Learning goals
- State Ohm’s law and rearrange it for V, I, or R
- Convert among milli-, base, and kilo- electrical units
- Choose consistent units before substituting into formulas
Three linked quantities
Voltage (V, volts) is electric potential difference — the “push” that can drive charge. Current (I, amperes) is the flow of charge. Resistance (R, ohms) measures how strongly a component opposes that flow.
For many DC and simple AC circuit estimates, Ohm’s law links them: V = I × R. If you know any two, you can find the third.
Engineering uses metric prefixes constantly: mV, mA, kΩ, MΩ. Convert to base units (V, A, Ω) before multiplying unless every factor shares a consistent scaled system.
Ohm’s law
V = I × R · I = V ÷ R · R = V ÷ I
Use volts, amperes, and ohms together. Example: 12 V across 6 Ω draws I = 12 ÷ 6 = 2 A.
Common prefix factors
1 mA = 0.001 A · 1 kΩ = 1000 Ω · 1 mV = 0.001 V
Move the decimal three places per milli/kilo step. Mega (M) is 10^6; micro (µ) is 10^−6.
Example 1 — Find current
Given
A 9 V battery feeds a 1.8 kΩ resistor. What is the current?
Steps
- Convert resistance: 1.8 kΩ = 1800 Ω.
- I = V ÷ R = 9 ÷ 1800 = 0.005 A.
- Express in milliamps: 0.005 A = 5 mA.
Answer
5 mA.
Example 2 — Milliamps to amps
Given
A USB device draws 500 mA. How many amperes is that?
Steps
- 1 mA = 0.001 A, so divide milliamps by 1000.
- 500 ÷ 1000 = 0.5.
Answer
0.5 A.
Tip: Power estimates often need amps in base units: P = V × I.
Example 3 — Find resistance
Given
A sensor drops 3.3 V while passing 10 mA. What is its resistance?
Steps
- Convert current: 10 mA = 0.01 A.
- R = V ÷ I = 3.3 ÷ 0.01 = 330 Ω.
Answer
330 Ω.
Mixing mA with ohms without converting
Common mistake
Computing R = 5 V ÷ 10 mA as 0.5 Ω by treating “10” as amps.
Better approach
10 mA = 0.01 A, so R = 5 ÷ 0.01 = 500 Ω. Or use V ÷ mA carefully: 5 ÷ 10 = 0.5 kΩ = 500 Ω.
Confusing kΩ and Ω on a schematic
Common mistake
Reading “4.7” next to a resistor marked k as 4.7 Ω.
Better approach
4.7 k means 4700 Ω. Prefixes change results by factors of a thousand.
Check your understanding
1.Convert 2200 Ω to kΩ.
Answer: 2.2 kΩ.
2.I = 12 V ÷ 4 Ω. What is I?
Answer: 3 A.
3.Convert 250 mV to volts.
Answer: 0.25 V.
Key takeaways
- Ohm’s law: V = I × R with volts, amps, and ohms.
- Convert milli/kilo prefixes before substituting blindly.
- Power and Ohm’s law need consistent base units.
- Schematic prefixes (k, M, m) are easy to miss — read them twice.