The shape of the lesson
By the end of the lesson, learners can
- Magnetic storage: A hard disk drive (HDD) contains one or more circular platters coated in a magnetic material, spinning at thousands of revolutions per minute.
- Optical storage: Optical discs store data on a single spiral track that runs from the centre to the edge of the disc.
- Solid-state storage: Flash memory stores data electronically in billions of transistors, with no moving parts at all.
- Magnetic: platters, tracks and sectors, electromagnet on a read/write head. HDD.
- Optical: laser creates and reads pits and lands. CD, DVD, Blu-ray.
- Solid-state: NAND/NOR flash, control and floating gates, no moving parts. SSD, SD card, USB drive.
- Choosing: link a feature (robust, capacity, cost, speed, portable) to the scenario.
One picture every idea returns to
Secondary storage is not directly accessed by the CPU and is non-volatile, so it is needed for more permanent storage of data and programs. Magnetic storage (HDD) uses spinning platters divided into tracks and sectors; data is read and written by electromagnets on a moving head. Optical storage (CD, DVD, Blu-ray) uses a laser to create and read pits and lands. Solid-state (flash) storage (SSD, SD card, USB drive) uses NAND or NOR technology, with transistors acting as control gates and floating gates, and has no moving parts.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Keeping data for good.. Your photos, games and homework must survive when the computer is switched off, so they live in secondary storage. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Magnetic storage | A hard disk drive (HDD) contains one or more circular platters coated in a magnetic material, spinning at thousands of revolutions per minute. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Optical storage | Optical discs store data on a single spiral track that runs from the centre to the edge of the disc. Your turn: Why can a Blu-ray disc hold much more data than a CD of the same size? | Pairs |
| 21 to 30 min | Teach and check: Solid-state storage | Flash memory stores data electronically in billions of transistors, with no moving parts at all. Model the worked example Choose the storage on the board, one line at a time. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 17 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Solid-state storage, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Data on a hard disk is stored magnetically, not with a laser. | Lasers belong to optical storage. |
| Solid-state does not use magnetism or lasers. | Do not write "the SSD spins"; it has no moving parts. |
Magnetic, optical and solid-state storage: worksheet
6 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
Explain why a computer needs secondary storage.
Describe how data is stored on a magnetic hard disk drive.
Describe how data is read from a DVD.
Describe how solid-state (flash) memory stores data.
A film company wants to send a copy of a new film to 5000 cinemas cheaply. Identify a suitable type of storage, give an example, and justify your choice.
Give two reasons why an SSD is used instead of an HDD in a smartphone.
Answer key and marking notes
- RAM is volatile, so data and programs would be lost when the power is turned off.
- Secondary storage is non-volatile, so it stores data and programs more permanently.
- The disk has platters coated in magnetic material that spin.
- Each platter is divided into tracks and sectors.
- A read/write head moves across the platter to the correct track.
- An electromagnet magnetises areas in one of two directions to represent 1s and 0s (and reads them by detecting the direction).
- The disc spins and a laser is shone onto its surface (along the spiral track).
- Pits and lands reflect the light differently (lands reflect, pits scatter).
- A sensor detects the reflected light and the changes are converted into binary.
- It uses NAND or NOR technology made of transistors.
- Each transistor has a control gate and a floating gate.
- Electrons are trapped in (or removed from) the floating gate to represent 1 or 0, and stay there without power.
- Optical storage.
- For example Blu-ray disc.
- Discs are cheap to produce in large numbers and portable/light to post (and Blu-ray has enough capacity for a high-definition film).
- No moving parts, so it is more robust if the phone is dropped.
- It is smaller/lighter, uses less power and has faster access (any second reason).
6 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
How is data stored on a hard disk drive (HDD)?
- Aas tiny areas of the platter magnetised in one of two directions
- Bas pits and lands burned into the surface by a laser
- Cas electrons trapped in the floating gates of transistors
- Das charges held in RAM chips that are kept powered on
How is data read from an optical disc such as a DVD?
- AA laser shines on the spinning disc; lands reflect light strongly and pits scatter it
- BA read head detects the direction of magnetisation of areas on the disc
- CAn electric current is passed through the transistors in each memory cell
- DA camera photographs the whole disc surface and decodes the pattern
A student needs storage for a laptop that is carried to school every day. Which is the best choice?
- Asolid-state drive, because it has no moving parts and is more robust
- Bhard disk drive, because it has the fastest access speed
- Coptical disc, because it is the most robust
- Dhard disk drive, because its platters are protected by spinning
How does solid-state (flash) storage represent a 1 or a 0?
- Aby trapping electrons in, or removing them from, the floating gate of a transistor
- Bby magnetising a small area of a spinning platter
- Cby burning a pit into a reflective layer with a laser
- Dby keeping a small current flowing through each memory cell
Why can a Blu-ray disc store much more data than a CD of the same size?
- AIt uses a blue-violet laser with a shorter wavelength, so pits are smaller and closer together
- BIt uses a red laser with a longer wavelength, so it can read deeper into the disc
- CIt stores data magnetically as well as optically on the same disc
- DIt spins faster, so more data passes the laser each second
A company needs to keep 200 TB of backup data on its own premises at the lowest possible cost per gigabyte. Fast access is not important. Which storage is most suitable?
- Amagnetic (hard disk drives)
- Bsolid-state drives
- CBlu-ray discs
- DUSB flash drives
Quiz key
An HDD's read/write head uses an electromagnet to magnetise tiny areas of the spinning platter in one of two directions, representing 1 and 0. Pits and lands made by a laser are optical storage, electrons in floating gates are solid-state (flash) storage, and powered RAM chips would be volatile primary storage.
A low-power laser is shone onto the spinning disc; lands reflect light back to a sensor and pits scatter it, and the changes are read as binary. Detecting magnetisation describes an HDD, current through transistors describes solid-state storage, and optical drives read the single spiral track with a laser, not a camera.
A solid-state drive has no moving parts, so it survives knocks better; it is also fast and uses little power. HDDs are slower than SSDs and their moving parts are easily damaged if dropped, so spinning is a weakness, not protection. Optical discs scratch easily and are slow.
In flash memory, electrons are trapped in or removed from the floating gate of each transistor to store 1 or 0, and they stay there without power, so flash is non-volatile. Magnetised platters are HDDs and laser pits are optical. Needing a constant current would make it volatile, which flash is not.
Blu-ray uses a blue-violet laser of shorter wavelength than the red laser of a CD, so its pits can be smaller and packed closer, fitting more data on the same area. A red, longer-wavelength laser is what CDs use. Optical discs are not magnetic, and spinning faster affects read speed, not how much data the disc holds.
HDDs give large capacity at a low cost per gigabyte, which suits big backups where speed does not matter. SSDs and USB flash drives are solid-state, with a higher cost per gigabyte. Blu-ray discs are cheap but each holds very little compared with an HDD, so storing 200 TB would need thousands of discs and be slow to manage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.