Electricity: Resistance

Resistance is a property that opposes or resists the flow of electrical current through a component in a circuit.
It is measured in ohms (Ω).
In a piece of conducting material, resistance is directly proportional to the length of the material and inversely proportional to its cross-sectional area. This relationship is defined by Ohm’s law.

The resistance of a conductor increases with an increase in its length. This is because the longer the conductor, the more collisions the electrons flowing through it will encounter, hence more resistance to current flow.
The resistance of a conductor is inversely proportional to its cross-sectional area. A larger cross-section allows for more paths for the electrical current, thus reducing resistance.
Resistance also depends on the type of material the conductor is made of. Some materials, like copper and aluminium, offer very little resistance to the flow of electric current and are called good conductors.
An increase in temperature also increases resistance. This is because atoms in the conductor vibrate more when the temperature increases, which causes more collisions with flowing electrons and therefore more resistance.

In a series circuit, the total resistance (Rt) is the sum of the individual resistances. For components R1, R2, and R3 this would be Rt = R1 + R2 + R3.
In a parallel circuit, the total resistance decreases as more components are added. The inverse of the total resistance is equal to the sum of the inverses of the individual resistances.
For example, in a parallel circuit with components R1 and R2, 1/Rt = 1/R1 + 1/R2.

Ohm’s Law states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance between them.
This is represented by the formula V = IR, where V is the voltage (or potential difference), I is the current, and R is the resistance.
Ohm’s law can be used to calculate the resistance in a circuit when the current and voltage are known, or vice versa.

Resistors are components used in circuits to control the current. They are designed to have specific resistances. They can be used to reduce the current to a safe level for other components in the circuit or to divide voltage in a circuit.
Variable resistors, such as a dimmer switch, allow the resistance to be changed, controlling the current and the power of a device, like the brightness of a light bulb.
Understanding of resistance and Ohm’s law is also fundamental to the design and use of many electronics and electrical systems, from household appliances to industrial machinery.