The physical quantity which is equal to the ratio of potential difference and current is RESISTANCE. 

R = V/I.

Electric Resistance (R)

Electric resistance (R) is the physical quantity which is equal to the ratio of voltage (V) to current (I). It is a fundamental concept in the study of electricity and plays a crucial role in various electrical systems and devices.

Explanation:

Electric resistance is a property of a material or a component that resists the flow of electric current through it. It is represented by the symbol 'R' and is measured in ohms (Ω). The relationship between voltage, current, and resistance is given by Ohm's Law, which states that the current flowing through a conductor is directly proportional to the voltage applied across it and inversely proportional to the resistance of the conductor.

Ohm's Law:
According to Ohm's Law, the relationship between voltage (V), current (I), and resistance (R) can be expressed by the equation:

V = I × R

where:
- V: Voltage (in volts)
- I: Current (in amperes)
- R: Resistance (in ohms)

Key Points:
- Electric resistance (R) is a physical quantity that measures how much a material or a component resists the flow of electric current.
- It is represented by the symbol 'R' and is measured in ohms (Ω).
- Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it and inversely proportional to the resistance of the conductor.
- The equation V = I × R represents the relationship between voltage (V), current (I), and resistance (R) according to Ohm's Law.
- In practical terms, resistance determines how much current will flow through a circuit for a given voltage. Higher resistance restricts the flow of current, while lower resistance allows more current to flow.
- Resistance is influenced by various factors such as the material used, length, cross-sectional area, and temperature of the conductor.
- Different materials have different resistivities, which determine their inherent resistance.
- Resistance plays a crucial role in electrical circuits, as it helps control the flow of current, protects components from damage due to excessive current, and allows for the functioning of various electrical devices and systems.