Heat from the sun reaches the earth mainly by which mode of transfer?
- A.Conduction
- B.Convection
- C.Radiation
- D.Conduction followed by convection
Correct answer
Explanation
The correct answer is C, radiation. The space between the sun and the earth is very nearly empty, and radiation is the only mode of heat transfer that needs no material medium: energy travels as electromagnetic waves at the speed of light and is absorbed when it strikes matter.
Option A, conduction, requires particles in contact to pass energy along, so it cannot work across empty space. Option B, convection, requires a fluid that can itself move and carry energy with it, which again is not available between the sun and the earth, though convection does distribute that heat once it has entered the atmosphere. Option D fails for both reasons. A good absorber of radiation is also a good emitter, which is why a blackened surface warms fastest in the sun.
Read the full article: Heat and Thermodynamics: Laws, Latent Heat and Transfer
Practice Questions
View allThe SI unit of heat is the
- A.Calorie
- B.Joule
- C.Kelvin
- D.Watt
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Heat is a form of energy in transit, so it is measured in the same unit as work and energy, the joule, which is one newton metre.
Read the full explanation →Water has its maximum density at a temperature of
- A.0 degrees Celsius
- B.4 degrees Celsius
- C.100 degrees Celsius
- D.Minus 4 degrees Celsius
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Water behaves abnormally between zero and four degrees: it contracts as it is heated in that range and expands again above four degrees, so the same mass occupies its least volume, and therefore has its greatest density, at four degrees Celsius.
Read the full explanation →The latent heat of vaporisation of water is about
- A.80 calories per gram
- B.540 calories per gram
- C.336 calories per gram
- D.2260 calories per gram
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This is the heat that one gram of water at a hundred degrees Celsius must absorb to become steam at the same temperature, with no rise in temperature at all; the same quantity is released when steam condenses back to water.
Read the full explanation →In a vacuum flask, the vacuum between the two walls prevents heat transfer mainly by
- A.Conduction and convection
- B.Radiation only
- C.Conduction only
- D.Neither conduction nor convection
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Both of these modes need matter: conduction passes energy from particle to particle and convection carries it by the movement of a fluid.
Read the full explanation →The first law of thermodynamics is essentially a statement of
- A.The conservation of energy
- B.The increase of entropy
- C.Thermal equilibrium
- D.The unattainability of absolute zero
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The first law says that the heat supplied to a system is equal to the increase in its internal energy plus the external work it does.
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