Motivation for Today’s Session

MS-PS4 Waves and their Applications in Technologies for Information Transfer

Students who demonstrate understanding can:MS-PS4-1: Use mathematical representations to describe a simple

model for waves that includes how the amplitude of a wave is related to the energy in a wave.

MS-PS4-2: Develop and use a model to describe that waves are reflected, absorbed, or transmitted through various materials.

MS-PS4-3: Integrate qualitative scientific and technical information to support the claim that digitized signals are a more reliable way to encode and transmit information than analog signals.

If the tension in a string is increased, then its natural frequency will a) increaseb) decrease

Two strings have the same length and tension. One string has a mass per length that is 4 times that of the other string. The fundamental frequency of the more massive string will be a) larger or b) smaller than that of the less massive string.

A musician shortens the length of a string by pushing it against the fingerboard. This causes the fundamental frequency to a) increase

b) decrease c) stay the same

Radio waves – analog transmission of information

• Travel at speed of light (approximately 3 x 108 m/s)

• Natural: lightning and astronomical objects• Artificial: fixed and mobile communication

broadcasting radar and navigation systems computer networks

• Different frequencies have different propagation characteristics.

Electromagnetic Spectrum

Microphone

• Converts sound waves (5 Hz – 20 kHz) into electrical impulses

• Strength of electrical impulses corresponds to sound diagram

Oscillator

• Produces AC – alternating current• Current flows in one direction and then back. • Current that switches 1 million times a

second, generates radio waves with a frequency of 1 million Hz.

AM vs. FM

Berserkerus/Wikimedia Commons

Carrier Signal – sine wave of constant frequency

Amplitude Modulation: Sounds are encoded by differences in amplitude. 500 – 1600 kHz

Frequency Modulation: Sounds are encoded by differences in frequency. 86 – 107 MHz

Receiving Radio TransmissionReceives lots of radio signals and converts to electrical current

Amplifies sine waves matching its resonance frequency

Converts current into sound wave

Extracts modulated signal from carrier frequency

Increases power (amplitude) without changing shape of signal. Modified from How

Stuff Works 2000.