The big idea: An analogue system is described with six quantities: voltage, current, resistance, power, frequency and time.
Voltage pushes, current flows, resistance opposes, power is the rate of energy use, frequency counts cycles a second, and time is the base unit the rest are built on.
The six quantities, the two relationships that link them, and the multiplier ladder from pico to tera.
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| Quantity | Symbol | SI unit | What it tells a designer |
|---|---|---|---|
| Voltage | V | volt (V) | What supply the product needs, and whether a part can survive it |
| Current | I | ampere (A) | How thick the wire must be, and how long the battery lasts |
| Resistance | R | ohm (Ω) | How much current a component will allow at a given voltage |
| Power | P | watt (W) | How much heat it makes, and how big the component must be |
| Frequency | f | hertz (Hz) | How fast a repeating signal changes — audio, radio, a clock |
| Time | t | second (s) | Delays, pulse widths, sample intervals |
Two relationships carry most of the marks: Ohm's law: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I.
Power: P = V × I. A 9 V supply across a 470 Ω resistor passes about 19 mA and dissipates about 0.17 W — comfortably inside a quarter-watt part.
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| Prefix | Name | Factor | Where you meet it |
|---|---|---|---|
| p | pico | ×10⁻¹² | Small capacitors — 22 pF in a crystal circuit |
| n | nano | ×10⁻⁹ | Capacitors again — 100 nF across every supply pin |
| µ | micro | ×10⁻⁶ | Larger capacitors, and sleep currents in microamps |
| m | milli | ×10⁻³ | Currents — an LED at 20 mA, a chip pin at 40 mA |
| k | kilo | ×10³ | Resistors — 4.7 kΩ, 10 kΩ; audio at 20 kHz |
| M | mega | ×10⁶ | Large resistors; a microcontroller clock at 16 MHz |
| G | giga | ×10⁹ | Processor clocks, radio bands, storage |
| T | tera | ×10¹² | Storage, and data totals |
One step is a factor of a thousand: Writing 4,700 Ω as 4.7 MΩ instead of 4.7 kΩ is an error of a thousand, and no amount of circuit knowledge recovers it.
The check that catches it: ask whether the value is plausible for that component. A 4.7 MΩ resistor in an LED circuit would pass about two microamps and light nothing at all.
How this is tested — describing analogue systems using voltage, current, resistance, frequency, power and SI units. It comes up two ways:
Paper 1 — multiple choice
- Choose the correct SI unit for a named quantity.
- Convert a value between SI multipliers.
Paper 2 — analysing a product
- Calculate a current or a power in a described circuit.
- Explain what a quoted value means for the product.
The trap: Losing a factor of a thousand in a prefix. Write the unit in full first, then apply the prefix, and check the answer is plausible for that component.
An indicator LED needs 20 mA at 2 V and is connected to a 9 V supply through a series resistor. Apply Ohm's law and the power equation to specify that resistor.
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