Radio and Television

Forces and Electric Charge

Electric Field

Electric field lines move from positive to negative charges. These lines represent the direction of the electric field at any point in space. A charged particle placed in the field will experience a force along the field lines: a positive charge will move in the direction of the lines, while a negative charge will move opposite to them.

Magnets and Electricity

• Electric charge creates an electric field (monopole), which acts as a source or sink for the field.
• Magnet creates a magnetic field (dipole), with distinct north and south poles.
• Electric current generates a magnetic field, forming the basis of electromagnetism.
• Changing magnetic field induces an electric field, as described by Faraday's Law of Induction.
• (NEW) Changing electric field induces a magnetic field, which is a key concept in Maxwell's equations and explains electromagnetic wave propagation.

Antenna

• Charge movement: Charge moves up and down the antenna, generating a polarized electric field. This electric field oscillates vertically, creating a vertically polarized wave.

• Electric and magnetic fields: A vertically polarized electric field induces a horizontally polarized magnetic field, as shown in the diagram. These perpendicular electric and magnetic fields propagate together as an electromagnetic wave.

This process illustrates the fundamental way antennas emit electromagnetic radiation, with oscillating electric and magnetic fields at right angles to each other.

Tank Circuit

1. Capacitor charged, no field: Initially, the capacitor is fully charged, and no current flows through the inductor. There is no magnetic field.
2. Capacitor discharging, current flowing, field growing: The capacitor begins to discharge, causing a current to flow through the inductor. The inductor generates a growing magnetic field as energy transfers from the capacitor to the inductor.
3. Capacitor discharged, maximum field: The capacitor is fully discharged, and the magnetic field in the inductor reaches its maximum strength.
4. Field reducing, capacitor charging: The magnetic field in the inductor begins to collapse, inducing a current that charges the capacitor in the opposite direction.
5. Field minimum, capacitor charged: The magnetic field reduces to zero, and the capacitor is fully charged again, but with opposite polarity.

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• Inductor: A coil that creates a magnetic field, storing energy in the field.
• Capacitor: Stores electric charge, holding energy in the electric field.
• Lenz's Law: Ensures the induced current opposes the change in magnetic flux.
• This is the foundation of a resonant LC circuit, where the oscillation frequency depends on the inductance and capacitance values.

Tank Circuit (Full Cycle)

Radio Transmitter-Receiver

1. Transmitter Tank Circuit:
   • Electric charge oscillates, draining energy from the transmitter's tank circuit.
   • These oscillations produce an alternating electric field, which radiates as an electromagnetic wave.
2. Receiver Tank Circuit:
   • The electromagnetic wave induces oscillations in the receiver's tank circuit.
   • Electric charge oscillates, feeding energy into the receiver's tank circuit, allowing signal detection.

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• Key Principle: The system works only if the frequencies of the transmitter and receiver tank circuits match.
• Tuning: Adjusting the receiver to match the transmitter frequency ensures proper signal reception and communication.

Since the wavelength is greater than the hole, so the holes can be a good block.

Sound Waves

• In a radio system, sound information is transmitted by encoding frequency and amplitude into electromagnetic waves.
• Alternatively, sound can be represented by changes in air pressure over time.

Key Concepts

• Compression: Regions where air molecules are pushed together, increasing pressure.
• Rarefaction: Regions where air molecules are spread apart, decreasing pressure.

These compressions and rarefactions form a longitudinal wave, which can be visualized as a sine wave representing pressure variations over time.

AM Radio Transmission

1. Carrier Frequency:
   • The AM station operates within a frequency range of 550 kHz to 1600 kHz.
   • This high-frequency carrier wave is used to transmit information.
2. Sound Wave:
   • Represents the audio signal (e.g., music or voice) to be transmitted.
   • It typically has a much lower frequency than the carrier wave.
3. Modulated Radio Wave:
   • The sound wave modulates the amplitude of the carrier wave, resulting in a combined signal.
   • This is known as amplitude modulation (AM), where the carrier's amplitude varies in proportion to the sound wave.

This modulated wave is broadcasted by the transmitter and later demodulated by the receiver to recover the original sound signal.

FM Radio Transmission

FM always broadcasts full power.

1. Carrier Frequency:
   • FM stations operate within a frequency range of 88 MHz to 108 MHz.
   • This high-frequency carrier wave acts as the medium for transmitting the signal.
2. Sound Wave:
   • Represents the audio information (e.g., voice or music) to be transmitted.
   • It has a lower frequency compared to the carrier wave.
3. Modulated Radio Wave:
   • In frequency modulation (FM), the frequency of the carrier wave is varied in accordance with the amplitude of the sound wave.
   • Unlike AM, the amplitude remains constant while the frequency changes to encode the audio signal.

FM offers higher sound quality and better noise resistance compared to AM, making it ideal for music and high-fidelity audio broadcasting.

Comparison of AM and FM Modulation

1. Sound Wave:
   • This represents the original audio signal, a low-frequency wave to be transmitted.
2. Carrier Frequency:
   • A high-frequency wave that carries the audio signal, remaining constant in both AM and FM before modulation.
3. AM (Amplitude Modulation):
   • The amplitude of the carrier wave varies in proportion to the sound wave.
   • The frequency of the carrier remains constant.
   • AM is more susceptible to noise but simpler to implement.
4. FM (Frequency Modulation):
   • The frequency of the carrier wave varies in accordance with the sound wave's amplitude.
   • The amplitude of the carrier stays constant.
   • FM provides better sound quality and noise resistance, ideal for high-fidelity broadcasts.

This comparison highlights the different ways audio information is encoded for radio transmission.

Why Does Sound Change When Rotating a Radio?

When you rotate a radio, the sound may become quieter or stronger due to the following reasons:

1. Signal Reception and Antenna Orientation

• Radios rely on their antenna to pick up signals. The effectiveness of the antenna depends on its orientation relative to the incoming radio waves.
• Best Reception: When the antenna aligns properly with the direction of the radio waves, the signal strength is maximized, resulting in clear and strong sound.
• Poor Reception: If the antenna is misaligned or in a position where it captures fewer radio waves, the signal weakens, causing the sound to become quieter or distorted.

2. Interference and Obstacles

• Interference from nearby electronic devices or physical objects (e.g., buildings, walls) can block or scatter radio signals.
• Rotating the radio may help find a position with fewer obstacles, improving signal strength.

3. Type of Modulation

• In AM radio, sound quality is more sensitive to signal strength fluctuations, so weak signals can cause a noticeable drop in volume or introduce static.
• In FM radio, weak signals may result in reduced sound clarity or complete loss of signal, but FM is less prone to static compared to AM.

Television Tube (CRT) - Cathode Ray Tube

Key Components and Function

1. Cathode:
   • Emits electrons when heated. These are the particles that form the image on the screen.
2. Cathode Grid:
   • Controls the number of electrons emitted by the cathode, adjusting the brightness of the display.
3. Anode:
   • Accelerates the electrons toward the screen by applying a high positive voltage.
4. Focusing Coil:
   • Focuses the electron beam into a fine point to improve image clarity and sharpness.
5. Deflecting Coils:
   • Steer the electron beam horizontally and vertically to scan the entire screen.
   • This deflection allows the beam to create images by lighting up specific points on the screen.
6. Phosphor Coating:
   • The inside of the screen is coated with phosphor material.
   • When the electron beam strikes the phosphor, it glows, producing visible light that forms the image.

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Process

• Electron Emission: Electrons are emitted from the cathode.
• Acceleration and Focusing: The anode accelerates them, and the focusing coil sharpens the beam.
• Beam Steering: Deflecting coils bend the beam to control where it hits the screen.
• Image Formation: The beam lights up different areas of the phosphor coating to produce the image, scanning line by line.

This system forms the basis of traditional CRT televisions and monitors.

Electron Gun

• Cathode: Emits electrons through heating.
• Grid: Controls electron beam intensity by adjusting voltage.
• Anode: Accelerates electrons toward the screen.
• Focusing Coil: Focuses the electron beam onto the phosphor-coated screen for a sharp image.

Magnetic Force

• A charged particle (e.g., an electron) moving through a magnetic field experiences a force.
• This force is perpendicular to both the particle's velocity and the magnetic field, as described by the right-hand rule.

Key Points

• The electron's path bends due to this force.
• In the diagram, the force on the electron is directed out of the paper, causing the electron to bend towards the viewer.

This principle is fundamental in devices like cathode ray tubes and particle accelerators.

Deflecting Coils and Electron Beam Path

• Deflecting Coils: Generate a magnetic field to bend the path of the electron beam.
• Path of Electron: The beam bends into the paper due to the perpendicular force from the magnetic field.

Key Points

• By varying the current through the deflecting coils, the beam can scan horizontally (left to right) and vertically (up and down) across the phosphor screen.
• Higher Current → Stronger magnetic field → Greater deflection of the electron beam.

Phosphor Screen

(Black and White Television)

Key Points

• Phosphor Screen: Glows when struck by the electron beam, creating visible light.
• Electron Beam Scanning: The beam is scanned horizontally across the screen, line by line, controlled by deflecting coils.

Process

1. Intensity Control: The brightness of each spot on the screen is determined by the intensity of the electron beam.
2. Image Formation: Bright and dark spots form patterns, such as the letter "H."
3. Interlaced Scanning: The entire screen is scanned 30 times per second, creating a smooth image for the viewer.

Colour Television

Key Points

• RGB Phosphor Dots: The screen is composed of tiny phosphor dots that emit red, green, or blue light when struck by electrons.
• Color Mixing: By combining different intensities of these three colors, the TV can produce all visible colors.

Process

1. Electron Beams: Three electron beams (one for each color) are precisely aimed at the corresponding phosphor dots.
2. Color Formation: The blending of red, green, and blue light at each pixel creates the desired color.

Colour Television

Key Components:

• Three Electron Guns: Emit electron beams for red, green, and blue colors.
• Shadow Mask: A metal sheet with a grid of tiny holes that ensures each beam strikes the correct color phosphor dot (red, green, or blue) on the screen.
• Phosphor Screen: Contains clusters of red, green, and blue phosphor dots.

Process

1. Electron Beams: Each gun fires a beam aimed at its respective color phosphor dot.
2. Shadow Mask Alignment: The mask aligns the beams so they only hit the corresponding phosphor dots.
3. Color Display: Combining the three colors at varying intensities produces the full range of colors for each pixel.