Radios and radiowaves Physics 1010: Dr. Eleanor Hodby Day 26: Radio waves Reminders: HW10 due Monday Nov 30th at 10pm. Regular help session schedule this week Final: Monday Dec 14 at 1.30-4pm
Midterm 1 topics - Motion Position, velocity and acceleration Definitions, Units Scalars and vectors Graphs of x, v, a vs time and relationships between graphs Equations of motion and how to use them: Constant velocity: x = x 0 + vt Constant acceleration: v = v 0 +at x = x 0 + v 0 t + ½ at 2 Forces Definition, units, vector F gravity = mg downwards F friction 0.3 weight in direction opposing motion F spring = -kx in direction opposing extension/compression F net = ma If a = 0, F net = 0 Free body diagrams and finding F net
Conservation of energy Midterm 2 summary W ext - W friction = DPE + DKE - Work done by a force = F d // - Looked at work done by external forces and by friction - GPE = mgh, KE = ½ mv 2, PPE = PV, SPE= ½ kx 2, Thermal energy = constant T - Ramps, roller coasters, balls. - Power = energy/s Bernoulli s equation E tpv = P + ½ rv 2 + rgh - Conservation of energy for an incompressible fluid Sound - Wave basics: f, A, T, l, v (and relationships) - How to get different notes from a violin - Harmonics on a violin string
Review Topics midterm 3 Blackbody spectrum - Introduction to EM waves and the EM spectrum - Kelvin temperature scale - Stefan-Boltzman law - Shape of BB spectrum at different temperatures why the sun produces visible light efficiently and incandescent lightbulbs don t. - Green house effect Static electricity - Coulomb s law for force between point charges - Voltage and electric potential energy (EPE) Electric circuits - Ohm s Law - Power dissipation law - Batteries in series - Bulbs in series and Parallel
Radio waves so far Electric charges surrounded by an electric field F = qe EM waves created by accelerating charges The oscillation frequency of the charge matches the frequency of the EM wave produced Radiowave Summary History of radio waves Creating and receiving a radio wave (and other EM waves) Electric fields and forces on charged particles Optimizing transmission/reception of a radio wave - Polarization - Power - Antenna length Tuning your radio: Tank circuits Carrying information with a radio wave - Modulation schemes (AM and FM) - Bandwidth Dangers of radio waves?
Creating radiowaves A 60 m The blue circles are electrons. What s the direction of the force on the test electron at A? a. b. c. d. e. What is the direction of the electric field at A? a. b. c. d. e.
Creating radiowaves Now the electron on the left moves suddenly down: 60 m A What s the direction of the force on the electron at A.? a. b. c. d. e. can t tell e. Can t tell unless I know when I measure the force at A. It takes time for the field (and hence force) at point A to know electron moved! This information travels the speed of light. It is an electromagnetic wave
Understanding radiowaves - several meters + c = 3 10 8 m/s - When the electron in the antenna is oscillating, the electric field at a distance is also constantly changing in time. The value of the field (and hence force on a charged particle) outside the antenna at any instant depends on a) Space - How far it is from the antenna b) Time - At what point in its oscillation the antenna electron is at
Understanding radiowaves - several meters + A c = 3 10 8 m/s - This picture shows the electric field of a radio wave at a certain instant in time. A very short time later, the strength of the field at A will be a. More downward, b. The same, c. Weaker in magnitude (down but smaller), d. Zero, e. Large upward
Understanding radiowaves - several meters + A B c = 3 10 8 m/s - This picture shows the electric field of a radio wave at a certain instant in time. What is the force on an electron at B at this instant? a. Zero b. Upwards but small c. Upwards and large d. Downwards and small e. Downwards and large
Understanding radiowaves - several meters + A B c = 3 10 8 m/s - This picture shows the electric field of a radio wave at a certain instant in time. What will the force be on the electron at B a fraction of a second later? a. Zero b. Down c. Up
Understanding radiowaves - several meters + A B c = 3 10 8 m/s - If the wavelength of the EM wave is 200m, at what frequency is the electron oscillating in the transmitting antenna? a. 3 10 8 Hz b. 1.5 10 8 Hz c. 1.5 10 6 Hz d. 6 10 10 Hz e. 1.7 10-11 Hz
Understanding radiowaves - several meters + A B c = 3 10 8 m/s - If the wavelength of the EM wave is 200m, at what frequency does the force on the electron at B oscillate? a. 3 10 8 Hz b. 1.5 10 8 Hz c. 1.5 10 6 Hz d. 6 10 10 Hz e. 1.7 10-11 Hz
So how do we receive a radio wave? What if we stick another metal spike (antenna) in the path of the wave. Oscillating E field in the wave will exert an oscillating force on free electrons in the antenna. They will be pushed up and down the antenna en mass Moving electrons are an electric current! We can detect this oscillating current and hence detect the incoming radio wave! Can be used to drive a loudspeaker Phet broadcast antenna c = 3 10 8 m/s
Receiving radio waves If the frequency of the transmitting antenna is increased from 530 Hz to 1060 Hz so that the electrons oscillate up and down transmitting antenna more times per second, the electrons in the receiving antenna will: a. be unaffected by the change and continue to oscillate up and down at the same frequency as before the change b. oscillate up and down at a higher frequency than before the change. c. oscillate up and down in a full cycle 1060 times per second. d. oscillate up and down at a lower frequency than before the change. e. b and c.
Understanding radiowaves What do the green arrows represent? a. The velocity of the electrons that are at each of those points, moving due to the electromagnetic wave. b. The positions of evenly spaced electrons moving up and down between the two antennae. c. The strength and direction of the electric field that is emitted by the antenna d. The force resulting from electrons moving off of the transmitting antenna towards the receiving antenna following the curved path.
Understanding radiowaves Where do these fields (and forces) come from again? a) Charged particles are surrounded by an electric field b) When an electron in the transmitting antenna is oscillating up and down its surrounding field is constantly changing c) It takes time for these changes to radiate outwards creating an EM wave that changes in time and space d) EM wave travels outwards at speed of light (c). e) E field in EM wave exerts oscillating force on electron in receiving antenna
Understanding radiowaves a b The speed of the wave (signal) is measured as a. how fast the peak moves towards antenna. b. how fast the peak moves up and down. c. both a and b
- Power - Polarization - Antenna length Optimizing transmission and reception Set up radio transmitter. Hook a flashlight bulb between two halves of receiving antenna (wires) The bulb will a. light up if the signal strong enough, b. not light up because there is no current through it, c. not light up because the current oscillates up and down so fast. Transmitter
Transmitting radio waves: Power A B Will the light bulb be: a. Brighter at A than at B b. Just as bright at A and B c. Dimmer at A than at B d. no way to tell
Transmitting radio waves: Power r A B Power in EM wave spread over surface of sphere, area 4 r 2. Power per area = P 0 /4 r 2 So signal gets weaker as 1/(distance from transmitter) 2
Cell phones emit high frequency radio wave/microwaves (1000 MHz) to communicate with the cell phone tower When I hold my cell phone to my ear, it is 1cm away from my head. When I put it in my pocket (and use an earpiece on a wire) it is 0.5m from my head. By what factor do I reduce the EM power near my head when I use the earpiece? a. 0.2 b. 0.04 c. 0.002 d. 0.0004 e. None of the above
Antenna orientation and polarization Consider a vertical broadcast antenna and a receiving antenna which we can orientate either parallel to broadcast antenna (vertically) or perpendicular to broadcast antenna (horizontally). How does the signal strength in the 2 receiving antennae compare: a. parallel to broadcast antenna has stronger signal than perpendicular b. parallel to broadcast antenna has same signal strength as perpendicular c. parallel to broadcast antenna has weaker signal than perpendicular do experiment to check
Optimal transmitting antenna size We find electrons oscillate up and down a transmitting antenna at a frequency of 890 khz. What is the wavelength of the radiowave? (Speed of light = 3 x 10 8 m/s) a. 330,000 m b. 337 m c. 2670 m d. 3.0 m e. 522 m
Antenna size and wavelength Optimum antenna length for broadcast is (l/4) = 84 m. AM broadcast antennas are tall! This is similar to the violin string, where the note (frequency) you produce depends on the length of the string.
Amplitude question If we increase amplitude of motion in transmitter, the wave will get to the receiver a) sooner than the small amplitude wave, b) at the same time, c) Later than the small amplitude wave If we increase the amplitude of motion in the transmitter, then: a) The electrons in the receiver move up and down with higher frequency. b) The force that the radiowave exerts on receiving electron will increase, c) The receiver electrons will move up and down with lower frequency, d) a and b e) b and c
Radio frequencies and channels Each radio station broadcasts at a particular carrier frequency. AM stations 530 to 1600 khz FM stations 88 to 108 MHz
Radio frequencies and channels Each radio station broadcasts at a particular carrier frequency. AM stations 530 to 1600 khz FM stations 88 to 108 MHz
Radio frequencies and channels 1490 AM Each radio station broadcasts at a particular carrier frequency. AM stations 530 to 1600 khz FM stations 88 to 108 MHz
Current in tank circuit Tuning your radio What are you doing when you tune your radio? You are getting it to selectively detect the broadcast frequency of your favorite channel. Inside the radio is a resonant circuit often called a tank circuit A circuit that is designed to respond wildly to radio waves at a specific frequency and ignore others Frequency of incoming radio wave Other resonant systems: Only respond to a specific driving frequency
Tuning your radio What are you doing when you tune your radio? You are getting it to selectively detect the broadcast frequency of your favorite channel. Inside the radio is a resonant circuit often called a tank circuit A circuit that is designed to respond wildly to radio waves at a specific frequency and ignore others receiving antenna I No tank circuit, Electrons go through once and gone. Same small response for all frequencies. R V radio electronics, converts V to sound
Current in tank circuit Tuning your radio: Tank circuits The weak incoming radio wave gently pushes on electrons in the antenna and attached resonant circuit at a regular frequency f w If f w matches the resonant frequency of the tank circuit (f 0 ), a large oscillating current builds up. This current produces a LARGE oscillating voltage across the capacitor If f w f 0, then no current builds up. The tank circuit selectively strengthens signals at a specific frequency, f 0, that are fed to radio electronics The tuning knob changes the electronic components which determine f 0 Inductor I Capacitor V f 0 Frequency of incoming radio wave Big oscillating current radio electronics, converts V to sound. Same approach used in reverse on broadcast to get big current in antenna at particular frequency.
Tuning your radio This system of transmitting information at high frequency between resonant circuits has 2 advantages a) Multiple radio stations can operate simultaneously at different frequencies The receiving circuit responds to just one of the incoming frequencies b) Random electric fields in the environment do not affect the signal because they do not push resonantly on the tank circuit. Other resonant systems You find resonant systems all over physics: Like pushing a child on a swing, or driving oscillations in this rope. Motor Variable mass Motor pushes gently on rope at a fixed frequency f M Resonant frequency of rope (f 0 ) can be tuned by adjusting tension (f 0 = (T/m) 1/2 /2L) f M = f 0 big response/oscillation f M f 0 no response/oscillation
How do we send the sound of a voice over the radio? Imagine we send out a steady radio wave at 100 MHz. E 3m Distance Is this wave carrying information about the human voice, song etc? a) Yes b) No c) Might be
Signal Carrying information on a radio wave. Radio wave High frequency 1 E8 oscillations/sec Time Sound wave Low frequency 1 E3 oscillations/second S Sound information (should the loudspeaker move forward or backward at this instant) can be carried as a slow modulation on the amplitude or frequency of the radio wave. e.g. Increase in amplitude/frequency: Move speaker forward Decrease in amplitude/frequency: Move the speaker backward. Note: Changes in frequency are SMALL. Modulated wave is still detected by the resonant circuit in the receiver
. Carrying information on a radio wave Signals that make a radiowave plotted as a function of distance
Bandwidth To carry sound information, radio stations are not transmitting at an exact single frequency - They are transmitting over a narrow range of frequencies The frequency range of 2 different stations must not overlap, otherwise your radio will output sounds from both at once (like when the tuner is between 2 channels). Each station is allocated a BANDWIDTH: a range of frequencies centered on the carrier frequency that they can use. AM bandwidth: 10 khz FM bandwidth: 200 khz To transmit a 4kHz note requires 8 khz of bandwidth: 4kHz above and 4kHz below the carrier frequency. Can an AM station transmit all of the audible range of sounds/frequencies? a. Yes b. No c. Maybe