MCAT-Style Passage Problems
Lightbulb Failure
You’ve probably observed that the most common time for an incandescent lightbulb to fail is the moment when it is turned on. Let’s look at the properties of the bulb’s filament to see why this happens.
The current in the tungsten filament of a lightbulb heats the filament until it glows. The filament is so hot that some of the atoms on its surface fly off and end up sticking on a cooler part of the bulb. Thus the filament gets progressively thinner as the bulb ages. There will certainly be one spot on the filament that is a bit thinner than elsewhere. This thin segment will have a higher resistance than the surrounding filament. More power will be dissipated at this spot, so it won’t only be a thin spot, it also will be a hot spot.
Now, let’s look at the resistance of the filament. The graph in Figure P22.70 shows data for the current in a lightbulb as a function of the potential difference across it. The graph is not linear, so the filament is not an ohmic material with a constant resistance, However we can define the resistance at any particular potential difference ∆V to be R = ∆V/I. This ratio, and hence the resistance, increases with ∆V and thus with temperature.
Figure P22. 70
When the bulb is turned on, the filament is cold and its resistance is much lower than during normal, high-temperature operation. The low resistance causes a surge of higher-than-normal current lasting a fraction of a second until the filament heats up. Because power dissipation is I2R, the power dissipated during this first fraction of a second is much larger than the bulb’s rated power. This current surge concentrates the power dissipation at the high-resistance thin spot, perhaps melting it and breaking the filament.
71. As the bulb ages, the resistance of the filament
A. Increases.
B. Decreases.
C. Stays the same.
Want to see the full answer?
Check out a sample textbook solutionChapter 22 Solutions
College Physics: A Strategic Approach Technology Update, Books a la Carte Plus Mastering Physics with Pearson eText -- Access Card Package (3rd Edition)
Additional Science Textbook Solutions
Lecture- Tutorials for Introductory Astronomy
College Physics
University Physics with Modern Physics (14th Edition)
University Physics Volume 1
Conceptual Physics (12th Edition)
An Introduction to Thermal Physics
- Four resistors are connected to a battery as shown in Figure P21.40. The current in the battery is I, the battery emf is , and the resistor values are R1 = R, R2 = 2R, R3 = 4R, and R4 = 3R. (a) Rank the resistors according to the potential difference across them, from largest to smallest. Note any cases of equal potential differences. (b) Determine the potential difference across each resistor in terms of . (c) Rank the resistors according to the current in them, from largest to smallest. Note any cases of equal currents. (d) Determine the current in each resistor in terms of I. (e) If R3 is increased, what happens to the current in each of the resistors? (f) In the limit that R3 , what are the new values of the current in each resistor in terms of I, the original current in the battery? Figure P21.40arrow_forwardRalph has three resistors, R1, R2, and R3, connected in series. When connected to an ideal emf source E1, current I1 flows through the resistors. a. If the resistors are instead connected to a second source with E2=2E1, what is the new current through the resistors in terms of the first current? b. Show that, if each resistance is doubled and the resistors are connected in series to the second emf source, the current through the resistors is equal to I1.arrow_forwardA 20.00-V battery is used to supply current to a 10-k resistor. Assume the voltage drop across any wires used for connections is negligible, (a) What is the current through the resistor? (b) What is the power dissipated by the resistor? (c) What is the power input from the battery; assuming all the electrical power is dissipated by the resistor? (d) What happens to the energy dissipated by the resistor?arrow_forward
- Three 100- resistors are connected as shown in Figure P21.41 The maximum power that can safely be delivered to any one resistor is 25.0 W. (a) What is the maximum potential difference that can be applied to the terminals a and b? (b) For the voltage determined in part (a), what is the power delivered to each resistor? (c) What is the total power delivered to the combination of resistors?arrow_forwardFigure 6 below shows an RC- circuit where the capacitor Is initially uncharged. 100n 502 12 V 1502 10 HF V. Figure 6 a) What is the current through the battery immediately after the switch S is closed? b) What is the current through the battery long time after the switch S is closed?arrow_forwardProblem 7: Why do lights dim when a large appliance is switched on? The answer is that the large current the appliance motor draws causes a significant potential drop in the wires and reduces the voltage across the lights. This is called an IR drop because it is simply due to the current and resistance of the device in question. Refrigerator Low P, Bulb dims R,= wire resistance Low R, large /, Motor Part (a) Assume the bulb in question dissipates P2 = 75 W when connected to the 120 V AC source. What average power will the bulb dissipate if I = 13.5 A passes through the wires when the motor comes on? Assume a negligible change in bulb resistance, while the resistance of the wires are R1 = 0.55 Q. Numeric : A numeric value is expected and not an expression. P2' = Part (b) How much power, P3, in watts, is consumed on average by the motor? Numeric : A numeric value is expected and not an expression. P3 =arrow_forward
- When a R = 92.4 Q resistor is connected across a 12.4 V battery, a current of 123.5 mA flows. What is the internal resistance of the battery and Terminal voltage of the battery? jon R (a) internal resistance of the battery(in ) = (b) Terminal voltage of the battery in V = My courses / PHYS1100 / Homework No.2/ Homework No.2 How many 19OW lightbulbs can you use in a 180-V circuit without tripping a are connected in parallel, which means that the potential difference across each lightbulb is 180V. Also calculate the resistance of the 190W lightbulb. circuit breaker? (The bulbs (a) No. of 190-W lightbulbs we can use without tripping a 8-A circuit breaker= (b) Resistance of 190W lightbulb in ohm =arrow_forwardQU5 a). A circuit consist of five resistors connected as below, determine the effective resistance in the circuit. R3 R5 R2 R4 Given that R1, R2,R3,R4 and R are given respectively as 500, 250, 300, 600 and 700Q, what is the current in the circuit if the potential across the resistors is 24 V?arrow_forwardimagearrow_forward
- A 10 Ω resistor and a 20 Ω resistor are connected in parallel with a power supply. If the power dissipated by the 10 Ω resistor is 100 W, the power dissipated by 20 Ω resistor is Blank 1 Watts. what is the wattsarrow_forwardIn the circuit, R = 17 kQ2, C = 0.3 μF, and V = 45 V. The capacitor is allowed to charge fully and then the switch is changed from position a to position b. a) What is the time constant? b) What is the current in the resistor 7.7 ms later? c) What is the voltage across the resistor 7.7 ms later? V a b R www To continue, please enter the result of c) in units of V. Round your answer to 1 decimal place.arrow_forwardConsider a resistor that is 5.0 mm in length and is part of a larger circuit you have previously measured that there is exactly 2.4 milliamps of conventional current (positive charges are moving) flowing through the resistor. There are colored bands on the resistor in the following order: BLUE-VIOLET-RED-(GAP)-SILVER. (We won't need the uncertainty/tolerance information, so you can ignore that.) What is the resistance of the resistor in ohms? You can ignore the tolerance/uncertainty part. I just want the resistance. Someone comes along and arbitrarily decides that the terminal of the resistor where current is flowing INTO has an electric potential value of 8.28 volts. (Since potential is completely arbitrary, there's nothing wrong with this choice.) Using this choice, what is the value of electric potential at the end of the resistor where current is flowing OUT? (Hint: The answer is not 16.08 volts.) What is the magnitude of the electric field inside the resistor, assuming it is…arrow_forward
- Principles of Physics: A Calculus-Based TextPhysicsISBN:9781133104261Author:Raymond A. Serway, John W. JewettPublisher:Cengage LearningPhysics for Scientists and Engineers: Foundations...PhysicsISBN:9781133939146Author:Katz, Debora M.Publisher:Cengage Learning
- Physics for Scientists and Engineers, Technology ...PhysicsISBN:9781305116399Author:Raymond A. Serway, John W. JewettPublisher:Cengage Learning