Question: If you took an electrically charged ball and shook it up - PDF document

Question: If you took an electrically charged ball and shook it up and down rapidly, charges in a nearby metal Radio object would move in response. How far away could that metal object be and still respond? 1. 1 meter 2. 1 kilometer 3. The other side of the universe Observations About Radio Electromagnets and Energy • Transmit sound long distances without wires • Electric and magnetic fields contain energy • Involve antennas • An electromagnet stores magnetic energy • Seem to involve electricity and magnetism • Electromagnet consumes energy as it turns on • Reception depends on antenna positioning – Current temporarily experiences a voltage drop • Electromagnet releases energy as it turns off • Reception weakens with distance – Current temporarily experiences a voltage rise • Two styles of radio: AM and FM • Electromagnet opposes current charges Inductors Tank Circuit • Inductor & • Inductors are electromagnets Capacitor share • Inductors store magnetic energy energy • Inductors oppose changes in current • Charge flows back and forth through inductor • Energy shifts back and forth between the two devices •1

Tank Circuit Oscillation Tank Circuits in Radio • Tanks are resonant devices • Tanks build up energy at a specific frequency • Tanks help radios emit radio waves • Tanks help radios detect radio waves Emitting Radio Waves 1 Emitting Radio Waves 2 • A transmitter uses a tank circuit • Accelerating charge emits radio waves to “slosh” charge up and down – Charge produces electric field its antenna – Current produces magnetic field • A receiver uses a tank circuit to – Changing current produces changing magnetic field, produces changing electric field, prod… detect charge “sloshing” on its • A radio wave consists only of an electric and antenna magnetic field • Transmitter antenna charge • A radio wave travels through empty space at the affects receiver antenna charge speed of light Structure of a Radio Wave AM Modulation • Electric field is • Information is encoded perpendicular to in the fluctuating magnetic field amplitude of the wave • Electric field creates • Pressure variations magnetic field and vice cause changes in the versa amount of charge • Electric field determines moving on the antenna polarization of the wave •2

FM Modulation Question: If you took an electrically charged ball and shook it • Information is encoded up and down rapidly, charges in a nearby metal in the exact frequency object would move in response. How far away of the charge motion could that metal object be and still respond? • Pressure variations cause slight shifts in the 1. 1 meter frequency of charge 2. 1 kilometer motion on the antenna 3. The other side of the universe •3