Theory — Quantities and Calculations
Back to Electronics hub GlossaryUnits and the core electrical quantities — the foundations for every calculation. Work through each part, try the examples, then test yourself at the end.
📐 The 5 steps for any calculation
- Find the formula — it's on your sheet.
- Fix the units — change any prefixes (see the number helper).
- Put the numbers in.
- Work it out — one line at a time.
- Write the answer — with its unit.
Units, scientific notation & prefixes
Key words
- Unit
- what you measured (for example volt or ohm).
- Prefix
- a letter in front of a unit that makes it bigger or smaller (k = ×1000).
- Base unit
- the plain unit with no prefix (for example Ω, A, V).
Every measurement has a unit. The unit tells you what you measured (for example volts or ohms).
Big or small numbers use a prefix — a letter in front of the unit (like k for kilo). You can also write them in scientific notation, a short way to write very big or very small numbers (for example 4 700 Ω = 4.7 × 10³ Ω).
- Current I — ampere (A) · Voltage / p.d. V — volt (V)
- Resistance R — ohm (Ω) · Power P — watt (W)
- Time t — second (s) · Capacitance C — farad (F) · Frequency f — hertz (Hz)
A prefix is a letter in front of a unit. k (kilo) means , while m (milli) means . The plain unit with no prefix is called the .
Write each value in its base unit (no prefix): (a) 4.7 kΩ (b) 22 µF (c) 3.3 mA (d) 470 nF.
Answer
✅ You can now…
- name the unit for current, voltage, resistance and power.
- change a value between a prefix and its base unit (× or ÷ 1000).
Current, voltage, resistance & power
Key words
- Voltage (p.d.)
- the push that moves the current. In volts (V).
- Current
- how much electricity flows. In amps (A).
- Resistance
- how hard it is for current to flow. In ohms (Ω).
- Power
- how fast energy is used (heat or light). In watts (W).
- Look at what the question gives you. Pick the power formula that uses those two things.
- A resistor turns electrical energy into heat — it gives out power.
- Always change prefixes to the base unit before you put numbers in (use the number helper).
Voltage is the that moves the current, measured in . Resistance is how hard it is for current to flow, measured in , and power is how fast energy is used, measured in .
See the full answer
A 12 V supply drives a current of 0.25 A through a lamp. Calculate (a) the resistance of the lamp and (b) the power it dissipates.
Answer
✅ You can now…
- use V = I R for a component or a whole circuit.
- pick and use the right power formula (P = IV, I²R or V²/R).
Number Cruncher challenge
Ten quick questions mixing prefixes & conversions with Ohm's law and power. Type each answer in the unit shown. Score 8 / 10 to earn the 🧮 Number Cruncher badge — tap New set for fresh numbers any time.
Check your understanding
Answer the multiple-choice questions, then mark yourself with the RAG self-check.
RAG self-check
Tap Red / Amber / Green for how confident you feel.