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A flashing lamp in a Christmas earring is based on an \(R C\) discharge of a capacitor through its resistance. The effective duration of the flash is \(0.250 \mathrm{~s}\), during which it produces an average \(0.500 \mathrm{~W}\) from an average \(3.00 \mathrm{~V}\). (a) What energy does it dissipate? (b) How much charge moves through the lamp? (c) Find the capacitance. (d) What is the resistance of the lamp?

Short Answer

Expert verified
Energy dissipated is 0.125 J, charge is 0.0417 C, capacitance is 13.9 mF, and resistance is 18.0 Ω.

Step by step solution

01

Calculate Energy Dissipated

The energy dissipated by the lamp during the flash can be calculated using the formula for power: \[ E = P \times t \]where \( E \) is the energy, \( P \) is the power (0.500 W), and \( t \) is the time duration of the flash (0.250 s). Substitute these values to find the energy:\[ E = 0.500 \, W \times 0.250 \, s = 0.125 \, J \]
02

Calculate Charge Moved Through Lamp

The charge that moves through the lamp can be calculated using the relationship between energy, charge, and voltage:\[ E = Q \times V \]Rearranging to solve for charge \( Q \):\[ Q = \frac{E}{V} \]Substitute the values for energy \( E = 0.125 \, J \) and voltage \( V = 3.00 \, V \):\[ Q = \frac{0.125 \, J}{3.00 \, V} = 0.0417 \, C \]
03

Calculate Capacitance

Capacitance \( C \) can be found using the relation between charge and voltage:\[ C = \frac{Q}{V} \]Substitute the charge \( Q = 0.0417 \, C \) and voltage \( V = 3.00 \, V \):\[ C = \frac{0.0417 \, C}{3.00 \, V} = 0.0139 \, F = 13.9 \, mF \]
04

Determine Resistance of the Lamp

Resistance \( R \) in this RC discharge circuit can be determined using the formula for the time constant \( \tau \), where \( \tau = R \times C \). Since the lamp effectively completes discharging in 0.250 s, assume \( \tau \approx 0.250 \) for simplicity:\[ R = \frac{\tau}{C} \]Substitute\( \tau = 0.250 \, s \) and \( C = 0.0139 \, F \):\[ R = \frac{0.250 \, s}{0.0139 \, F} = 17.99 \, \Omega \]

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Energy Dissipation
In an RC circuit, energy dissipation refers to the energy lost, often in the form of heat, when current flows through a resistor. Understanding this phenomenon helps us grasp how electrical devices convert electrical energy into other forms of energy, like light and heat. For our specific problem, the energy dissipation of the flashing lamp is determined by using the formula for power:
  • Energy (\(E\)) is equal to power (\(P\)) multiplied by time (\(t\)).
So, if the average power is 0.500 W and the duration of the flash is 0.250 s, the energy dissipated is \(E = 0.500 \, \text{W} \times 0.250 \, \text{s} = 0.125 \, \text{J}\).
This tells us that 0.125 Joules of energy is lost, mainly as heat, during each flash of the earring. Understanding energy dissipation is crucial as it helps in designing circuits with appropriate resistive elements that can withstand the heat generated in the process.
Charge Calculation
Calculating the amount of charge that moves through a circuit component is key for gauging how much current is flowing over time. We use this to understand the behavior of electric charges in an RC circuit. You can find the charge (\(Q\)) by understanding its relationship with energy and voltage:
  • The energy (\(E\)) is the product of charge (\(Q\)) and voltage (\(V\)).
Rearranging gives:\(Q = \frac{E}{V}\).
We know the energy dissipated is 0.125 J, and the applied voltage is 3.00 V, leading to:\(Q = \frac{0.125 \, \text{J}}{3.00 \, \text{V}} = 0.0417 \, \text{C}\).
This means 0.0417 Coulombs of charge flows through the lamp each time it flashes. This calculation is vital in ensuring that the circuit is capable of handling the charge flow without damaging any components.
Capacitance Calculation
Capacitance is a measure of how much electric charge a capacitor can store at a given voltage. It's a key parameter in RC circuits, impacting how quickly circuits can charge or discharge. To find the capacitance (\(C\)) of a circuit, we use the relationship:
  • Capacitance (\(C\)) equals charge (\(Q\)) divided by voltage (\(V\)).
Substituting the known values gives:\(C = \frac{0.0417 \, \text{C}}{3.00 \, \text{V}} = 0.0139 \, \text{F}\).
This is equivalent to 13.9 milliFarads (mF), indicating the capacity of the earring lamp to store electrical energy. Understanding the capacitance helps in designing circuits with appropriate storage capabilities, ensuring efficient and effective energy use.
Resistance Calculation
Resistance quantifies how much a component resists the flow of electric current, playing a critical role in determining the speed of current flow in the circuit. The resistance (\(R\)) in an RC circuit can be inferred from the time constant (\(\tau\)), which describes how fast the circuit responds to changes. The time constant is given by:
  • The time constant (\(\tau\)) equals resistance (\(R\)) times capacitance (\(C\)).
In our case, the time constant is approximately 0.250 s, the same as the duration of the lamp's flash. Rearranging the time constant formula gives:\(R = \frac{\tau}{C}\).
Using \(\tau = 0.250 \, \text{s}\) and \(C = 0.0139 \, \text{F}\), we find:\(R = \frac{0.250 \, \text{s}}{0.0139 \, \text{F}} = 17.99 \, \Omega\).
The resistance value of 17.99 ohms indicates how much the lamp thwart the flow of electric charge. Lower resistance means faster discharge and thus shorter flash durations, crucial for timing the lamp's flashes accurately.

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Most popular questions from this chapter

(a) What is the terminal voltage of a large 1.54-V carbon-zinc dry cell used in a physics lab to supply \(2.00 \bar{A}\) to a circuit, if the cell's internal resistance is \(0.100 \Omega ?\) (b) How much electrical power does the cell produce? (c) What power goes to its load?

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