A converging lens has focal length 0.25 m. What is its power?
From Paper 1: Fundamentals of physics
21 mark
An object is 0.30 m from a lens of focal length 0.10 m. Using 1/v = 1/u + 1/f (u = −0.30 m), the image distance is:
From Paper 1: Fundamentals of physics
31 mark
How many bits are needed to code 256 different levels?
From Paper 1: Fundamentals of physics
41 mark
An image has 1000 × 800 pixels, each coded with 8 bits. How many bits are needed?
From Paper 1: Fundamentals of physics
51 mark
A signal contains frequencies up to 20 kHz. What is the minimum sampling rate?
From Paper 1: Fundamentals of physics
61 mark
What is the conductance of a 50 Ω resistor?
From Paper 1: Fundamentals of physics
71 mark
Conductances of 0.010 S and 0.040 S are connected in parallel. The total conductance is:
From Paper 1: Fundamentals of physics
81 mark
A sensor's output changes by 0.50 V when the temperature changes by 10 K. Its sensitivity is:
From Paper 1: Fundamentals of physics
31(a)3 marks
A camera image has 2048 × 1536 pixels, each coded with 24 bits. Calculate the number of bytes needed to store the image.
From Paper 1: Fundamentals of physics
31(b)3 marks
Explain how replacing each pixel's value by the median of its neighbours removes 'salt-and-pepper' noise, and one disadvantage.
From Paper 1: Fundamentals of physics
31(c)4 marks
A camera lens of focal length 50 mm focuses on a person 1.8 m tall standing 2.0 m away. Calculate the image distance and the height of the image.
From Paper 1: Fundamentals of physics
32(a)4 marks
Music is sampled at 44.1 kHz with 16 bits per sample in two channels. Calculate the bit rate and the storage for a 3.0-minute track in megabytes, and explain the choice of 44.1 kHz.
From Paper 1: Fundamentals of physics
32(b)3 marks
A signal varies over 2.0 V and the noise is 0.010 V. Calculate the number of distinguishable levels and the number of bits worth recording.
From Paper 1: Fundamentals of physics
32(c)4 marks
A temperature sensor is a thermistor in series with a fixed 2.2 kΩ resistor across 5.0 V. The output is across the fixed resistor. The thermistor's resistance is 4.7 kΩ at 20 °C and 2.2 kΩ at 40 °C. Calculate the output at each temperature and the mean sensitivity.
From Paper 2: Scientific literacy in physics
5(a)4 marks
A GPS satellite is 2.0 × 10⁷ m from a receiver. Calculate the signal travel time, the position error caused by a 1.0 μs timing error, and explain why satellites carry atomic clocks.
Advance Notice article (written for this paper): Seeing inside the body with antimatter
(1) Positron emission tomography (PET) builds a three-dimensional map of where a radioactive tracer collects in the body. The most common tracer is a sugar labelled with fluorine-18, which decays by β⁺ emission with a half-life of 110 minutes.
(2) Because fluorine-18 decays so quickly, it is made in a cyclotron close to the hospital. A typical dose leaves the cyclotron at 08:00 with an activity of 2.0 GBq and is injected about two hours later.
(3) Each positron travels about a millimetre through tissue before it meets an electron. The pair annihilates, producing two gamma photons, each of energy 511 keV, which travel in almost exactly opposite directions.
(4) A ring of detectors surrounds the patient. When two detectors on opposite sides register photons within a few nanoseconds of each other, the annihilation must have happened somewhere on the line joining them. Modern scanners also measure the tiny difference in arrival times to locate the event along that line.
(5) A scan typically collects about 4 × 10⁸ such coincidences in 20 minutes. Like all counting experiments, the image is noisy: the uncertainty in N counts is about √N.
(6) A PET scan gives the patient an effective dose of about 7 mSv, compared with an average background dose in the UK of 2.7 mSv per year. The commonly used risk estimate is a 5% chance of a fatal cancer per sievert.
(7) PET shows what tissues are doing (for example, fast-growing tumours take up more sugar), while X-ray CT shows their structure. Many scanners combine the two.
From Paper 2: Scientific literacy in physics
8(f)*6 marks
Discuss the benefits and risks of PET scanning for a patient who may have a tumour, using information from the article.
From Paper 2: Scientific literacy in physics
8(g)4 marks
Explain two physical factors that limit how sharp a PET image can be.
From Paper 3: Practical skills in physics
2(a)3 marks
Describe how to calibrate a thermistor as a temperature sensor between 10 °C and 70 °C.
From Paper 3: Practical skills in physics
2(b)5 marks
Results:
T / K
283
298
313
328
343
R / Ω
18 636
10 000
5696
3416
2142
The model is R = R₀e^(B/T). Show that the data fit the model and find B.
From Paper 3: Practical skills in physics
2(c)4 marks
Explain why the thermistor is less sensitive at high temperatures, and how the resolution of the measuring instrument limits the temperature resolution.
Score 0 / 55 · 0 of 20 marked
This paper is written by Plasmo in the style of the OCR A-Level Physics exam. It is not a past paper and is not produced or endorsed by the exam board.