Energy Production & Thermal Energy Transfer Flashcards

Flashcards about Energy Production and Thermal Energy Transfer based on lecture notes by Dr. Amjad Al-Refai.

What is black-body radiation?

Radiation emitted from the theoretical perfect emitter for a given temperature; it depends on the object's temperature and size, not its surface nature.

What happens to the color of radiation emitted from an enclosed container as its temperature rises?

It goes from infra-red at low temperatures, to red, then yellow, and eventually white at very high temperatures.

What is Wien's displacement law?

It relates the wavelength at which the intensity of radiation is a maximum to the Kelvin temperature of the black body: λmax × T = constant = 2.9 × 10-3

According to Wien's Displacement Law, are the wavelength at which the intensity of the emitted radiation by a black-body and its Kelvin temperature directly or inversely proportional?

Inversely proportional; lower temperature means longer wavelength.

How can the surface temperature of a star be determined?

By analyzing the light from a star.

What color do hot stars tend to appear and why?

White, because they emit all frequencies of visible light.

What color do cooler stars tend to appear and why?

Red, because they emit the higher wavelengths (lower frequencies) of visible light.

How does increasing the temperature of a black-body affect emitted radiation?

The overall intensity at each wavelength increases, the total power emitted per square meter increases, and the peak of the curve shifts towards shorter wavelengths.

Define Intensity of Radiation (I)

Power per unit area received by an object (W/m^2).

What is the formula for the intensity of radiation at a distance d from the radiation source?

I = P/A = P/(4πd^2), where P is the power of the source.

What does Luminosity (L) of a star refer to?

The total energy emitted by the star per second, measured in Watts (W).

What is Apparent Brightness (b)?

The incident power per unit area received by an observer at the surface of the earth measured in (W.m-2).

What is the formula for apparent brightness?

b = L/(4πd^2) where L is luminosity and d is the distance from the star.

What happens if two stars are at the same distance away from the earth?

The one with the greatest luminosity would be brighter

What is the Stefan-Boltzmann Law?

It links the total power radiated by a black body per unit area to the Kelvin temperature of the black-body. It states that (energy emitted per unit time (power) per unit area is proportional to (Kelvin temperature in K) 4

What does the Stefan-Boltzmann Law state?

I = σT^4, where I is the intensity, σ is the Stefan-Boltzmann constant (5.67 x 10^-8 Wm^-2K^-4), and T is the temperature in Kelvin.

What is the formula for the total power radiated (L) by a black-body?

L = σAT^4, where A is the surface area and T is the temperature.

If a star is assumed to be a sphere with radius r, what is the formula for its luminosity?

L = (4πr^2)σT^4

Define Grey Body.

Objects that can be very close to the behavior of a black body but not quite (100)% perfect in the way they behave. At a particular temperature will emit less energy per second than a perfect black body of the same dimensions at the same temperature.

Define Emissivity (e)

The measure of the ratio between the power emitted by a radiating object and the power emitted by a black body of the same dimensions at the same temperature. It is given by: e = (power emitted by a radiating object)/(power emitted by a black body of the same dimensions at the same temperature)

What is the emissivity of a perfect black body?

1

What is the emissivity of an object that completely reflects radiation?

0

What range of emissivity do real objects have?

Between 0 and 1.

What is the solar constant?

The amount of the incident solar power (energy radiated per second) per unit perpendicular area at the average distance of the earth from the sun along the line joining the center of the sun and the center of the earth. It is approximately 1400 W/m^2.

Average power received per unit area of the Earth

1/4 (intensity of the Sun’s radiation at the position of the Earth) = 1400/4 =(350) W m–2

Why does the value of Solar Constant varies periodically?

1. The output of the sun varies by about (0.1)% during its principal 11-years sunspot cycle. 2. The Earth’s orbit is elliptical with the earth slightly closer to the sun in January compared to July, this accounts for a difference of about (7)% in the solar constant.

For a real object, what is the formula to calculate the power emitted?

P = e × σ × A × T^4

What formula calculates the net power lost by a body to the surroundings?

∆P = σ × A × (T1^4 - T2^4)

Define Albedo

The measure of the ratio between the total scattered power and the total incident power or (the ratio between the energy reflected by a given surface in a given time and the total energy incident on the surface in the same time).

What is the formula for Albedo?

α = (Total scattered power)/(Total incident power) = (Total energy reflected by a given surface in a given time)/(Total energy incident by a given surface in the same time)

What is Earth's global annual average Albedo?

30%

Do Oceans or Ice caps have a greater Albedo?

Polar ice caps.

What is the Green House Effect?

Natural process by which short wavelength radiation received from the sun causes the surface of the Earth to warm up, and thus it emits longer wavelength in the form of infra-red radiation because the Earth is cooler than the Sun. Some of this infra- red radiation will be absorbed by green house gases in the atmosphere and re-radiated in all directions.

Name common greenhouse gases

Carbon dioxide (CO2), Methane (CH4), Water (H2O), Nitrous oxide (N2O), Ozone (O3) and Chlorofluorocarbons (CFCs).

Mechanism in which the Earth’s surface radiation is absorbed by greenhouse gases

The Earth radiates photons of infra-red frequency of the electromagnetic spectrum; the greenhouse gas molecules have energy level differences corresponding to infra-red energies and so the bonds within the molecules of the gas will oscillate/vibrate with natural frequencies in the infrared region; therefore the infra-red photons radiated by the surface of the earth will be absorbed as a result of resonance.

Explain why the incoming solar radiation is not affected by the green house mechanism

Most incoming radiation consists of photons in the visible/ultraviolet region that have much shorter wavelength than infra-red radiation emitted by the Earth, thus resonance will not occur and therefore these photons cannot be absorbed.

What is a likely cause for global warming?

Changes in the composition of greenhouse gases in the atmosphere caused by natural effects or human activities, changes in the intensity of the radiation emitted by the sun and Cyclical changes in the Earth’s orbit.

Global Warming effect on the Average Albedo of the Earth

Albedo decreased because carbon dioxide reduces the amount of reflected Radiation

Outline the difference, if any, that the melting of oceanic ice sheets makes to the mean sea level of the Earth

There is no change in mean sea level because Oceanic ice floats on sea water will displace same volume which is equal to the volume of the floating ie sheet when it melts

What is likely the most significant way to reduce the enhanced greenhouse effect?

Replace coal-fired power stations with nuclear power stations

Why are scientists concerned with the increased rate of evaporation caused by continued global warming?

Temperature increase reduces the concentration of CO2 in the sea due to the rate of evaporation that increases and thus increases atmospheric concentrations. Continued global warming will increase both evaporation and the atmosphere’s ability to hold water vapor. Water vapor is a greenhouse gas.

What is the enhanced greenhouse effect

an increase in the concentration of infra-red radiation from the atmosphere to the Earth, thus the greenhouse effect will increase due to human activities.