Convert every quantity to base units before using a formula, and state the simplifying assumptions in a sentence. These two habits fix the commonest lost marks in circuit calculations.
This lesson belongs to SPM subjects and learning resources. The work is paper-based, and any building or testing is done at school with supervision.
Which prefixes matter?
| Prefix | Meaning | Example | Base unit |
|---|---|---|---|
| k (kilo) | × 1 000 | 4.7 kΩ | 4 700 Ω |
| m (milli) | ÷ 1 000 | 28 mA | 0.028 A |
| μ (micro) | ÷ 1 000 000 | 100 μF | 0.0001 F |
Write the converted value on the line before the formula. This also shows the marker which conversion you made.
Worked example: Ohm’s law with a kilo-ohm resistor
An original problem: a 12 V supply is connected across a 4.7 kΩ resistor. Find the current and the power.
Convert. 4.7 kΩ = 4 700 Ω.
Current. I = V ÷ R = 12 ÷ 4 700 = 0.002 55 A, which is 2.55 mA.
Power. P = V × I = 12 × 0.002 55 = 0.0306 W, which is 30.6 mW.
Sense check. A few volts across thousands of ohms gives milliamps. So 2.55 mA is sensible.
Assumptions. The supply voltage is steady, and the wires have negligible resistance.
The mistake that costs marks
The usual slip is to divide 12 by 4.7 directly, which gives 2.55 A. That is 1 000 times too large. A 12 V supply across 4.7 kΩ cannot push 2.55 A.
| Step | Wrong | Right |
|---|---|---|
| Resistance used | 4.7 (kΩ left as is) | 4 700 Ω |
| Current | 12 ÷ 4.7 = 2.55 A | 12 ÷ 4 700 = 2.55 mA |
| Sense check | Not done | Milliamps expected |
The fix is mechanical. Convert first, then calculate, then check the size.
Worked example: two resistors in series
An original problem: a 9 V battery is connected to a 100 Ω and a 220 Ω resistor in series. Find the current.
Total resistance = 100 + 220 = 320 Ω. Current = 9 ÷ 320 = 0.028 125 A, which is about 28.1 mA.
Assumptions: the battery is ideal and the wires have no resistance. A real battery has some internal resistance, so the measured current would be slightly smaller.
Check yourself
A 6 V supply is connected across 2.2 kΩ. Find the current in mA.
Answer
Convert: 2.2 kΩ = 2 200 Ω.
I = 6 ÷ 2 200 = 0.002 727 A, which is about 2.73 mA.
Sense check: a few volts across a few kilo-ohms gives a few milliamps, so the answer is reasonable. Stated assumptions: an ideal 6 V supply and wires with negligible resistance.
What to study next
Try the original mixed practice. The structured answer self-review tool helps you check prefixes and assumptions in your own working.
For a teacher to go through your calculations, see online one-to-one Electrical and Electronic Engineering Studies tuition.