Are Electricity & Magnetism really that Different ? What produces electric fields ? - the presence of electric charges What produces magnetism ? - 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 v

Now what happens if you accelerate charges ? 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’ v +v AC Can these be useful ? What if you pass an AC Current thru a metal rod ? This can be an ‘antenna’ ! Current I

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 TransmitterTankReceiverTank Antenna 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.

current Tank Circuit Half-Cycle

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

What does a Radio Wave Look like ? 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) Oscillating charges Electric Field above is vertically-polarized. Direction of propagation

Optimizing Reception of Radio Waves 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) TransmitterTankReceiverTank Antenna II’ 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 ) Frequency Modulation (FM) information encoded in the fluctuating amplitude or envelope of modulation signal pressure variations causes Changes in intensity of carrier wave information encoded in the Exact frequency of the charge motion presure variations causes shifts in Frequency of charge motion