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
Pick an answer to see feedback on it. You can then try the others. Your score counts first tries only.
RAG self-check
Tap Red / Amber / Green for how confident you feel.