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Unformatted text preview: On the Brightness of Bulbs
Resistance Blackbody Radiation Ohm's Law UCSD: Physics 8; 2 Review: What makes a bulb light up? The critical ingredient is closing a circuit so that current is forced through the bulb filament more on filaments and what is physically going on later The more the current, the brighter the bulb The higher the voltage, the brighter the bulb Power "expended" is P = VI this is energy transfer from chemical potential energy in the bulb to radiant energy at the bulb UCSD: Physics 8; 2 Bulb Design Basics
Tungsten Filament Sealed Bulb Electrical contacts
120 W bulb at 120 V must be conducting 1 Amp (P = VI) Bulb resistance is then about 120 Ohms (V = IR) UCSD: Physics 8; 2 What makes the bulb light up? Bulb contains a very thin wire (filament), through which current flows The filament presents resistance to the current electrons bang into things and produce heat a lot like friction Filament gets hot, and consequently emits light gets "red hot" UCSD: Physics 8; 2 Everything is Aglow All objects emit "light" Though almost all the light we see is reflected light The color and intensity of the emitted radiation depend on the object's temperature Not surprisingly, our eyes are optimized for detection of light emitted by the sun, as early humans saw most things via reflected sunlight no light bulbs, TVs We now make some artificial light sources, and ideally they would have same character as sunlight better match to our visual hardware (eyes) UCSD: Physics 8; 2 Color Temperature Object You Heat Lamp Candle Flame Bulb Filament Sun's Surface Temperature ~ 30 C 300 K ~ 500 C 770 K ~ 1700 C 2000 K ~ 2500 C 2800 K ~ 5500 C 5800 K Color Infrared (invisible) Dull red Dim orange Yellow Brilliant white The hotter it gets, the "bluer" the emitted light The hotter it gets, the more intense the radiation UCSD: Physics 8; 2 The "Blackbody" Spectrum UCSD: Physics 8; 2 Blackbody spectra on logarithmic scale Sun peaks in visible band (0.5 microns), light bulbs at 1 m, we at 10 m. (note: 0C = 273K; 300K = 27C = 81F) UCSD: Physics 8; 2 Bulbs aren't black! Blackbody??!! Black in this context just means reflected light isn't important Hot charcoal in a BBQ grill may glow bright orange when hot, even though they're black Sure, not everything is truly black, but at thermal infrared wavelengths (250 microns), you'd be surprised your skin is 90% black (absorbing) even white paint is practically black metals are still shiny, though This property is called emissivity: radiated power law modified to P = AT4, where is a dimensionless number between 0 (perfectly shiny) and 1.0 (perfectly black) , recall, is 5.67 10-8 in MKS units, T in Kelvin Why do we use aluminum foil? UCSD: Physics 8; 2 What Limits a Bulb's Lifetime Heated tungsten filament drives off tungsten atoms heat is, after all, vibration of atoms: violent vibration can eject atoms occasionally Tradeoff between filament temperature and lifetime Brighter/whiter means hotter, but this means more vigorous vibration and more ejected atoms "Halogen" bulbs scavenge this and redeposit it on the filament so can burn hotter Eventually the filament burns out, and current no longer flows no more light! How "efficient" do you think incandescent bulbs are? Ratio between energy doing what you want vs. energy supplied Efficiency = (energy emitted as visible light)/(total supplied) UCSD: Physics 8; 2 Predicting Brightness in Bulb Networks This is a very instructive (and visual) way to learn about the behavior of electronics, how current flows, etc. The main concept is Ohm's Law: V = IR voltage = current resistance We've already seen voltage and current before, but what's this R? R stands for resistance: an element that impedes the flow of current measured in Ohms () Remember the bumper-cars nature of a bulb filament? Electrons bounce off of lattice atoms this constitutes a resistance to the flow of current UCSD: Physics 8; 2 Interpretation of Ohm's Law The best way to think about Ohm's law is: when I have a current, I, running through a resistance, R, there will be a voltage drop across this: V = IR "voltage drop" means change in voltage Alternative interpretations: when I put a voltage, V, across a resistor, R, a current will flow through the resistor of magnitude: I = V/R if I see a current, I, flow across a resistor when I put a voltage, V, across it, the value of the resistance is R = V/I Ohm's Law is key to understanding how current decides to split up at junctions try to develop a qualitative understanding as well as quantitative UCSD: Physics 8; 2 Bulbs in Series Each (identical) light bulb presents a "resistance" to the circulating electrical current Adding more bulbs in series adds resistance to the current, so less current flows
_ + Which bulb is brighter? WHY? A B UCSD: Physics 8; 2 Answer There is only one current flowing, and it goes through both bulbs. They will therefore shine with equal brightness. Imagine exchanging bulbs. Does this change anything? + _ Bulbstravaganza
Exploration of Circuits & Ohm's Law UCSD: Physics 8; 2 Reminder: Ohm's Law There is a simple relationship between voltage, current and resistance: V is in Volts (V) I is in Amperes, or amps (A) R is in Ohms () V=IR
Ohm's Law V I R UCSD: Physics 8; 2 Numerical examples of Ohm's Law (V = IR) How much voltage is being supplied to a circuit that contains a 1 Ohm resistance, if the current that flows is 1.5 Amperes? If a 12 Volt car battery is powering headlights that draw 2.0 Amps of current, what is the effective resistance in the circuit? UCSD: Physics 8; 2 Answer #1:
(How much voltage is being supplied to a circuit that contains a 1 Ohm resistance, if the current that flows is 1.5 Amperes?) Use the relationship between Voltage, Current and Resistance, V = IR. Total resistance is 1 Ohm Current is 1.5 Amps So V = IR = (1.5 Amps)(1 Ohms) = 1.5 Volts UCSD: Physics 8; 2 Answer #2
(If a 12 Volt car battery is powering headlights that draw 2.0 Amps of current, what is the effective resistance in the circuit?) Again need V = IR Know I, V, need R Rearrange equation: R = V/I = (12 Volts)/(2.0 Amps) = 6 Ohms UCSD: Physics 8; 2 Conductors are at Constant Voltage Conductors in circuits are idealized as zeroresistance pieces so V = IR means V = 0 (if R = 0) Can assign a voltage for each segment of conductor in a circuit 1.5 V 1.5 V 3.0 V 0V
batteries in parallel add energy, but not voltage 0V
batteries in series add voltage UCSD: Physics 8; 2 Multi-bulb circuits
Rank the expected brightness of the bulbs in the circuits shown, e.g. A>B, C=D, etc. WHY?! _ + _ + A B C UCSD: Physics 8; 2 Answer: Bulbs B and C have the same brightness, since the same current is flowing through them both. Bulb A is brighter than B and C are, since there is less total resistance in the single-bulb loop, so A > B=C. UCSD: Physics 8; 2 Adding Bulbs Where should we add bulb C in order to get A to shine more brightly? _ + P A C R B Q Answer UCSD: Physics 8; 2 The only way to get bulb A to shine more brightly is to increase the current flowing through A. The only way to increase the current flowing through A is to decrease the total resistance in the circuit loop Since bulbs in parallel produce more paths for the current to take, the best (and only) choice is to put C in parallel with B:
_ + A B C UCSD: Physics 8; 2 A more complex example!
A B + _ C D E F Predict the relative brightness of the bulbs UCSD: Physics 8; 2 Answer The entire current goes through bulb F so it's going to be the brightest The current splits into 3 branches at C,D,E and they each get 1/3 of the current The current splits into 2 branches at A,B and they each get half the current, so F>A=B>C=D=E If I disconnect bulb B, does F get brighter or fainter? UCSD: Physics 8; 2 A B + _ C D E F UCSD: Physics 8; 2 Answer By disconnecting B, the resistance of the (AB) combination goes up, so the overall current will be reduced. If the current is reduced, then F will be less bright. UCSD: Physics 8; 2 Power Dissipation How much power does a bulb (or resistor) give off? P = VI but V = IR so P = I2R and P = V2/R are both also valid Bottom line: for a fixed resistance, power dissipated is dramatic function of either current OR voltage UCSD: Physics 8; 2 How about multiple resistances? Resistances in series simply add Voltage across each one is V = IR
R1=10 V = 3.0 Volts R2=20 Total resistance is 10 + 20 = 30 So current that flows must be I = V/R = 3.0 V / 30 = 0.1 A What are the Voltages across R1 and R2? UCSD: Physics 8; 2 Parallel resistances are a little trickier.... Rule for resistances in parallel: 1/Rtot = 1/R1 + 1/R2 10 Ohms 10 Ohms 5 Ohms Can arrive at this by applying Ohm's Law to find equal current in each leg. To get twice the current of a single10 , could use 5 . UCSD: Physics 8; 2 A Tougher Example What is the voltage drop across the 3 resistors in this circuit? R2=20 R1=5 V = 3.0 Volts R3=20 UCSD: Physics 8; 2 Answer First, need to figure out the current that flows in the circuit. This depends on the total resistance in the loop. Combine the parallel resistors into an equivalent single series resistor, the parallel pair are equal to a single resistor of 10 Ohms The total resistance in the loop is 5 + 10 = 15 Ohms So the total current is I = V/R = 3/15 = 0.20 Amps Voltage across R1 is V = IR = 0.2A 5 Ohms = 1 Volt Voltage across R2, R3 is equal, V = IR = 0.2A 10 = 2 V Note that the sum of the voltage drops equals battery voltage! R2=20 R1=5 V = 3.0 Volts R3=20 UCSD: Physics 8; 2 Complex Example Say battery is 5.5 Volts, and each bulb is 6 AB combo is 3 CDE combo is 2 total resistance is 11 current through battery is 5.5V/11 = 0.5 A A gets 0.25 A, so V = 1.5V C gets 0.1667 A, so V = 1.0 V E F gets 0.5 A, so V = 3.0 V note voltage drops add to 5.5 V Use V2/R or I2R to find: PAB = 0.375 W each PCDE = 0.167 W each PF = 1.5 W A B + _ C D F UCSD: Physics 8; 2 Assignments Read pp. 224231, 332333, 407 for this lecture HW #3: Chapter 10: E.2, E.10, E.32, P.2, P.13, P.14, P.15, P.18, P.19, P.23, P.24, P.25, P.27, P.28, P.30, P.32 Next Q/O (#2) due next Friday: only submit one this week if you missed it last week. ...
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