13.1 Radios and 13.1 Radios and Electromagnetic wavesElectromagnetic waves

New ideas for today:New ideas for today:

•Electrical resonators (tank circuits)•Electromagnetic waves (light)•How FM and AM radio works

What do all radios have in common?What do all radios have in common?

Antennas!Antennas!• Charges move in the

transmitting antenna

• Those charges exert forces on charges in receiving antenna

• Charges in the receiving antenna move

Tank circuits are used to turn small amounts of sloshing charges into large amounts of sloshing charges!

Tank circuits are all around Tank circuits are all around you:you:

• Cell phone• Any watch with a battery /

electronic clock• Computers…

Tank circuitTank circuit

CapacitorCapacitor

Just two plates of metal

•Stores charges

•Stores energy – it takes work to separate charges that attract!

Capacitor and oilCapacitor and oil

CapacitorsCapacitors

Capacitor Capacitor and and generatorgenerator

InductorInductor

Just a loop of wire

Moving charges (current) store energy in magnetic field

InductorsInductors

Tank circuitTank circuit

Electric energy

Electric energy

Magnetic energy

Magnetic energy

Tank circuitTank circuit

This is just like a pendulum!This is just like a pendulum!

Potential energy

Kinetic energy

Potential energy

Ever wonder how a traffic light “knows” that a car is waiting?

That loop of wire is part of a tank circuit…and your car changes its inductance!

Alice and Bob each use a hand-cranked generator to charge a capacitor. Who has done more work?

+ + +

- - -

+ + ++++++

- - - - - - - -

(A) (B)

Clicker questionClicker question

““Capacitive touch switch”Capacitive touch switch”Switch measures how long it takes to charge a capacitor

What’s the capacitor?

•Just like pendulums, tank circuits have a frequency•We call that the “resonant frequency”•Driving current through the tank circuit at the resonant frequency results in big changes in voltage across the capacitor•This is the trick to radios! Antennas are tuned to listen to specific frequencies…

WILL FM: 90.9 MHzWILL AM: 580 kHz cellphone: 900 MHz

90.9 millionoscillations per second

580 thousandoscillations per second900 millionoscillations per second!!!

Antennas that are far apart…Antennas that are far apart…

……are connected are connected by by electromagnetic electromagnetic radio waves!radio waves!•Accelerating charges make electromagnetic waves•Electromagnetic waves cause charges to accelerate

Tuning forksTuning forks

Radio broadcastRadio broadcast

Electromagnetism IIElectromagnetism II

• Magnetic fields created by– Fundamental particles (dipoles) ― electrons,

protons, neutrons…– Moving electric charges (current)

• Electric fields created by– Charges– Changing magnetic fields (induction)

Electromagnetism IIElectromagnetism II

• Magnetic fields created by– Fundamental particles (dipoles) ― electrons,

protons, neutrons…– Moving electric charges (current)– Changing electric fields

• Electric fields created by– Charges– Changing magnetic fields (induction)

•Electromagnetic waves and light are the same thing!•They move at the speed of light: 186,282 miles/sec•They have two “polarizations”•Are not like sound waves: do not require a “medium”

Electromagnetic wavesElectromagnetic waves PolarizationPolarization

Heinrich Hertz: discovered radio waves (1885) and showed that they are light

Hertz radio wavesHertz radio waves

Tesla coilsTesla coils

Tesla coil Tesla coil and radioand radio

Alice wants to send a message to Bob. Which method will get her message to Bob faster?A) Using a laser beamB) Using a walkie talkie (radio)C) Using a cell phoneD) All of the above are the same

Clicker questionClicker question

EM waves with AM frequencies bounce off of the ionosphere and can travel really far (1000s of miles) at night

•37-190 miles above the earth•Very electrically charged!•FM frequency waves do not reflect from the ionosphere

Faraday cageFaraday cage

Information is transmitted by modulation

Microphone receives sound wave and converts to oscillating electrical current (20 Hz – 20 kHz)

An electrical “carrier” signal is produced (90.9 MHz for WILL FM) indepedently

modulator

AM

FM

sound electronic

AM = Amplitude modulation

FM = Frequency modulation

Frequency modulation: information encoded in slight changes in the frequency

Amplitude modulation: information is encoded in changes in the size of the electrical signal

demodulator

carrier

signal

sound

See you next class!

For next class: Read Section 13.2