Radio and Television

What is an electric field?

• A region of space where an electric charge feels a force

What is an electric field line?

• They move from positive to negative charges

• A changed particle experiences a force along the field lines

Strong electric fields can…

• Accelerate quickly 

How do charges behave when they’re placed in an electric field?

• When a charge sits inside an electric field, the field exerts a force on it.

• This tells us two things:

  1. The force is proportional to the size of the charge.
     A bigger charge experiences a stronger push; a smaller charge feels a weaker one.

  2. The force is proportional to the strength of the electric field.
     Stronger electric field → larger force; weaker field → smaller force.

How are electricity and magnetism related?

• A changing electric field always produces a magnetic field

• A changing magnetic field always produces an electric field

• Electric and magnetic fields are linked and cannot change one without generating the other

How do we use an antenna to create an electric field?

• Putting a charge on an antenna creates an electric field

  • it can be positive or negative

• If the charge doesn’t change, then the electric field is constant in time (static) - not enough to send out radio waves

What do antennas need to radiate electromagnetic waves?

• Need a changing charge or current (moves back and forth)

What happens if the antennas has a positive charge?

• Electric field points away from the antenna

What happens if the antennas has a negative charge?

• Electric field points toward the antenna

How can we change the electric field coming off the antenna?

• If we accelerate the charge up and down on the antenna

How do we make the charges accelerate on the antenna?

• Connect the antenna to electronics that produce an alternating current

• Alternating current means the flow of charges switches direction back and forth many times per second

• The charges move up and down in the antenna at the same frequency as the electronics 

Why does accelerating charge produce an electromagnetic wave?

• A changing electric field must also produce a changing magnetic field - and thats why accelerating charge gives us an electromagnetic field

What is the main point about what leaves the antenna?

• What is coming off the antenna is an electric field that is changing with time

• It becomes stronger and stronger (in one direction), then changes direction and gets weaker and weaker, then builds up again - repeating over and over

• Whenever we have a changing electric field, we also have a changing magnetic field

What is a vertically polarized electric field?

• If we have a vertical antenna, the electric field coming off of it will point in the same direction as the antenna 

• The electric field points up or down, changing its intensity, and it will not have any horizontal component

  • The horizontal component is a horizontally polarized magnetic field

Can electromagnetic waves travel without matter?

• Can travel in a vacuum

What are examples of electromagnetic waves?

• visible light, radio waves, microwaves, X-rays, gamma rays

• the only thing differentiating them is the wavelength or frequency

What is a tank circuit and how does it store energy?

• Can be used to create an oscillating electric current

• It stores energy in two forms: 

  • Magnetic field energy (in the inductor)

  • Electric field energy (in the capacitor)

How does an inductor store energy?

• Basically a coil of wire

• when current flows through the coil, it creates a magnetic field around it

• the more current that flows, the stronger the magnetic field

• the magnetic field represents the energy stored in the inductor 

How does a capacitor store energy?

• Thought of two parallel plates separated by some distance

• if we put a positive charge on one plate and negative charge on the other, an electric field forms between the plates

• the capacitor stores energy in this electric field

• so a capacitor both stores charge and creates an electric field

How does a tank circuit oscillate?

1. At first, the capacitor slowly charges the coil because Lenz’s law resists a sudden change in the magnetic field.

2. As current flows through the coil, the magnetic field grows.

3. When the current in the coil is maximum, the capacitor is completely discharged.

4. The magnetic field in the coil now begins to collapse, inducing a current in the wire that continues to flow.

5. This current charges the capacitor in the opposite direction (the top plate becomes negative, the bottom plate positive).

• The cycle then repeats, with energy constantly oscillating between the electric field of the capacitor and the magnetic field of the inductor

How do radio transmitters and receivers work?

• Transmitter (for the radio station) and a receiver (for the radio)

• Both transmitter and receiver use a tank circuit

What does the tank circuit do in a transmitter?

• You need to put power into the tank circuit.

• The transmitter gives small boosts to the tank circuit to compensate for energy lost.

• Since electromagnetic waves are emitted from the antenna, the energy in the tank circuit must match the energy flowing off the antenna.

• The transmitter converts sound into an oscillating signal in the tank circuit, which then produces electromagnetic wavesthat travel through space.

What happens in the receiver?

• The receiver also has an antenna attached to a tank circuit.

• Electronics in the receiver convert the signal from the tank circuit into something we can hear.

• When the electromagnetic wave hits the receiver’s antenna, it pushes charges up and down in the antenna at the same frequency.

• If this matches the resonance frequency of the receiver’s tank circuit, the oscillating current builds up amplitude, making the signal easier to detect.

• Even though the charges moving in the antenna are small, the tank circuit amplifies the signal enough to use it.

How does the radio antennas interact?

• The transmitter’s tank circuit charges oscillate and feed energy into its antenna.

• The antenna emits an electromagnetic wave:

  • Electric field vertically polarized

  • Magnetic field horizontal

• The receiver’s antenna collects this wave:

  • The vertical electric field moves charges in the receiver antenna up and down.

  • This drives the tank circuit in the receiver at the same frequency

What happens if antenna orientations dont match?

• If the transmitter antenna is vertical but the receiver antenna is horizontal, the receiver will not pick up the signal.

• Correct polarization alignment is necessary for the electromagnetic wave to effectively induce current in the receiving antenna.

What is AM radio?

• Assigned a specific carrier frequency determined by law

• the sound that we want to transmit has constant frequency and its own amplitude

How does amplitude modulation work?

• we take the carrier wave and change its amplitude according to the amplitude of the sound wave:

  • Where the sound wave is loudest, the carrier wave’s amplitude is maximum.

  • Where the sound wave is quietest, the carrier wave’s amplitude is minimum.

• Loud sounds are broadcast at full power; quiet sounds are broadcast at lower power.

• This variation in amplitude encodes the sound information onto the carrier wave.

What are some characteristics of AM waves?

• AM waves can bounce off the atmosphere, which allows them to travel long distances.

• AM radio is good for sending sound, but the varying amplitude makes it more susceptible to noise and interference.

What is FM radio

• FM radio uses a higher frequency range than AM, which allows it to carry more information.

• This extra information is why FM can broadcast in stereo.

• Frequency modulation

How does frequency modulation work?

• Unlike AM, where you change the amplitude, in FM you change the frequency of the carrier wave according to the sound signal.

• The amplitude of the FM wave stays constant — it always broadcasts at full power.

What are some characteristics of FM waves?

• The receiver picks up the frequency changes of the wave to reconstruct the sound.

• When driving away from an FM station, the signal either comes through completely or is lost entirely — there’s no gradual fading like AM.

What is the electron gun in a CRT (old tv)

• The electron gun is the part of a CRT that creates, shapes, and controls a beam of electrons, so it can strike the phosphor screen and form an image.

What does a cathode do?

• Heated wire

• when it gets hot, electrons gain enough energy to spill off the surface

• the electron source, the starting point of the beam

What is the control grid and how does it work?

• metal structure with a hole in it, positioned after the cathode

• held at negative voltage relative to the cathode, which repels electrols

• by changing the grids voltage, the electron gun allows more or fewer electrons to pass through the hole

• this controls the brightness: more electrons → stronger beam → phosphor glows brighter 

What does the anode do?

• accelerates electrons

• it is positively charged and pulls electrons forward, accelerating them to high speeds

• helps shape the initial path of the beam

• without the anodes strong electric field, electrons would drift and never reach the screen

What is the role of the focusing coil?

• after acceleration, electrons tend to spread out

• the focusing coil uses a magnetic field to squeeze the beam into a thin sharp spot

• a blurred beam → fuzzy picture

• a tight beam → sharp pixels

How do deflection optics work?

• Deflection coils create magnetic fields that sweep the electron beam in a fixed pattern across the screen.

• The beam moves slowly across the screen, zips back quickly, drops down, then repeats.

• By changing the intensity of the electron beam as it moves, you can create bright and dark spots, forming different images.

What is an interlaced image?

• The beam sweeps the screen in a way that produces interlaced lines, so the image appears smooth without flickering.

• The magnetic field is the key tool used to steer the beam.

• Steering is done with horizontal and vertical sweeps.

How do colour tvs produce images?

• If we use different phosphors that glow red, green, and blue, and the electron beam hits them, we can create a full-color image.

• The front of the TV has tiny dots of phosphor, each glowing red, green, or blue.

• To make an image, we hit the correct phosphor dots with electrons, making them glow.

What is the problem with the electron beam?

• The electron beam is a little thick, so it might accidentally hit the wrong color phosphor (for example, hitting blue and red at the same time).

• To solve this, TVs use a shadow mask, which ensures the electron beam hits only the correct colored phosphor dot.

What is a shadow mask?

• A shadow mask is a piece of metal with lots of tiny holes - basically a mesh.

• It sits just in front of the phosphor-coated screen in a color CRT.

How does the shadow mask work with three electron guns?

• Color CRTs have three electron guns: red, green, and blue.

• The shadow mask ensures that:

  • The red electron gun hits only the red phosphor dots

  • The green gun hits only the green dots

  • The blue gun hits only the blue dots

• This allows each electron gun to strike only its corresponding colored phosphor, producing accurate colors

What are some precautions with a shadow mask?

• Do not put a magnet near the TV screen.

• A magnetic field can magnetize the shadow mask, causing the electron beams to miss the correct phosphor, which distorts colors.

• Black-and-white TVs do not have a shadow mask, so this isn’t a problem for them.