Correct Option
The correct option is 1 joule per coulomb.
Explanation
Electric potential difference, or voltage, is a derived physical quantity representing the work done per unit charge to move a test charge between two points in an electric field. The SI unit for this potential difference is the Volt.
Option Analysis
- 1 joule per coulomb: Correct. Mathematically, electric potential ($V$) is defined as $V = \frac{W}{Q}$, where $W$ is the work done (measured in Joules) and $Q$ is the electric charge (measured in Coulombs). Therefore, 1 Volt is defined as 1 Joule per Coulomb ($1 \text{ V} = 1 \text{ J/C}$).
- 1 coulomb per second: Incorrect. 1 Coulomb per second ($1 \text{ C/s}$) represents the rate of flow of electric charge, which is the definition of Electric Current (the Ampere).
- 1 ampere per ohm: Incorrect. According to Ohm’s Law ($V = I \times R$), 1 Volt is equal to 1 Ampere multiplied by 1 Ohm ($1 \text{ A} \cdot \Omega$). The expression "1 ampere per ohm" ($A/\Omega$) does not define voltage.
- 1 watt per second: Incorrect. Power is defined as the rate of doing work. 1 Watt is equal to 1 Joule per second. "1 watt per second" would represent the rate of change of power, which is unrelated to the definition of the Volt.
Key Takeaway: The Volt is the SI derived unit for electric potential, potential difference, and electromotive force. It is strictly defined as energy per unit charge ($1 \text{ V} = 1 \text{ J} \cdot \text{C}^{-1}$).