Understanding the Electron-Volt Unit

The question asks us to identify the physical quantity measured by the Electron-Volt (eV). Let's break down what the Electron-Volt represents.

Defining the Electron-Volt (eV)

An Electron-Volt (eV) is a unit of energy. It is defined as the amount of kinetic energy gained or lost by a single electron when it moves across an electric potential difference of one volt in a vacuum.

Mathematically, we can express this relationship. If an electron with charge '$e$' moves through a potential difference '$V$', the energy '$E$' gained or lost is given by:

$E = qV$

Where:

• $E$ is the energy
• $q$ is the charge
• $V$ is the potential difference

Substituting the charge of an electron ($q = e$) and a potential difference of 1 volt ($V = 1 \text{ V}$), we get:

$E = (1 \text{ e}) \times (1 \text{ V}) = 1 \text{ eV}$

The elementary charge '$e$' is approximately $1.602 \times 10^{-19}$ Coulombs. Therefore, 1 eV is equal to:

$1 \text{ eV} \approx 1.602 \times 10^{-19} \text{ C} \times 1 \text{ V} = 1.602 \times 10^{-19} \text{ Joules (J)}$

Since the Joule (J) is the standard SI unit for energy, this shows that the Electron-Volt is also a unit of energy, commonly used in atomic, nuclear, and particle physics where energies involved are very small.

Comparing eV with Other Options

Let's look at why the other options are incorrect:

• Electric Potential: Measured in Volts (V). It represents the potential energy per unit charge.
• Electric Power: Measured in Watts (W). It is the rate at which energy is transferred or converted, calculated as $P = V \times I$ (Voltage times Current).
• Electric Force: Measured in Newtons (N). It's the force experienced by charges due to an electric field, often described by Coulomb's Law.

Based on the definition and comparison, the Electron-Volt (eV) is fundamentally a unit of energy.

Conclusion

The Electron-Volt (eV) quantifies the energy change experienced by an electron moving through a potential difference. Thus, it is a unit of electric energy.