In physics, resistance is defined as the ability of a substance to prevent or resist the flow of electrical current. A substance resists electrical current because of a collision between electrons and atoms, which slows the electrons down and converts some of their energy to heat energy. In some cases, the energy is also converted into light.

Some substances resist electrical current more than others. For example, copper resists less than plastic, which means electricity flows more readily through copper, a good conductor, than it does through plastic. Likewise, copper resists less than glass, which is considered an insulator.

German physicist Georg Ohm (1789-1854) is credited with discovering the effect of a material's make up, length and thickness on its resistance. In fact, the standard unit of measuring resistance, the ohm, is named after him. Ohm's law became publicly known in 1827 and states that a circuit's resistance is one ohm if a one-volt voltage makes one amp of current. According to Ohm's law, a circuit's resistance is equal to its voltage divided by the amount of current.

A voltage of two volts would cause a two-amp voltage to flow if a circuit had a resistance of one ohm. However, if a circuit had a resistance of two ohms, two volts would instead produce only a one amp of current. This is because of the following relationship between voltage, current, and resistance: Voltage can be found by calculating current times resistance, and current is found by dividing voltage by resistance.

Physics equations typically use letters and symbols to express resistance. The letter symbol for resistance is R. An equation used in measuring resistance is R equals V over (divided by) I. In this equation, R equals the resistance (ohms), V equals the potential difference of the object (volts), and I is the current that flows through the object (amperes).

When it comes to conductive wires, their length and thickness impacts their electrical resistance. For example, a thick wire allows electrical current to flow more readily. This is due to the amount of electrons contained in a cross-section of the wire. Thicker wires have a higher number of free electrons to transport electrical current. On the other hand, long wires resist current more, as they contain more free atoms that block the way of the free electrons.