Energy

Radiant Energy Radiant: think light…. How does light carry energy through space???

Waves Light waves –electromagnetic –consist of an electric & magnetic fields oscillating at right angles to each other in direction of motion

WAVE CHARACTERISTICS WavelengthWavelength – distance between crests AmplitudeAmplitude – height of wave from origin to crest or peak –Brightness FrequencyFrequency – how fast a wave oscillates –The number of times a wave completes an up and down cycle –Measured in cycles per second –s -1 = 1/s = hertz = Hz

Speed of Light constant speed in space c = 3.00 x 10 8 m/s For all waves, The shorter the wavelength ( ) of a wave, higher the frequency ( ) c = x = c/

Practice Problem Calculate the wavelength of a radio wave with a frequency of 93.1 x 10 6 s -1 = c/ = 3.00 x 10 8 m/s / 93.1 x 10 6 s -1 = 3.22 m

Electromagnetic Spectrum

We see light in the visible spectrum Each color that we see has a different wavelengths. –400 nm (violet) – 700 nm ( red) Visible light is a very small portion of the electromagnetic spectrum. Much of the spectrum is below and above the Visible portion m – 10 1 m Electromagnetic Spectrum

Beyond red is the infrared portion. This is also called radiant heat –Used in microwaves, radio waves, tv waves Electromagnetic Spectrum

Beyond violet is the ultraviolet radiation (UV) –Responsible for sunburns and skin cancer. –X-rays are even shorter in wavelength Electromagnetic Spectrum

Quantum Theory

Planck’s Theory Max Planck predicted accurately how the spectrum of radiation emitted by an object changes with temperature

Proposed that there is a fundamental restriction on amounts of energy that an object emits or absorbs, called quantum. –Means FIXED AMOUNTS

Related the Frequency of the radiation to the amount of energy. E = h Energy = Planck’s Constant x Frequency Know Planck’s Constant (h) = x J-s (Joule x second)

Energies absorbed or emitted by atoms are quantized, which means that their values are restricted to certain quantities. Energy is not continuous!

Example Imagine a car’s fundamental quantum of energy corresponds to a speed of 10 km/hr. If the car has 7 quanta of energy, it will have a speed of 70 km/hr. If the car has 9 quanta of energy, it will have a speed of 90 km/hr. This shows that a car can only move in multiples of 10 km/hr (in this case). Speeds such as 88 km/hr and 41 km/hr are impossible!

QuantizedContinuous

Planck’s Theory Applied Determining temperatures of distant planets and stars by measuring the wavelengths of the EM radiation that they emit Why can’t we see quantized energy –Planck’s Constant is very small. –Quanta are too small for every day notice, but to a very small atom they make a big difference!

The Photoelectric Effect (Einstein thanks Planck…) electrons ejected from surface of a metal when light shines on it Einstein realized that Planck’s idea of energy quanta could explain this Light consists of quanta of energy that behave like tiny particles, called PHOTONS –Each photon carries an amount of energy given by Planck’s equation.

Red light will not cause electrons to flow in a sheet of sodium metal, no matter how long or bright the source is. –Not enough quanta—cannot escape from the metal… Violet light will cause electrons to flow. Violet light has a greater frequency, and a greater amount of energy per photon

From the previous picture--Notice where the colors are on the EM spectrum and their frequencies (energies)… The frequency of light and the energy of the photon explains the effects of electromagnetic radiation. X-rays contain high amounts of energy because of their high frequency, while radio waves have low frequency and low energy.

Dual Nature of Radiant Energy (particles & waves…) Arthur Compton later demonstrated that photons can collide with electrons. Photons behave like a particle that always travels at the speed of light. –E = mc 2 = h (particle) (wave) Like saying “are books made of words or pages?”