In an operating X-ray tube, a high p.d is applied between the cathode and the anode for the purpose of

Analyze the question: It asks why a high potential difference (p.d.) is applied between the cathode and anode of an X-ray tube.

Concept: An X-ray tube has two main electrical circuits:

1. A low-voltage circuit that heats the cathode filament. This produces electrons by thermionic emission.
2. A high-voltage circuit (tens of kilovolts) connected between the cathode and the anode (target).

The high p.d. gives the emitted electrons kinetic energy:

eV = ½mv²

The electrons are accelerated toward the target at very high speed. When they strike the metal target (e.g. tungsten), they are suddenly stopped. Their kinetic energy is converted into X-rays (about 1% of it) and heat (about 99%).

Checking each option:

• A. Accelerating the electrons to a very high energy: Correct. This is the exact role of the high p.d.
• B. Concentrating electrons on the target: This is done by the focusing cup/shield around the cathode, not by the high p.d.
• C. Heating the anode: The anode heats up as a side effect of electron bombardment. This is not the purpose of the p.d.
• D. Heating the cathode: This is done by a separate low-voltage supply through the filament.
• E. Producing a stream of electrons: This is done by thermionic emission from the heated cathode, not by the high p.d.

Answer: A. The high p.d. accelerates the electrons to a very high energy. The maximum energy of the X-ray photons is determined by this accelerating voltage (eV = hf_max).

Exam tip: Remember the pairing "low voltage heats, high voltage accelerates." The filament's low voltage produces the electrons, and the high voltage speeds them up. The intensity of the X-rays depends on the filament current (the number of electrons), while the penetrating power (hardness) depends on the accelerating p.d.