In a traditional cathode-ray television picture tube, which type of field is used to produce deflection of the electron beam on the screen?

Introduction / Context:
Older television sets used picture tubes called cathode-ray tubes (CRTs). Inside a CRT, an electron beam is fired towards the fluorescent screen, and this beam has to be bent or deflected so that it scans the entire screen line by line. This question tests your understanding of which physical field is actually used in a television picture tube to deflect the electron beam and form the image on the screen, a basic idea from electron optics and electromagnetism.

Given Data / Assumptions:

• The device considered is a traditional television picture tube (CRT type).
• Electrons are emitted and accelerated to form a narrow electron beam.
• The beam must be deflected horizontally and vertically to scan the screen.
• The options mention electric fields, magnetic fields, both fields, or none of these.

Concept / Approach:
Charged particles like electrons experience a force when they move through electric or magnetic fields. In oscilloscopes, either electrostatic (electric) plates or magnetic coils can be used. However, in television CRTs, the conventional design uses pairs of electromagnetic deflection coils placed around the neck of the tube. When current flows through these coils, they create magnetic fields that bend the moving electron beam. Therefore, the correct field used for deflection in a standard television picture tube is a magnetic field produced by these coils.

Step-by-Step Solution:
Step 1: Identify the moving charged particles: the electron beam in the CRT.Step 2: Recall that a TV picture tube uses deflection yokes, which are coils of wire around the neck of the tube.Step 3: When current flows through these coils, they produce magnetic fields.Step 4: A moving electron entering a magnetic field experiences a force perpendicular to its velocity and to the field, causing the beam to bend.Step 5: By varying the current in the horizontal and vertical coils, the beam is scanned over the entire screen.Step 6: Conclude that the operational deflection in TV CRTs is achieved using magnetic fields, not electrostatic plates.

Verification / Alternative check:
In electronics textbooks that describe CRT televisions, you will find references to the horizontal and vertical deflection yoke, which is clearly an electromagnetic component. Oscilloscopes often use electrostatic plates, but television CRTs almost always use magnetic deflection because it allows a wider deflection angle in shorter tube lengths. This cross-check confirms that the practical answer in the context of TV picture tubes is the use of a magnetic field for deflection.

Why Other Options Are Wrong:
Using only an electric field is typical in some lab CRTs and oscilloscopes, not in standard television picture tubes. The option stating both fields are used does not match the dominant design of TV CRTs, where deflection is purely magnetic. The option none of the above is incorrect because we clearly rely on a physical field, namely a magnetic field, to bend the electron beam in a TV picture tube.

Common Pitfalls:
Students sometimes confuse TV CRTs with the electrostatic CRTs used in physics labs and assume that deflection must be electrostatic. Another common confusion is to think that both fields are always involved whenever electrons are present. To avoid such mistakes, remember this key pairing: oscilloscopes often use electric plates for deflection, whereas television picture tubes use magnetic deflection coils around the tube for scanning the beam.

Final Answer:
In a television picture tube, deflection of the electron beam is produced by a magnetic field.