Voltage, current and resistance
Ohm’s law formula
The statement of the law
For an ohmic conductor, the voltage U across its terminals is proportional to the current I flowing through it. This relationship is expressed as:
U = R × I
where U is in volts (V), R in ohms (Ω) and I in amperes (A).
Two other useful forms
From U = R × I, we can isolate each quantity:
- To find the current: I = U / R
- To find the resistance: R = U / I
Practical example
A 100-ohm resistor is connected across a generator supplying a voltage of 5 V. What is the current?
We use I = U / R = 5 / 100 = 0.05 A, or 50 mA.
A table for practice
| U (V) | R (Ohm) | I (A) |
|---|---|---|
| 12 | 6 | 2 |
| 9 | 3 | 3 |
| 4.5 | 9 | 0.5 |
Common pitfalls to avoid
- Do not confuse R = U / I with U = R / I: resistance is calculated by dividing the voltage by the current, never the other way round.
- Be careful with units: if the current is given in milliamps (mA), you must convert it to amps (divide by 1,000) before calculating.
- Ohm’s law, U = R × I, applies only to ohmic conductors (such as resistors), not to all components (a light bulb or a diode does not follow this law exactly).
Method for solving a problem
- Identify the known quantities and the unit required.
- Choose the correct form of the formula (U, I or R depending on the unknown).
- Check the units before calculating.
- Calculate and give the result with its unit.