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How many days does it take for a perfect blackbody cube \((0.0100 \mathrm{m}\) on a side, \(30.0^{\circ} \mathrm{C}\) ) to radiate the same amount of energy that a one hundred-watt light bulb uses in one hour?

Short Answer

Expert verified
The cube takes approximately 44.8 days to radiate the same energy.

Step by step solution

01

Calculate the Energy Radiated by the Light Bulb

To find the energy used by the 100-watt light bulb in one hour, use the formula for energy: \[ E = P \times t \]where:- \( E \) is the energy in joules,- \( P = 100 \text{ watts} \),- \( t = 3600 \text{ seconds (1 hour)} \).Substitute the values:\[ E = 100 \times 3600 = 360,000 \text{ joules} \].
02

Calculate the Surface Area of the Cube

The cube has sides of length 0.01 m. The surface area \( A \) of a cube is given by:\[ A = 6s^2 \]where \( s \) is the side length. Substitute the given side length:\[ A = 6 \times (0.01)^2 = 0.0006 \text{ m}^2 \].
03

Use the Stefan-Boltzmann Law to Find the Energy Radiated by the Cube per Second

The Stefan-Boltzmann Law states that the power radiated by a blackbody is given by:\[ P = \sigma A T^4 \]where:- \( \sigma = 5.67 \times 10^{-8} \text{ W/m}^2\cdot\text{K}^4 \) is the Stefan-Boltzmann constant,- \( A = 0.0006 \text{ m}^2 \),- \( T \) is the temperature in Kelvin.Convert \( 30.0^{\circ} \text{C} \) to Kelvin:\[ T = 30.0 + 273.15 = 303.15 \text{ K} \].Substitute these into the formula:\[ P = 5.67 \times 10^{-8} \times 0.0006 \times (303.15)^4 \approx 0.093 \text{ watts} \].
04

Calculate the Time Required for the Cube to Radiate the Equivalent Energy

Now find the time \( t \) for the cube to radiate 360,000 joules at a rate of 0.093 watts:\[ t = \frac{360,000}{0.093} \approx 3,870,968 \, \text{seconds} \].
05

Convert the Time from Seconds to Days

Convert the time from seconds to days:\[ 1 \, \text{day} = 24 \, \text{hours} \times 3600 \, \text{seconds/hour} = 86,400 \, \text{seconds} \].Calculate the number of days:\[ \frac{3,870,968}{86,400} \approx 44.8 \, \text{days} \].

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Key Concepts

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

Stefan-Boltzmann Law
The Stefan-Boltzmann Law is a fundamental principle in the physics of blackbody radiation. It describes how the power radiated by an object depends on its temperature and surface area. This law can be represented by the formula:\[ P = \sigma A T^4 \]- **\( P \)**: Power radiated in watts (W)- **\( \sigma \)**: Stefan-Boltzmann constant, \( 5.67 \times 10^{-8} \text{ W/m}^2\cdot\text{K}^4 \)- **\( A \)**: Surface area of the object in square meters (\(\text{m}^2\))- **\( T \)**: Temperature of the object in Kelvin (K)
This law indicates that the energy emitted by a perfect blackbody is directly proportional to the fourth power of its absolute temperature and the surface area. It helps understand how objects like stars or heated metal emit energy as electromagnetic radiation.
Radiative Heat Transfer
Radiative heat transfer is a mode of heat transfer that occurs through electromagnetic waves. It doesn't require a medium and is how, for instance, the sun's energy reaches Earth. In the context of a perfect blackbody like the cube in this problem, we can visualize this transfer through thermal radiation. Key aspects:
  • **Blackbody:** An idealized object that absorbs all incident radiation, with no reflection. It radiates energy perfectly according to its temperature.
  • **Emission:** Due to its temperature, the blackbody emits radiation across different wavelengths.
  • **Radiance:** The amount of energy emitted is influenced by the object's temperature and characteristics.
This concept is essential in understanding how objects transfer heat through the radiation process, which is critical in technologies like thermal imaging or climate science.
Energy Calculation
Energy calculation is fundamental when analyzing how much energy an object, like a blackbody, radiates over time or when comparing energy usage, such as with a light bulb.Here's how you can calculate energy:- **Formulation:** The energy \( E \) is the product of power \( P \) and time \( t \). Thus, \[ E = P \times t \]
- **Example:** A 100-watt light bulb that runs for 3600 seconds (or 1 hour) uses an energy of: \[ E = 100 \times 3600 = 360,000 \text{ joules} \]
Calculating energy is crucial for understanding efficiency and equivalency between energy sources, ensuring that comparisons make sense when applied in real-world contexts.
Temperature Conversion
Temperature conversion is often necessary in physics problems when dealing with heat and radiation, due to the need for a temperature in Kelvin. Kelvin is the SI unit for temperature and is used in many scientific formulas related to thermal dynamics.Here's how to convert Celsius to Kelvin:- The formula is simple: \[ T_{K} = T_{C} + 273.15 \] - **Example Conversion:** To convert \(30.0^{\circ} \text{C} \) to Kelvin: \[ T = 30.0 + 273.15 = 303.15 \text{ K} \]
In the context of blackbody radiation, having the correct temperature scale aligns with the requirements of the Stefan-Boltzmann Law and ensures calculations are accurate for the given conditions.
Surface Area of a Cube
The surface area of a cube is a straightforward concept but crucial in calculations involving radiative properties because it affects how much energy the object can emit.For a cube with a side length \( s \):- **Surface Area \( A \):** The formula is \[ A = 6s^2 \] - **Calculation Example:** For a cube with side length \(0.01 \text{ m} \): \[ A = 6 \times (0.01)^2 = 0.0006 \text{ m}^2 \]
The surface area determines the extent to which an object can absorb or emit radiation, making it a critical factor in thermodynamics and radiative heat transfer calculations.

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Most popular questions from this chapter

Multiple-Concept Example 8 reviews the approach that is used in problems such as this. A person eats a dessert that contains 260 Calories. This "Calorie" unit, with a capital \(C,\) is the one used by nutritionists; 1 Calorie \(=4186 \mathrm{J}\) . See Section \(12.7 .\) The skin temperature of this individual is \(36^{\circ} \mathrm{C}\) and that of her environment is \(21^{\circ} \mathrm{C}\) . The emissivity of her skin is 0.75 and its surface area is 1.3 \(\mathrm{m}^{2}\) How much time would it take for her to emit a net radiant energy from her body that is equal to the energy contained in this dessert?

Two pots are identical except that the flat bottom of one is aluminum, whereas that of the other is copper. Water in these pots is boiling away at \(100.0^{\circ} \mathrm{C}\) at the same rate. The temperature of the heating element on which the aluminum bottom is sitting is \(155.0^{\circ} \mathrm{C}\) . Assume that heat enters the water only through the bottoms of the pots and find the temperature of the heating element on which the copper bottom rests.

Sirius \(B\) is a white star that has a surface temperature (in kelvins) that is four times that of our sun. Sirius \(\mathrm{B}\) radiates only 0.040 times the power radiated by the sun. Our sun has a radius of \(6.96 \times 10^{8} \mathrm{m} .\) Assuming that Sirius \(\mathrm{B}\) has the same emissivity as the sun, find the radius of Sirius \(\mathrm{B}\) .

The concrete wall of a building is 0.10 \(\mathrm{m}\) thick. The temperature inside the building is \(20.0^{\circ} \mathrm{C},\) while the temperature outside is \(0.0^{\circ} \mathrm{C}\) . Heat is conducted through the wall. When the building is unheated, the inside temperature falls to \(0.0^{\circ} \mathrm{C},\) and heat conduction ceases. However, the wall does emit radiant energy when its temperature is \(0.0^{\circ} \mathrm{C}\) . The radiant energy emitted per second per square meter at \(0.0^{\circ} \mathrm{C}\) is the same as the heat lost per second per square meter due to conduction when the temperature inside the building is \(20.0^{\circ} \mathrm{C}\) . What is the emissivity of the wall?

A closed box is filled with dry ice at a temperature of \(-78.5^{\circ} \mathrm{C}\) while the outside temperature is \(21.0^{\circ} \mathrm{C} .\) The box is cubical, measuring 0.350 \(\mathrm{m}\) on a side, and the thickness of the walls is \(3.00 \times 10^{-2} \mathrm{m} .\) In one day, \(3.10 \times 10^{6} \mathrm{J}\) of heat is conducted through the six walls. Find the thermal conductivity of the material from which the box is made.

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