Two bodies at different temperatures are in thermal contact within an isolated system. Net energy transfer stops when
their temperatures become equal
the hotter body reaches absolute zero
all energy has been destroyed
the cooler body starts to radiate
Tap an answer to check it.
WhyThermal equilibrium is the state of equal temperature with no net energy flow.
2Multiple choice · Medium
Metal blocks X and Y of the same mass are in good thermal contact, X initially at 40 C and Y at 30 C. The specific heat capacity of X is greater than that of Y. Assuming no heat loss, at steady state
their temperature becomes the same and is higher than 35 C
their temperature becomes the same and is lower than 35 C
their temperature becomes the same and equals 35 C
block X stays hotter than block Y
Tap an answer to check it.
WhyBecause c_X > c_Y, X falls less than Y rises, so the common temperature exceeds 35 C.
3Multiple choice · Hard
A copper block (mass m, c = 390 J kg^-1 K^-1) at 90 C is mixed in isolation with an equal-mass copper block at 30 C. The final temperature is
60 C
45 C
75 C
50 C
Tap an answer to check it.
WhySame substance and mass mix to the average: (90+30)/2 = 60 C.
4Fill in the blank · Easy
A state in which two objects in contact are at the same temperature with no net heat flow is called thermal .
Answer:
equilibrium
WhyNo net energy transfer occurs at thermal equilibrium.
5Fill in the blank · Medium
For equal masses mixed in isolation, the block with the specific heat capacity undergoes the smaller temperature change.
Answer:
larger / greater / higher
WhyA larger c resists temperature change per unit energy.
6Fill in the blank · Hard
Equal masses are mixed with no losses. Block P (c = 3c of block Q) starts at 40 C and Q at 20 C. The final common temperature is C.
Answer:
35 / 35.0
Why3(40-T)=(T-20) gives 140=4T, so T=35 C.
7Multiple choice · Easy
A patch of alcohol on the skin feels cool as it evaporates. The best explanation is that
the evaporation of alcohol absorbs heat from the skin
the alcohol releases latent heat to the skin
the motion of all alcohol molecules slows down
air molecules remove heat from the alcohol by conduction
Tap an answer to check it.
WhyEvaporation absorbs latent heat from the skin.
8Multiple choice · Medium
When a liquid evaporates below its boiling point, the temperature of the remaining liquid tends to fall because
the more energetic molecules escape, lowering the average kinetic energy
the slower molecules escape first
the liquid gains latent heat from the vapour
the number of molecules increases
Tap an answer to check it.
WhyThe higher-energy molecules escape, reducing the mean kinetic energy of those remaining.
9Multiple choice · Hard
A refrigerant evaporates and removes 4.0 x 10^4 J from a cabinet while 0.16 kg vaporises. The specific latent heat of vaporisation of the refrigerant is
2.5 x 10^5 J kg^-1
6.4 x 10^3 J kg^-1
2.5 x 10^6 J kg^-1
1.6 x 10^5 J kg^-1
Tap an answer to check it.
WhyL = E/m = 4.0 x 10^4 / 0.16 = 2.5 x 10^5 J kg^-1.
10Fill in the blank · Easy
Evaporation of a liquid absorbs latent heat, so the temperature of the remaining liquid tends to .
Answer:
fall / decrease / drop
WhyRemoving energetic molecules cools the liquid.
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Temperature: A measure of the degree of hotness of a body, related to the average kinetic energy of its particles.
Heat: Energy transferred from a hotter body to a colder body because of the temperature difference between them.
Internal energy: The total of the random kinetic energy and potential energy of all the particles making up a body.
Heat capacity: The energy needed to raise the temperature of a whole body by one kelvin.
Specific heat capacity: The energy needed to raise the temperature of one kilogram of a substance by one kelvin, in J per kg per K.
Specific latent heat: The energy needed to change the state of one kilogram of a substance without any change in temperature.
Specific latent heat of fusion: The energy needed to change one kilogram of a solid into liquid at its melting point without change of temperature.
Specific latent heat of vaporization: The energy needed to change one kilogram of a liquid into vapour at its boiling point without change of temperature.