Ohm’S Law

Ohm’s law is an equation describing the relationship between the voltage across an electrical component, the electrical resistance of the component, and the current flowing through the component. It is named after its discoverer, Georg Simon Ohm (1789–1854). Ohm found that for most electric circuits, the voltage across the circuit was equal to the current flowing through the circuit times the electrical resistance of the circuit. For the same voltage, a circuit with a low resistance will have a higher current than a circuit with a higher resistance. The voltage, properly called the potential difference, is measured in volts, and the current in amperes. The resistance therefore has units of volts per ampere, defined as ohms (Ω).

Ohm’s law is not a fundamental law that always applies, such as the law of gravity. Rather, it is an empirical law that has been found by experiment to work well enough most of the time. There are times, however, usually in extreme cases, when Ohm’s law breaks down. For example, if a high voltage is applied across a circuit, Ohm’s law will not predict the correct value for the current. Even though Ohm’s law does not always apply, it works for most everyday situations and is therefore very useful.

For example, why will a short circuit blow a fuse or circuit breaker? When a short circuit occurs, most of the electrical resistance in the circuit is bypassed. In effect, a new circuit with a very low resistance is created. So, according to Ohm’s law if the resistance is very low the current must be very high. Fuses and circuit breakers are designed to protect the circuit by blowing (melting or snapping open) when the current becomes too high. Hence, a short circuit produces a current high enough to open the circuit.

Electronic devices often have resistors placed in the circuit to increase the resistance and therefore limit the current. Further, whenever current flows through a resistance, energy is dissipated as heat. Unwanted resistance therefore wastes energy. Some resistive devices, such as the heating coils found in hair driers, are designed to convert large amounts of electrical energy directly into heat.