Are Electricity & Magnetism really that Different ?

What produces electric fields ?

- the presence of electric charges - - - - - - - - - - - What produces magnetism ?

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v

- the presence of moving electric charges (current!) If someone runs carrying a charge rod, what would you see?

What would the runner be seeing ?

In the frame of the runner, he sees Electric Fields due to the stationary charges.

In your frame, you see Magnetism due to the moving charges!

Electricity & Magnetism – two sides of the same coin; intimately related

Now what happens if you

accelerate

charges ?

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-v +v

• produce a mixture of changing electric and magnetic fields • these ‘fluctuations of electric, magnetic fields are periodic and can travel in empty space • these fluctuations have a frequency identical to the • frequency at which you jiggle the charges.

fluctuations are called ‘radio waves’ Can these be useful ?

What if you pass an AC Current thru a metal rod ?

AC

Current I

This can be an ‘antenna’ !

Quick Observations about Radios

• Ability to transmit sound wirelessly over long distances, even thru vacuum!

• Involves transmitter, receiver and antennas • Reception depends on distance, antenna orientation • Appears to involve electricity and magnetism as basic driving principles • Two typical modes; AM, FM

Antennas and Tank Circuits

Antenna Antenna Transmitter Tank Tank Receiver • • Electric charges sloshing up/down transmitting antenna propagate thru space and causes identical motion of charges on the receiver antenna.

What’s inside a Tank Circuit ?

+ + + + + - - - - - Capacitor – two parallel plates w/ opposite charges. Stores energy in its electric field.

Inductor – solenoid, stores energy in its magnetic field • Energy sloshes back and forth between the capacitor (as electric field) and inductor (as magnetic field).

• This ‘sloshing’ occurs at a frequency characteristic of of capacitor, inductor used. The larger they are, the more energy then can store and the lower the frequency of sloshing.

• Tanks are ‘ resonant devices ’ that build up energy at a particular frequency.

+ + + + + - - - - - -

Tank Circuit Half-Cycle

current + + - - + + - - - - - + + + + +

Emitting Radio Waves

How are Radio Waves generated, transmitted and received ? Changing current Changing Magnetic Fields Changing Electric Fields Radio Waves – fluctuations in electric fields and magnetic fields, that can travel in vacuum at the speed of light!

Antenna I I’ Antenna Transmitter Tank Tank Receiver

What does a Radio Wave Look like ?

Oscillating charges Direction of propagation Electric Field above is vertically-polarized.

• Electric, magnetic fields are mutually perpendicular • The polarization of the electric field – depend on how the charges are oscillating.

• Maxima, minima of E fields coincide with maxima, minima of magnetic fields, respectively.

• Fluctuations travel in vacuum (require no elastic medium!) at the speed of light (c=3 x 10 8 m/s) • Radio waves propagated at right angles to oscillation axis (never along the axis)

Optimizing Reception of Radio Waves

Antenna I I’ Antenna TransmitterTank Tank Receiver • For maximum reception, receiving tank circuit must be resonant at the radio wave’s frequency.

• For resonant transfer of energy, antenna length = ¼ the wavelength of the tank’s broadcast frequency.

• Receiving antenna must be perpendicular to direction of radio wave propagation (or parallel to polarization) To ensure good reception, some radio stations transmit a circularly polarized wave – a superposition of vertical, horizontal polarizations.

Embedding Sound(info) in AM/FM Radio Waves

Amplitude Modulation (AM ) • information encoded in the fluctuating amplitude or envelope of modulation signal • pressure variations causes Changes in intensity of carrier wave Frequency Modulation (FM) • information encoded in the Exact frequency of the charge motion • presure variations causes shifts in Frequency of charge motion