Whether you are studying physics at a Polish liceum, taking an electronics course, or troubleshooting a simple circuit at home, Ohm's Law is the fundamental rule you need to understand. Formulated by German physicist Georg Simon Ohm in 1827, it describes the relationship between voltage, current and resistance — the three core quantities in any electrical circuit.
The Formula
Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance:
V = I × R
Where:
- V = Voltage, measured in volts (V)
- I = Current, measured in amperes (A)
- R = Resistance, measured in ohms (Ω)
This single equation can be rearranged to solve for any one unknown when the other two are known:
- Current: I = V / R
- Resistance: R = V / I
Understanding the Three Quantities
Voltage (V)
Voltage is the electrical potential difference between two points in a circuit. Think of it as the "pressure" pushing electrons through a conductor. A standard AA battery provides 1.5 V. A household socket in Poland provides 230 V AC. A car battery provides 12 V DC.
Current (I)
Current is the flow rate of electric charge — specifically, the number of coulombs of charge passing a point per second. One ampere equals one coulomb per second. Low-power electronics like LED circuits may use milliamperes (mA); industrial motors may draw hundreds of amperes.
Resistance (R)
Resistance is the opposition a material offers to the flow of current. Conductors like copper have very low resistance; insulators like rubber have extremely high resistance. Resistors — small components in electronic circuits — have fixed resistance values, typically expressed in ohms (Ω), kilohms (kΩ) or megaohms (MΩ).
Worked Examples
Example 1: Finding Current
A 9 V battery is connected to a 180 Ω resistor. What current flows?
I = V / R = 9 / 180 = 0.05 A (50 mA)
Example 2: Finding Voltage
A current of 2 A flows through a 15 Ω resistor. What is the voltage across it?
V = I × R = 2 × 15 = 30 V
Example 3: Finding Resistance
A lamp draws 0.5 A when connected to a 230 V supply. What is its resistance?
R = V / I = 230 / 0.5 = 460 Ω
Ohm's Law Triangle
A popular memory aid is the "Ohm's Law triangle." Draw a triangle and write V at the top, I on the bottom-left and R on the bottom-right. Cover the quantity you want to find and the remaining two show you the correct operation: covering V gives I × R; covering I gives V / R; covering R gives V / I.
Power and Ohm's Law
Ohm's Law is closely linked to the power formula P = V × I. Combining these two equations gives useful variants:
- P = I² × R
- P = V² / R
For example, a 60 W bulb on a 230 V supply draws I = P / V = 60 / 230 ≈ 0.26 A, and has a resistance R = V / I = 230 / 0.26 ≈ 885 Ω (when hot).
Practical Applications
LED circuits: LEDs require a current-limiting resistor. If you want to run a red LED (forward voltage 2 V, target current 20 mA) from a 5 V USB source: R = (5 − 2) / 0.02 = 150 Ω.
Fuse selection: Knowing the current draw of a device helps you select the correct fuse rating — just enough above normal operating current to protect the circuit without nuisance tripping.
Heating elements: Electric kettles and space heaters rely on resistive heating. A 2 000 W kettle on 230 V draws 8.7 A and has a heating element resistance of about 26.5 Ω.
Battery life estimation: If a circuit draws 50 mA from a 2 000 mAh battery, it will run for approximately 2 000 / 50 = 40 hours.
Limitations of Ohm's Law
Ohm's Law applies to ohmic (linear) conductors — materials where resistance stays constant regardless of voltage or current. Many components are non-ohmic: diodes, transistors and incandescent bulbs (whose resistance increases with temperature) do not obey Ohm's Law across their full operating range.
Need to calculate resistance, current or voltage quickly? Try the physics calculators at Liczbnik.pl for Ohm's Law and other electrical formulas.