A Level Physics Practice Exam

What is a key component of how a mercury discharge tube operates?

1. Gas atoms absorb energy and become ionized

2. Gas atoms emit thermal energy

3. The pressure inside remains constant

4. Tungsten filaments generate heat

The correct answer is: Gas atoms absorb energy and become ionized

In a mercury discharge tube, the operation relies heavily on the ionization of gas atoms. When an electrical current is applied across the tube, it provides energy to the mercury vapor present inside. This energy excites the mercury atoms, and once they absorb sufficient energy, they can lose one or more electrons, resulting in ionization. This ionization process is fundamental because it creates free electrons within the tube. The presence of these free electrons allows for the conduction of electricity through the gas, leading to further ionization, and establishing a chain reaction. As the gas continues to ionize, it produces light and other forms of radiation, which is why mercury discharge tubes are commonly used in lighting applications. The other options do not capture the primary mechanism of operation. Gas atoms emitting thermal energy relates to heat generation rather than ionization. The pressure inside a discharge tube can vary during operation due to the conversion between gas atoms and ions, which does not remain constant. Tungsten filaments, typically found in incandescent bulbs, are not a component of mercury discharge tubes; instead, they use gases like mercury vapor to produce light. Thus, the correct understanding hinges on how gas atoms absorb energy leading to ionization.