Are Electricity & Magnetism really that Different ?

1. 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

2. - - - - - - - - - - - - -v +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’ Can these be useful ? What if you pass an AC Current thru a metal rod ? Current I AC This can be an ‘antenna’ !

3. 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

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

5. 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.

6. + + + + + - - - - - - - - + + Tank Circuit Half-Cycle current + + - - - - - - - - + + + + +

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

8. What does a Radio Wave Look like ? Direction of propagation Electric Field above is vertically-polarized. Oscillating charges • 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 108m/s) • Radio waves propagated at right angles to oscillation axis (never • along the axis)

9. Antenna Antenna I I’ Transmitter Tank Tank Receiver 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) To ensure good reception, some radio stations transmit a circularly polarized wave – a superposition of vertical, horizontal polarizations.

10. 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