[PDF] - Radio Radio nearby metal object would move in response. nearby PDF Document

SLIDE 1

1

Radio 1

Radio Radio

Radio 2

Introductory Question Introductory Question

If you took an electrically charged ball and

If you took an electrically charged ball and shook it up and down rapidly, charges in a shook it up and down rapidly, charges in a nearby metal object would move in response. nearby metal object would move in response. How far away could that metal object be and How far away could that metal object be and still respond? still respond?

A. A.

1 meter 1 meter

B. B.

1 kilometer 1 kilometer

C. C.

The other side of the universe The other side of the universe

Radio 3

Observations about Radio Observations about Radio

It can transmit sound long distances wirelessly

It involve antennas

It apparently involves electricity and magnetism

Its reception depends on antenna positioning

Its reception weakens with distance

There are two styles of radio: AM and FM

Radio 4

3 Questions about Radio 3 Questions about Radio

How can a radio wave exist?

How is a radio wave emitted and received?

How can a radio wave represent sound?

Radio 5

Question 1 Question 1

How can a radio wave exist?

How can a radio wave exist?

What is an electromagnetic wave anyway?

How can an electric field exist without charge?

H

n m n ti fi ld i t ith t p l ? H n m n ti fi ld i t ith t p l ?

How can a magnetic field exist without pole?

Radio 6

Electromagnetic Waves Electromagnetic Waves

Radio waves are a class of electromagnetic waves

Electromagnetic waves

are structures in space that consist only of electric

are structures in space that consist only of electric fields and magnetic fields fields and magnetic fields fields and magnetic fields. fields and magnetic fields.

They are emitted or received by charge or pole,

but are self

but are self-

sustaining while traveling in vacuum

sustaining while traveling in vacuum

because their electric and magnetic fields endlessly

because their electric and magnetic fields endlessly recreate one another. recreate one another.

SLIDE 2

2

Radio 7

Electromagnetism Electromagnetism (Version 3)

(Version 3)

Magnetic fields are produced by

magnetic poles (but free poles don’t seem to exist),

moving electric charges,

nd h n in

l tri fi ld nd h n in l tri fi ld

and changing electric fields

and changing electric fields. .

Electric fields are produced by

electric charges,

moving magnetic poles,

and changing magnetic fields.

Radio 8

Structure of a Radio Wave Structure of a Radio Wave

Electric field is perpendicular to magnetic field

Changing electric field

Changing electric field creates magnetic field creates magnetic field Ch i i fi ld Ch i i fi ld

Changing magnetic field

Changing magnetic field creates electric field creates electric field

Polarization of the wave

Polarization of the wave is associated with the is associated with the wave’s electric field wave’s electric field

Radio 9

Clicker Questions Clicker Questions

If you place a positive test charge in a radio

If you place a positive test charge in a radio wave while there are no other charges nearby, wave while there are no other charges nearby, will that test charge experience a force? will that test charge experience a force?

A.

A. Yes

Yes

B.

B. No

No

Radio 10

Question 2 Question 2

How is a radio wave emitted and received?

Radio 11

Launching a Radio Wave Launching a Radio Wave

Accelerating charge emits electromagnetic waves

Accelerating charge emits electromagnetic waves

The more charge and the more it accelerates, the

The more charge and the more it accelerates, the stronger the resulting electromagnetic wave stronger the resulting electromagnetic wave R di i d i R di i d i

Radio stations need to emit strong waves

so they move large amounts of charge

with the help of resonant devices: tank circuits

Radio 12

A Tank Circuit A Tank Circuit

A tank circuit

is a harmonic oscillator for charge

its charge flows rhythmically through the inductor,

its charge flows rhythmically through the inductor, from one capacitor plate to the other and back again from one capacitor plate to the other and back again from one capacitor plate to the other and back again. from one capacitor plate to the other and back again.

Its energy alternates between

magnetic field in its inductor

electric field in its capacitor

SLIDE 3

3

Radio 13

Tank Circuit Oscillation Tank Circuit Oscillation

The frequency of the tank

The frequency of the tank circuit is determined only by its circuit is determined only by its inductance and its capacitance. inductance and its capacitance.

It can store a great deal of energy

It can store a great deal of energy and have an enormous amount and have an enormous amount

f charge moving through it.
f charge moving through it.
Tank circuits come in many

Tank circuits come in many shapes, including straight lines! shapes, including straight lines!

Radio 14

Tank Circuits in Radio Tank Circuits in Radio

Tanks build up energy at a specific frequency

Two tanks help radios

Two tanks help radios emit emit radio waves radio waves

The first tank circuit feeds the antenna

The first tank circuit feeds the antenna Th d k i i IS h Th d k i i IS h

The second tank circuit IS the antenna

Two similar tanks help radios

Two similar tanks help radios detect detect radio waves radio waves

Radio 15

Emitting Radio Waves Emitting Radio Waves (Part 1)

(Part 1)

A transmitter uses a tank circuit to “slosh”

A transmitter uses a tank circuit to “slosh” charge up and down its antenna, charge up and down its antenna, which acts as a second tank. which acts as a second tank.

A receiver uses a tank circuit to

A receiver uses a tank circuit to detect charge “sloshing” on its detect charge “sloshing” on its tank tank-

circuit antenna.

circuit antenna.

Transmitter antenna charge

Transmitter antenna charge affects receiver antenna charge affects receiver antenna charge

Antenna orientations matter!

Radio 16

Emitting Radio Waves Emitting Radio Waves (Part 2)

(Part 2)

Accelerating charge emits radio waves

Stationary charge produces an electric field

Steady current produces a magnetic field

Changing current produces a changing magnetic

Changing current produces a changing magnetic g g p g g g g g p g g g field, produces a changing electric field, prod… field, produces a changing electric field, prod…

A radio wave consists only of an electric and

A radio wave consists only of an electric and magnetic field magnetic field

A radio wave travels through empty space at the

A radio wave travels through empty space at the speed of light speed of light

Radio 17

Clicker Question Clicker Question

Does a flashlight emit radio waves?

A.

A. No

No

B.

B. Yes, whenever it is on.

Yes, whenever it is on.

C.

C. Yes, but only as you turn it on or off.

Yes, but only as you turn it on or off.

Radio 18

Introductory Question Introductory Question (revisited)

(revisited)

If you took an electrically charged ball and

If you took an electrically charged ball and shook it up and down rapidly, charges in a shook it up and down rapidly, charges in a nearby metal object would move in response. nearby metal object would move in response. How far away could that metal object be and How far away could that metal object be and still respond? still respond?

A. A.

1 meter 1 meter

B. B.

1 kilometer 1 kilometer

C. C.

The other side of the universe The other side of the universe

SLIDE 4

4

Radio 19

Question 3 Question 3

How can a radio wave represent sound?

Radio 20

AM Modulation AM Modulation

Information can be encoded as a fluctuating

Information can be encoded as a fluctuating amplitude of the radio wave amplitude of the radio wave

The air pressure variations

The air pressure variations that are so nd ca se changes that are so nd ca se changes that are sound cause changes that are sound cause changes in the amount of charge in the amount of charge moving on the antenna and moving on the antenna and thus the intensity of the wave thus the intensity of the wave

The receiver detects these

The receiver detects these changes in radio wave intensity. changes in radio wave intensity.

Radio 21

FM Modulation FM Modulation

Information can be encoded as a fluctuating

Information can be encoded as a fluctuating frequency of the radio wave frequency of the radio wave

The air pressure variations

The air pressure variations that are so nd ca se slight that are so nd ca se slight that are sound cause slight that are sound cause slight shifts in the frequency of shifts in the frequency of charge motion on the antenna charge motion on the antenna and the frequency of the wave and the frequency of the wave

The receiver detects these

The receiver detects these changes in radio wave frequency. changes in radio wave frequency.

Radio 22

Summary about Radio Summary about Radio

Radio waves are emitted by charge accelerating

n a transmitting antenna
n a transmitting antenna

Radio waves are detected when they cause

Radio waves are detected when they cause charge to accelerate on a receiving antenna charge to accelerate on a receiving antenna charge to accelerate on a receiving antenna charge to accelerate on a receiving antenna

Radio waves consist only of self

Radio waves consist only of self-

sustaining

sustaining electric and magnetic fields electric and magnetic fields

Radio waves can represent sound information as