# Waves Sending energy at different wavelengths. Outline Waves and Solar Energy Definitions and Descriptions Sources and disturbances Velocity and the medium.

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Waves Sending energy at different wavelengths

Outline Waves and Solar Energy Definitions and Descriptions Sources and disturbances Velocity and the medium Frequency and Wavelength

Solar Energy What is Solar Energy? Nuclear fusion in the Sun’s core. Heating up the core to 40,000 K, and the surface to 6,000 K. How does Solar Energy get here? Conduction? – heat transfer through contact Convection? – through transport of matter Radiation! – through the emission of electromagnetic waves EM waves can transport energy through a vacuum and over billions of miles!

Waves -- A disturbance in a medium which carries energy but not matter. Disturbance – can be a single pulse or a repeated (periodic) vibration. Medium – Something through which waves can travel (water, air, strings, etc.) Transverse waves – the disturbance of the medium is perpendicular to the motion of the wave. Longitudinal waves – motion of the medium is in the direction of the motion of the wave.

Characteristics of Waves A = amplitude is the displacement of the medium from equilibrium. λ = wavelength is the distance along the direction of motion between repeating points on the wave. T = period is the time it takes for the wave to repeat itself. v = velocity is the rate at which the wave is moving through the medium. A λ velocity

Wave velocity and the medium The velocity of a regular or periodic wave is just v = λ / T Example: λ = 2 m and T = 4 seconds. v = 2m / 4s =.5 m/s. The velocity of waves is primarily a function of the medium. v sound in air = 340 m/s v sound in water = 1500 m/s v surface water waves = 2 m/s v light = 3 x 10 8 m/s

Wave Speed: a comparison Certain events produce both light and sound simultaneously. We detect them at different times because of the different wave speeds in the media. You’re a distance of 1 mile (1.6km) away. v = d / t --- so, t = d / v. t sound = 1600 m ÷ 340 m/s = 5 sec t light = 1600 m ÷ 3 x 10 8 m/s = 5 x 10 -6 s

Wave frequency Frequency, f, is the inverse of the period, T. f = 1 / T. Period is the time it takes for a wave to repeat itself – frequency is the number of times a wave repeats itself in one second Measured in Hertz: 1 Hz = 1 cycle per sec. Now, v = λ / T = λ f

v = λf; λ = v/f; f = v/λ Example 1: AM radio 1000 has f =1000 kHz or 1 MHz. Since v = 3 x 10 8 m/s, λ = 3 x 10 8 m/s ÷ 1 x 10 6 Hz = 300 m! Example 2: The highest frequency we can hear is 20,000 Hz. λ = 340 m/s ÷ 20,000 Hz =.017 m = 1.7 cm

The Audible Spectrum C above

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