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
Correct answer
Explanation
The correct answer is A, conduction and convection. Both of these modes need matter: conduction passes energy from particle to particle and convection carries it by the movement of a fluid. A vacuum has almost no particles, so neither can work across it.
Option B is wrong because radiation travels through a vacuum perfectly well, being an electromagnetic wave; the flask stops it by a different device, the silvering of the facing surfaces, which reflects the radiation back. Option C is incomplete, since the vacuum stops convection as well as conduction. Option D states the opposite of the truth. The stopper of the flask completes the design by preventing the escape of hot vapour, and this three part answer is what examiners look for. The same flask keeps a cold liquid cold for exactly the same reasons, because it blocks heat moving in as well as heat moving out.
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 →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.
Read the full explanation →Which law of thermodynamics provides the basis for the concept of temperature?
- A.The zeroth law
- B.The first law
- C.The second law
- D.The third law
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It states that if body A is in thermal equilibrium with body C, and body B is also in thermal equilibrium with body C, then A and B are in thermal equilibrium with each other.
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