Ohm's Law Explained
What is Ohm’s law actually about? And how can we use it to understand electricity and electronics?
The article is written in connection with our "Ohm’s law calculator".
Ohm’s law describes the relationship between three quantities in an electric circuit: voltage, current and resistance. It says that the current through most materials is directly proportional to the voltage applied to them. The German physicist Georg Simon Ohm (1787–1854) was the first to show experimentally that the current in a metal wire is proportional to the voltage, and he described his experiment in a paper published in 1827, according to the OpenStax textbook University Physics.
In practice, this means that as long as the resistance stays the same, doubling the voltage doubles the current, and halving the voltage halves the current. If you want more or less current to flow through a circuit, you need to change either the voltage or the resistance.
The Ohm’s law formula
Ohm’s law is written as:
V = I × R
- V is the voltage, in volts (V).
- I is the current, in amperes (A). Current is the rate at which electric charge flows, as OpenStax defines it.
- R is the resistance, in ohms (Ω). One ohm is one volt per ampere, according to OpenStax.
Rearranging the formula gives the two other forms, which you use to find the quantity you are missing:
- I = V / R, to find the current
- R = V / I, to find the resistance
Ohm’s law examples
Finding the current. A 12 V battery is connected to a 24 Ω resistor. The current is I = 12 / 24 = 0.5 A.
Finding the resistance. A 9 V source drives a current of 0.03 A (30 mA) through a component. Its resistance is R = 9 / 0.03 = 300 Ω.
Finding the voltage. A current of 0.02 A (20 mA) flows through a 470 Ω resistor. The voltage across it is V = 0.02 × 470 = 9.4 V.
Finding the power. Ohm’s law is often combined with the formula for electric power, P = V × I, which you can work out with our "Watt’s law calculator". In the first example, the resistor dissipates P = 12 × 0.5 = 6 W. Substituting Ohm’s law into the power formula gives two more forms, P = I² × R and P = V² / R, as OpenStax explains.
What can Ohm’s law be used for?
Ohm’s law lets you calculate one of the three quantities when you know the other two. You can use it to work out how much current will flow through a circuit, to choose a resistor that limits the current, or to find the resistance of a component. If you measure the voltage across a component and the current through it, you find its resistance by dividing the voltage by the current.
Ohm’s law also helps you check whether a component can handle the load you put on it. Our Ohm’s law calculator takes any two of voltage, current, resistance and power, calculates the other two, and, if you enter your resistor’s wattage rating, warns you when the calculated power exceeds it.
When doesn’t Ohm’s law work?
According to OpenStax, Ohm’s law is an empirical law, which means it is based on what is observed in experiments. The linear relationship between voltage and current does not always hold. Materials and components that obey it are called ohmic, and those that don’t are called nonohmic.
Nonohmic components. One example is the diode, a semiconductor component that lets current flow in only one direction. In a diode, the current is not proportional to the voltage.
Temperature. The resistance of a material depends on its temperature. In most metals, such as copper, resistivity increases as the temperature rises. In other materials, such as carbon and semiconductors, it decreases, according to the OpenStax chapter on resistivity and resistance. The same chapter shows how large the effect can be: a tungsten filament with a resistance of 0.350 Ω at 20 °C has a resistance of about 4.8 Ω at 2850 °C, more than ten times as much. In an incandescent light bulb, that means the current is higher when the bulb is first switched on than once the filament has reached its operating temperature.