Practice problems
Units and prefixes practice problems
Electronics values span a huge range: a resistor can be 10 Ω or 10 MΩ, and a capacitor 10 pF or 1000 µF. Prefixes keep the numbers short, so you'll convert between them all the time. These ten problems start with milliamps and kilohms and work up to a two-step conversion and a current in microamps.
The method
Each prefix is a power of 1000:
| Prefix | Symbol | Multiplies by |
|---|---|---|
| mega | M | 1,000,000 |
| kilo | k | 1000 |
| milli | m | 0.001 |
| micro | µ | 0.000001 |
| nano | n | 0.000000001 |
| pico | p | 0.000000000001 |
- To a smaller unit (A → mA, kΩ → Ω): multiply by 1000 for each step
- To a bigger unit (mA → A, pF → nF): divide by 1000 for each step
The problems
Type your answer and tap Check. Every one has the working, step by step.
Problem 1 of 10
A motor draws 0.35 A. How many milliamps is that?
Show the working
- Milli means one-thousandth, so there are 1000 mA in 1 A. Going to the smaller unit, multiply by 1000.
- 0.35 A × 1000 = 350 mA
Answer: 350 mA
Problem 2 of 10
A resistor is marked 6.8 kΩ. What is that in ohms?
Show the working
- k (kilo) means × 1000.
- 6.8 kΩ × 1000 = 6,800 Ω
Answer: 6800 Ω
Problem 3 of 10
An LED runs at 45 mA. What is that in amps?
Show the working
- Going from milliamps to amps, divide by 1000: the number gets smaller because the unit is bigger.
- 45 mA ÷ 1000 = 0.045 A
Answer: 0.045 A
Problem 4 of 10
A button is held down for 250 ms. How long is that in seconds?
Show the working
- There are 1000 ms in 1 s, so divide by 1000.
- 250 ms ÷ 1000 = 0.25 s
Answer: 0.25 s
Problem 5 of 10
A meter reads 850 mV across a diode. What is that in volts?
Show the working
- Millivolts to volts: divide by 1000.
- 850 mV ÷ 1000 = 0.85 V
Answer: 0.85 V
Problem 6 of 10
A capacitor is marked 0.47 µF. What is that in nanofarads?
Show the working
- Each prefix step is × 1000: 1 µF = 1000 nF.
- 0.47 µF × 1000 = 470 nF
Answer: 470 nF
Problem 7 of 10
A tiny capacitor is marked 2200 pF. What is that in nanofarads?
Show the working
- 1 nF = 1000 pF, so going up to nanofarads, divide by 1000.
- 2200 pF ÷ 1000 = 2.2 nF
Answer: 2.2 nF
Problem 8 of 10
A light sensor reads 1.2 MΩ in the dark. What is that in kilohms?
Show the working
- M (mega) is × 1,000,000 and k (kilo) is × 1000, so 1 MΩ = 1000 kΩ.
- 1.2 MΩ × 1000 = 1200 kΩ
Answer: 1200 kΩ
Problem 9 of 10
A capacitor is marked 47,000 pF. What is that in microfarads?
Show the working
- Pico to micro is two prefix steps: pF → nF → µF. Divide by 1000 for each step.
- 47,000 pF ÷ 1000 = 47 nF
- 47 nF ÷ 1000 = 0.047 µF
Answer: 0.047 µF
Problem 10 of 10
A 3.3 V supply sits across a 1.5 MΩ resistor. What current flows, in microamps?
Show the working
- Use I = V over R. Volts divided by megohms gives microamps, just as volts divided by kilohms gives milliamps.
- I = 3.3 V over 1.5 MΩ = 2.2 µA, which is 0.0000022 A.
Answer: 2.2 µA
“Resistance: the squeeze” is lesson 4 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 3 before it first.
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Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Resistance: the squeeze” is lesson 4 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 3 before it first.
Or start from the very first lesson, no account neededCheck it with a calculator
The ideas behind it
- What is electric current?How much charge flows past a point each second, in amps, and why it's the same all round a loop.
- What is resistance?The squeeze on the flow, in ohms: how resistors set the current and protect an LED.
- Ohm's law explainedV = I × R: set any two of voltage, current and resistance and the third follows.
Questions people ask
How do I convert milliamps to amps?
Divide by 1000, because 1 mA is one-thousandth of an amp. So 45 mA = 0.045 A and 250 mA = 0.25 A. Going the other way, from amps to milliamps, multiply by 1000: 0.35 A = 350 mA.
Which way do I move the decimal point?
Going to a smaller unit (A to mA, kΩ to Ω), the number gets bigger: multiply by 1000. Going to a bigger unit (mA to A, pF to nF), the number gets smaller: divide by 1000. Each prefix step is a factor of 1000.
What is the difference between µF, nF and pF?
They're all fractions of a farad, each 1000 times smaller than the last: 1 µF = 1000 nF, and 1 nF = 1000 pF. So 0.1 µF, 100 nF and 100,000 pF are the same capacitor, written three ways.
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