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Chapter 11: Problem 10

Why can a nearby magnet distort a cathode ray tube television picture?

Short Answer

Expert verified
A nearby magnet can distort a cathode ray tube television picture because its magnetic field alters the path of the electron beam responsible for creating the image on the phosphor-coated screen. The magnetic field affects the direction of the electrons, causing them to deviate from their intended path according to the Lorentz force equation. This deviation leads to color and brightness mismatches, resulting in a distorted image.

Step by step solution

01

Cathode Ray Tube (CRT) Television Working Principle

In a cathode ray tube television, an electron gun emits a beam of electrons, which are then accelerated and focused on a phosphor-coated screen by an electric field. As the electrons strike the phosphor coating, they emit light, producing an image on the screen. The electron beam scans across the screen in a raster pattern, covering the entire screen in lines from top to bottom. The intensity of the electron beam is varied to create different levels of brightness in the picture.
02

Magnetic Field Effects on Electron Beam

When a nearby magnet is brought close to a CRT television, it creates a magnetic field which has a direct effect on the electrons within the CRT. Electrons, being charged particles, experience a force when they travel through a magnetic field. According to the Lorentz force equation, the force on a charged particle is given by \[ F = q(E + v 脳 B) \] where F is the force on the particle, q is its charge, E is the electric field, v is the velocity of the particle, and B is the magnetic field. In a CRT television, the electric field is responsible for the acceleration and focusing of the electron beam, while the magnetic field induced by the nearby magnet affects the direction of the electrons, causing them to deviate from their intended path.
03

Distortion of TV Picture

As the electron beam inside the CRT deviates from its intended path due to the magnetic field, it causes a distortion in the television picture. The electrons no longer align with the correct spots on the phosphor-coated screen, causing the colors and brightness levels to be misplaced or altered. This results in the television picture being distorted and displaying a poor or incorrect image. In summary, a nearby magnet distorts a cathode ray tube television picture because its magnetic field alters the path of the electron beam responsible for creating the image on the phosphor-coated screen. The deviation from the intended electron path leads to color and brightness mismatches, resulting in a distorted image.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Electron Beam
The electron beam is a stream of electrons that plays a crucial role in a cathode ray tube (CRT) television. In the CRT, an electron gun generates this beam by emitting electrons into a vacuum. These electrons are then directed towards the screen. The screen is covered with a phosphor coating that lights up when hit by the beam. This lighting creates the images you see on the screen.
The
  • electron gun controls the beam's intensity to display different brightness levels,
  • an electric field accelerates these electrons as they travel to the screen,
  • and the beam moves in a raster pattern, systematically covering the entire screen.
Understanding the electron beam is vital because it interacts with magnetic fields, which can change its direction, thereby impacting the image on the television. The precise control of this beam enables clear and accurate picture projection.
Magnetic Field
A magnetic field is an invisible force around magnets, electric currents, or certain materials. In the context of a CRT television, this field can significantly affect the path of the electron beam. When a magnet is near a CRT, its magnetic field interacts with the flowing electrons.
Here's what happens:
  • The field creates a force on the electron beam.
  • This force can alter the direction of the electrons.
  • As a result, the electrons may not strike the intended spots on the phosphor-coated screen.
The magnetic field essentially acts as an external influence that can divert the electron beam from its normal path, leading to changes in the picture displayed on the TV.
Lorentz Force
The Lorentz force is a fundamental concept that explains how charged particles like electrons behave in electric and magnetic fields. According to the Lorentz force law, a charged particle, such as an electron, experiences a force when moving through a magnetic field. The formula used to describe this force is: \[ F = q(E + v \times B) \]where
  • \( F \) is the force experienced by the particle,
  • \( q \) is the electric charge,
  • \( E \) is the electric field,
  • \( v \) is the velocity of the particle, and
  • \( B \) is the magnetic field.
In a CRT television, the Lorentz force comes into play when a magnetic field is present near the television. This external magnetic field can redirect the electron beam by exerting a force on it, based on its velocity and the charge of the electrons. This misdirection is a key cause of distortion in the television picture.
Television Picture Distortion
Television picture distortion in CRTs happens when the electron beam does not follow its intended path. This misalignment means that:
  • electrons may hit the wrong spots on the phosphor screen,
  • this results in altered colors and brightness levels,
  • and the overall image becomes blurred or skewed.
Such distortion usually occurs when an external magnetic field, perhaps from a magnet placed near the television, interacts with the electron beam. The result is a misrepresentation of images on the screen, with possible color mismatches and image warping. Understanding the nature of this distortion helps appreciate why CRT TVs were sensitive to magnetic field interference, highlighting the delicate relationship between technology and environmental factors in early television designs.

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Most popular questions from this chapter

(a) Aircraft sometimes acquire small static charges. Suppose a supersonic jet has a \(0.500-\mu C\) charge and flies due west at a speed of \(660 .\) m/s over Earth's south magnetic pole, where the \(8.00 \times 10^{-5}-\mathrm{T}\) magnetic field points straight down into the ground. What are the direction and the magnitude of the magnetic force on the plane? (b) Discuss whether the value obtained in part (a) implies this is a significant or negligible effect.

What is the direction of the magnetic field that produces the magnetic force shown on the currents in each of the three cases, assuming \(\overrightarrow{\mathbf{B}}\) is perpendicular to $$I ?$$

$$\text { An alpha-particle } \quad\left(m=6.64 \times 10^{-27} \mathrm{kg}\right.$$, $$\left.q=3.2 \times 10^{-19} \mathrm{C}\right)$$ travels in a circular path of radius 25 \(\mathrm{cm}\) in a uniform magnetic field of magnitude \(1.5 \mathrm{T}\). (a) What is the speed of the particle? (b) What is the kinetic energy in electron-volts? (c) Through what potential difference must the particle be accelerated in order to give it this kinetic energy?

A 4.0-cm by 6.0-cm rectangular current loop carries a current of \(10 \mathrm{A} .\) What is the magnetic dipole moment of the loop?

Hall potentials are much larger for poor conductors than for good conductors. Why?
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