Chapter 3 — Physics for computing
Lesson map
Lesson 03 published. Map, example, EX007.
flowchart TD
N0["Learning outcomes"]
N1["3.1 Bits are physical"]
N2["3.2 Flash memory and wear levelling"]
N3["3.3 Energy and retention"]
N4["3.4 Signals and side channels"]
N5["Synthesis"]
N0 --> N1 --> N2 --> N3 --> N4 --> N5
N5 --> EX["Worked example"]
EX --> Q["Exercise EX007"]
Logical drawing
flowchart LR
N0["Learning outcomes"]
N1["3.1 Bits are physical"]
N2["3.2 Flash memory and wear levelling"]
N3["3.3 Energy and retention"]
N4["3.4 Signals and side channels"]
N5["Synthesis"]
N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> Q["EX007"]
[Learning outcomes]
|
v
[3.1 Bits are physical]
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v
[3.2 Flash memory and wear levelling]
|
v
[3.3 Energy and retention]
|
v
[3.4 Signals and side channels]
|
v
[Synthesis]
|
v
[Worked example] --> [Exercise EX007]
Learning outcomes
You will (1) explain how data persist on physical media; (2) reason about deletion at the hardware level; (3) link physical facts to erasure obligations.
3.1 Bits are physical
In physics for computing Zara learns that stored bits are physical states: charge in flash cells, magnetised regions on disks. Deleting a file usually removes only a pointer, leaving the states intact until overwritten. For Pellmore this matters because the GDPR right to erasure in Article 17 requires that data be erased without undue delay in certain cases. If engineers equate deleting a database row with erasing data, copies may persist on disks, in logs and in backups, which is a gap between what the system reports and what is physically true.
3.2 Flash memory and wear levelling
Solid-state drives spread writes across cells to avoid wearing out any single area. A consequence is that overwriting a file may write to new cells and leave the old ones untouched. Zara reads that secure deletion on such media relies on built-in sanitisation commands or encryption. With encryption, data are stored only in encrypted form and destroying the key renders all copies unreadable. This approach, often called cryptographic erasure, is attractive for Pellmore's video archive, where physical overwriting of every copy would be impractical across many drives.
3.3 Energy and retention
Physics also teaches that storage consumes energy and space. Keeping years of video consultations has a cost in power and cooling as well as in risk. Zara writes a short note for Pellmore's facilities team linking retention to energy use. Shorter retention reduces both the environmental footprint and the amount of health data exposed in a breach. The argument is not decisive on its own, but it shows that privacy engineering often aligns with other engineering goals, which can make proposals easier to accept for managers.
3.4 Signals and side channels
Finally, Zara studies how physical signals leak information. Video calls send packets whose sizes and timing can reveal activity even when content is encrypted, and screens emit faint signals. For Pellmore the practical lesson is modest: encrypting video protects content, but metadata such as call duration and time still reveal that a patient consulted a doctor. She notes this as a theme to revisit when she studies networks and threat models, because protecting metadata often requires design choices rather than encryption alone.
Worked example
Zara explains that row deletion leaves physical traces, proposes cryptographic erasure for the video archive, links retention to energy use and notes metadata leakage from encrypted calls.
Synthetic case. Pellmore's archive is synthetic; GDPR Article 17 is real.
Errors and limits
Cryptographic erasure depends on keys never being copied in plaintext; key management becomes the critical control.
Exercises
- EX007 — Pointer deletion. Why does deleting a file often fail to erase the data? Rubric: 3 = only the pointer is removed; the physical states remain until overwritten;; 2 = mentions overwrite;; 1 = says it erases;; 0 = none.
- EX008 — Cryptographic erasure. How does cryptographic erasure work? Rubric: 3 = data stored encrypted; destroying the key makes every copy unreadable;; 2 = mentions keys;; 1 = describes overwriting;; 0 = none.
- EX009 — Metadata leak. What still leaks from an encrypted video call? Rubric: 3 = metadata such as time and duration reveal that a consultation happened;; 2 = mentions metadata;; 1 = vague;; 0 = none.
Synthesis
Chapter 3 grounds erasure in physics. Chapter 4 starts programming for privacy.
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