with multiple readings, applying wave equations (c = fλ), understanding nodal spacing (λ/2), and recognising wave properties such as polarisation.
Drawing a Line of Best Fit & Identifying Minima
✅ Correct Answer
- A smooth, single-pass curve or straight line of best fit drawn within half a grid square of all plotted points.
- The minimum value identified correctly on the graph between 32.6 cm and 32.8 cm .
🧠 Exam Technique
When plotting lines of best fit for curved data (like microammeter readings near a node), ensure a smooth curve is sketched balancing points evenly on both sides. Avoid kinky or segmented lines.
State a Value for the Position of P
✅ Correct Answer
A value matching your graph's minimum position, typically between 32.6 cm and 32.8 cm (inclusive, depending on your drawn line).
❌ Common Errors
Reading the lowest plotted data point directly rather than using your line of best fit. Always follow the turning point of your drawn curve/line.
Percentage Uncertainty Calculation
📐 Step-by-Step Calculation
- Identify raw values: Distance PQ = 50.9 cm . Absolute uncertainty in each reading = ±0.2 cm .
- Account for multiple readings/half-range: Because a distance measurement between two points involves two separate position readings (P and Q), the absolute uncertainty doubles: 0.2 cm × 2 = 0.4 cm (Alternatively, calculate percentage uncertainty for a single 0.2 uncertainty and double it).
- Calculate percentage:
(0.4 / 50.9) × 100 = 0.7858% - Round appropriately: 0.8% (to 1 or 2 sig figs).
❌ Common Errors
Forgetting to double the absolute uncertainty ( ±0.2 cm ) when dealing with a distance calculated from two separate point measurements on a ruler. Examiners penalised students who used 0.2 instead of 0.4.
Deducing Microwave Frequency
💡 Key Knowledge
In a stationary wave system, the distance between adjacent nodes (minima) is equal to half a wavelength ( λ / 2 ).
📐 Step-by-Step Calculation
- Determine wavelength (λ): Distance PQ ( 50.9 cm or 0.509 m ) spans 8 minima (node-to-node intervals). Therefore, node spacing = 0.509 / 8 = 0.063625 m .
Since node spacing = λ / 2 , wavelength λ = (0.509 × 2) / 8 = 0.12725 m . - Apply wave speed equation: c = fλ , where c = 3.00 × 10⁸ m s⁻¹ (speed of EM waves).
- Rearrange for frequency (f):
f = c / λ = (3.00 × 10⁸) / 0.12725 = 2.3575 × 10⁹ Hz . - Final Answer: 2.36 × 10⁹ Hz (to 3 significant figures).
Wave Properties: Polarisation
✅ Correct Answer
Polarised (or microwaves are polarised).
💡 Examiner Commentary
Rotating the microwave detector probe by 90 degrees changes the received signal from maximum to zero current because the electric field vector of the microwaves oscillates in a single plane (i.e., they are plane-polarised). When the detector wire is turned perpendicular to this electric field, no emf is induced.
Topics
Physics · Practical skills · 3.3 Waves · Data analysis · Uncertainty and evaluation
Question and mark scheme from the AQA AS Level Physics examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.