It seems hard to justify spending \$5.00 for an LED lightbulb when an ordinary incandescent bulb costs 50¢. To see if this makes sense, compare a 60 W incandescent bulb that lasts 1000 hours to a 10 W LED bulb that has a lifetime of 15,000 hours. Both bulbs produce the same amount of visible light. If electricity costs \$0.15/kWh, what is the total cost—purchase price plus energy—to get 15,000 hours of light from each type of bulb? This is called the life-cycle cost.
Ch 28: Fundamentals of Circuits
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook
Chapter 28, Problem 42
An electric eel develops a 450 V potential difference between its head and tail. The eel can stun a fish or other prey by using this potential difference to drive a 0.80 A current pulse for 1.0 ms. What are (a) the energy delivered by this pulse and (b) the total charge that flows?
Verified step by step guidance1
To calculate the energy delivered by the pulse, use the formula for electrical energy: , where is the potential difference (450 V), is the current (0.80 A), and is the time duration of the pulse (1.0 ms or 0.001 s). Substitute the given values into the formula to find the energy.
To calculate the total charge that flows, use the relationship between current, charge, and time: , where is the charge, is the current (0.80 A), and is the time duration of the pulse (1.0 ms or 0.001 s). Substitute the given values into the formula to find the total charge.
Ensure that the time is converted to seconds before substituting into the formulas, as the SI unit for time in these equations is seconds. For example, 1.0 ms = 0.001 s.
After substituting the values into the formulas, perform the multiplication to find the energy delivered and the total charge. Remember to keep track of the units to ensure consistency.
Finally, verify the results by checking the units of the calculated values. The energy should be in joules (J), and the charge should be in coulombs (C). This ensures that the calculations are dimensionally correct.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Ohm's Law
Ohm's Law states that the current (I) flowing through a conductor between two points is directly proportional to the voltage (V) across the two points and inversely proportional to the resistance (R) of the conductor. It is mathematically expressed as V = I × R. This principle is essential for understanding how electric circuits operate and is fundamental in calculating current, voltage, and resistance in various scenarios.
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Resistance and Ohm's Law
Electrical Energy
Electrical energy is the energy derived from electric potential energy or kinetic energy of charged particles. It can be calculated using the formula E = V × Q, where E is energy, V is voltage, and Q is charge. In the context of the electric eel, the energy delivered by the pulse can be determined by multiplying the potential difference by the total charge that flows during the pulse.
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Electric Potential Energy
Charge Flow
Charge flow refers to the movement of electric charge, typically measured in coulombs (C). The total charge (Q) that flows can be calculated using the formula Q = I × t, where I is the current in amperes and t is the time in seconds. In the case of the electric eel, understanding how to calculate the total charge that flows during the pulse is crucial for determining the energy delivered to its prey.
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Flow Continuity
Related Practice
Textbook Question
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What is the time constant for the discharge of the capacitors in FIGURE EX28.34?
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A capacitor is discharged through a 100 Ω resistor. The discharge current decreases to 25% of its initial value in 2.5 ms. What is the value of the capacitor?
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A 2.0-m-long, 1.0-mm-diameter wire has a variable resistivity given by where x is measured from one end of the wire. What is the current if this wire is connected to the terminals of a 9.0 V battery?
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You have a 2.0 Ω resistor, a 3.0 Ω resistor, a 6.0 Ω resistor, and a 6.0 V battery. Draw a diagram of a circuit in which all three resistors are used and the battery delivers 9.0 W of power.
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To which two points in the circuit of FIGURE P28.45 should a 12 V battery be connected to dissipate the most power?
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