INTRODUCTION. 5. Electromagnetic Waves

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1 INTRODUCTION An electric current produces a magnetic field, and a changing magnetic field produces an electric field Because of such a connection, we refer to the phenomena of electricity and magnetism together as electromagnetism Electric and magnetic fields work together to create travelling waves called electromagnetic waves Such waves are responsible for everything from radio and TV signals, to the visible light we see all around us, to X-rays that reveal our internal structure Electromagnetic waves can be produced by (and detected as) oscillating electric currents (AC) in a wire or similar conducting element An antenna is a device designed to transmit and receive EM waves 1

2 PRODUCTION OF EM WAVES (1) Consider a simple circuit where an AC generator of period T is connected to the centre of an antenna The antenna is basically a long straight wire with a break in the middle At time t = 0, the generator gives the upper part of the antenna a maximum positive charge, and the lower segment a maximum negative charge (a) A positive test charge placed on the x axis at point P experiences a downward force, hence the electric field there is also downwards A short time later, when the charge on the antenna is reduced (due to smaller amplitude of AC waveform), the electric field at P also has a smaller magnitude (b) In (b), the electric field produced at t = 0 has not vanished, nor been replaced, but has moved further from the antenna to Q 2

3 PRODUCTION OF EM WAVES (2) At t = T/4, the antenna is uncharged, and the field vanishes (c) In (d) the charges on the antenna segments change sign, giving rise to an electric field that point upwards The field vanishes again at t = 3T/4 (f), after which the field begins to point downwards once more The net result is a wavelike electric field moving steadily away from the antenna The electric field produced by an antenna connected to an AC generator propagates away from the antenna, analogous to a wave on a string moving away from your hand as you shake it up and down The wave produced by the electric field is only half of the EM wave there is also a magnetic field wave that is produced and are perpendicular to each other 3

4 DETECTION OF EM WAVES Suppose an EM wave moves to the right as shown As the wave continues to move, its electric field exerts a force on electrons in the antenna that is alternately up and down, resulting in an alternating current If the antenna is connected to an LC circuit as shown, the resulting current can be relatively large if the resonant frequency (the frequency of a signal at which the maximum amplitude is produced) of the circuit matches the frequency of the wave This is the basic principle behind radio and television tuners when you turn the tuner, you are actually changing the capacitance or inductance of an LC circuit, and thus changing the resonant frequency Whenever an electric charge is accelerated it will radiate an EM wave The intensity of the radiated EM waves depends on the orientation relative to the viewer 4

5 PROPAGATION OF EM WAVES Sound waves require a medium through which to propagate When the air is pumped out of a glass jar containing a ringing bell, its sound can no longer be heard, but we can still see the bell is ringing! Thus light and other types of EM waves can propagate through a vacuum All EM waves travel through a vacuum with precisely the same speed, c = m/s To quantify, a beam of light could travel around the world 7 times in a single second The speed of light is slightly less in air and in denser materials such as glass or water Danish astronomer Ole Romer ( ) was the first to estimate c When Earth is at its greatest distance from Jupiter, it takes light 16 minutes longer to travel between them 5

6 THE DOPPLER EFFECT The Doppler effect studied for sound waves is also applicable to EM waves, but with two differences First, sound waves require a medium to propagate, whereas light does not Second, the speed of sound can be different for different observers, e.g.. an observer approaching a source of sound measures an increased speed of sound, whereas an observer detecting sound from a moving source measures the usual speed of sound In contrast the speed of EM waves is independent of the motion of the source and observer Thus there is just one Doppler effect for EM waves, which depends only on the relative speed between the observer and source For source speeds u that are small compared to c, the observed frequency f from a source with frequency f is given by: f = f (1± u/c) u is always positive, and the plus sign applies to a source approaching the observer and the minus sign is for a receding source Example: An FM radio station broadcasts at a frequency of 88.5MHz. If you drive your car toward the station at 32.0 m/s, what change in frequency do you observe? 6

7 EXAMPLE: DOPPLER RADAR The Doppler effect for EM waves is used in applications such as the radar units used to measure the speed of cars, and to monitor the weather In Doppler radar, EM waves are sent out into the atmosphere and are reflected back to the receiver The change in frequency in the reflected beam relative to the outgoing beam provides a way of measuring the speed of clouds and precipitation that reflected the beam A typical Doppler weather radar operates at a frequency of 2.7GHz. If a wave transmitted by the weather station reflects from an approaching weather system moving with a speed of 28m/s, find the difference in frequency between the outgoing and returning waves. 7

8 THE EM SPECTRUM When white light passes through a prism is spreads out into a rainbow of colours, with red at one end and violet at the other These various colours of light are all EM waves, and only differ in their frequency and thus their wavelength The relationship between frequency and wavelength for any wave with a speed v is v = fλ For EM waves, c = fλ, where c in a vacuum is constant Thus as the frequency of an EM wave increases, its wavelength decreases Wavelengths are given in units of nanometres (nm) 1nm = 10-9 m, and occasionally the angstrom is used where 1Å = m Example: Find the frequency of red light, with a wavelength of 700.0nm, and violet light with a wavelength of 400.0nm. 8

9 REGIONS OF EM SPECTRUM (1) Radio : f ~ 10 6 Hz to 10 9 Hz, λ = 300m to 0.3m Lowest frequency EM waves of practical importance, and used for TV and radio signals which are produced by alternating currents in metal antennas Radio astronomers use large dish receivers to detect molecules and accelerated electrons in space Microwaves: f ~ 10 9 Hz Hz, λ = 300mm - 0.3mm in this frequency range are used to carry long distance telephone communications, and to cook food Infrared : f ~ Hz Hz, λ = 0.3mm - 700nm IR waves can be felt as heat, can t be seen with eyes Many creatures have specialised infrared receptors that allow them to see the infra red rays given off a warm blooded animal, even in total darkness IR rays are often generated by rotations and vibrations of molecules Thus when IR rays are absorbed by an object, its molecules rotate and vibrate more vigorously, thus raising the object s temperature Remote controls operate on a beam of IR light, but with low intensity, and thus can t be felt as heat 9

10 REGIONS OF EM SPECTRUM (2) Visible Light: f ~ Hz Hz, λ = 700nm 400nm Represented by the full range of rainbow colours Each colour is simply an EM wave with a different frequency Such waves are produced by electrons changing their positions within an atom Ultraviolet Light: f ~ Hz Hz, λ = 400nm to 3nm Such rays are invisible, but can cause suntans with moderate exposure Prolonged exposure can have harmful consequences such as development of skin cancer Most UV radiation from the sun is absorbed in the upper atmosphere by ozone (O 3 ) A significant reduction in the ozone concentration in the stratosphere could result in an unwelcome increase of UV radiation on Earth s surface 10

11 REGIONS OF EM SPECTRUM (3) X-Rays: f ~ Hz to Hz, λ = 3nm to 0.003nm The X-rays used in medicine are generated by the rapid deceleration of high speed electrons projected against a metal target These energetic rays are weakly absorbed by the skin and soft tissue and pass through our bodies quite freely, except when they encounter bones, teeth or other dense material X-rays can cause damage to human tissue, and so it is desirable to reduce exposure as much as possible Gamma (γ) Rays: f > Hz, λ < 0.003nm More energetic than X-rays, produced as neutrons and protons rearrange themselves within the nucleus of an atom, or when particles and antiparticles collide and annihilate each other Highly destructive to living cells, and are used to kill cancer cells and micro organisms in food 11