6.3 Heat Transfer and Thermal Equilibrium

1. AThe total thermal energy of each block
   A student who thinks energy is transferred from the body with more energy in total picks this. A large cool block can hold more energy in total than a small hot one, yet the net transfer is from the hot block to the cool one.
2. BThe mass and size of each of the blocks
   A student who thinks a bigger or heavier object is the hotter one, and so gives out energy, picks this. Size does not fix the temperature: the net transfer is from whichever block is at the higher temperature.
3. CThe material each block is made from
   A student who thinks each material has its own natural temperature, with metals being cold, picks this. Any material can be at any temperature; the direction depends on which block is warmer, not on what the blocks are made of.
4. DThe temperature of each of the blocks Correct
   The warmer block is the one whose particles have the greater average kinetic energy, and collisions transfer energy on balance from that block to the cooler one. Knowing the two temperatures is therefore enough to give the direction.

Working No calculation. Collisions transfer energy, on balance, from the body whose particles have the greater average kinetic energy to the body whose particles have the lower average kinetic energy. The average kinetic energy of the particles is indicated by the temperature, so the two temperatures are sufficient: the net transfer is from the block at the higher temperature to the block at the lower temperature. Total energy, size and material do not fix the direction.

CED 6.3.A.1 · Read this in Fix

1. AHeat, a substance held between the copper atoms, flows out of the metal and spreads among the water molecules
   A student who thinks heat is a substance stored in hot objects picks this. Nothing material passes from the copper to the water; energy is transferred in collisions and appears as faster motion of the water molecules.
2. BCold leaves the water molecules and passes into the copper, so the water molecules are left warmer than before
   A student who thinks cold is something that moves out of cold objects picks this. Cold is not a substance or a form of energy; the water warms because energy is transferred to it from the copper.
3. CCopper atoms collide with water molecules, which leave the collisions with more kinetic energy on average Correct
   The atoms of the hot copper have a greater average kinetic energy than the water molecules. In collisions at the surface of the metal, energy is transferred on average from the copper atoms to the water molecules, whose average kinetic energy, and so the water's temperature, rises.
4. DCopper atoms pass on their high temperature to the water molecules, so each molecule becomes hot in itself
   A student who thinks each particle of a hot object is itself hot picks this. A single atom or molecule has kinetic energy, not a temperature; what is passed on in collisions is kinetic energy, and temperature describes the average for the whole sample.

CED 6.3.A.2 · Read this in Fix

1. AAverage kinetic energy: He = Ar; average speed: He > Ar Correct
   At thermal equilibrium the two samples have the same temperature, so their atoms have the same average kinetic energy. Because kinetic energy is ½mv², the lighter He atoms must move faster on average than the Ar atoms to have the same average kinetic energy.
2. BAverage kinetic energy: He < Ar; average speed: He = Ar
   A student who thinks equal temperatures mean equal particle speeds picks this, and then gives the heavier Ar atoms the greater kinetic energy. Equal temperatures mean equal average kinetic energies, so the lighter He atoms are the faster ones.
3. CAverage kinetic energy: He > Ar; average speed: He > Ar
   A student who thinks faster particles always have more kinetic energy picks this. The He atoms are faster on average, but their smaller mass offsets their greater speed: at the same temperature the average kinetic energies are equal.
4. DAverage kinetic energy: He = Ar; average speed: He = Ar
   A student who treats average kinetic energy and average speed as interchangeable picks this, taking equal kinetic energies to mean equal speeds. With equal average kinetic energies, atoms of different mass have different average speeds; the lighter He atoms are faster.

CED 6.3.A.3 · Read this in Fix

1. AFrom Y to X, as the nine atoms in Y have more kinetic energy in total
   A student who thinks energy is transferred from the body with more energy in total picks this, counting the nine atoms in Y against the three in X. The direction of net transfer is set by the average kinetic energy per atom, which is greater in X, not by the total.
2. BFrom X to Y, as the atoms in X have the greater average kinetic energy Correct
   The atoms are all neon atoms, so the faster atoms in X have the greater average kinetic energy: X is the warmer sample. Collisions with the shared wall pass energy, on balance, from the sample with the greater average kinetic energy to the sample with the lower, so the net transfer is from X to Y.
3. CFrom Y to X, as the atoms in Y are closer together and collide more
   A student who takes the more crowded sample, with more collisions, to be the hotter one picks this. Temperature reflects the average kinetic energy of the atoms, which is lower in Y, so Y is the cooler sample and gains energy.
4. DFrom X to Y, as the faster atoms in X pass through the wall into Y
   A student who thinks heat is transferred by fast particles moving into the cooler body picks this. The direction is right, but no atoms cross the wall: energy is passed on in collisions between the atoms and the wall.

CED 6.3.A.1 · Read this in Fix

1. A0.72
   A student who takes average kinetic energy to be proportional to the Celsius temperature picks this: 527/727 = 0.72. The proportionality holds for the Kelvin temperature: 800 K/1000 K = 0.80.
2. B0.60
   A student who thinks the Kr(g) cools by as many degrees as the Ne(g) warms, 400°C, picks this: the Kr(g) would end at 327°C = 600 K, giving 600/1000 = 0.60. At thermal equilibrium the two samples have the same temperature, 527°C; their temperature changes need not be equal.
3. C0.89
   A student who treats average speed and average kinetic energy as changing by the same factor picks this: √(800/1000) = 0.89 is the factor by which the typical speed of the atoms changes. Kinetic energy depends on the square of the speed, and its ratio is 0.80.
4. D0.80 Correct
   At thermal equilibrium the Kr(g) has the same temperature as the Ne(g), 527°C, which is 800 K. Initially it was at 727°C, which is 1000 K. Average kinetic energy is proportional to the Kelvin temperature, so the ratio is 800/1000 = 0.80.

Working At thermal equilibrium both samples have the same temperature, so the Kr(g) is at 527°C = 527 + 273 = 800 K. Initially the Kr(g) is at 727°C = 1000 K. Average kinetic energy is proportional to the Kelvin temperature, so the ratio is 800 K ÷ 1000 K = 0.80.

CED 6.3.A.3 · Read this in Fix

1. AThe steel spoon gives a lower reading than the air in the room
   A student who thinks metals are colder than their surroundings picks this, because metal objects feel cold. After a night in the room the spoon is in thermal equilibrium with the air and is at 22°C; it feels cold only because it conducts energy away from the skin quickly.
2. BThe wooden spoon gives a lower reading than the big steel pan Correct
   Each object has had time to reach thermal equilibrium with the air around it, so the steel pan and the steel spoon are at 22°C and the wooden spoon is at 4°C. The wooden spoon therefore gives the lower reading, whatever the objects are made of and whatever their sizes.
3. CThe wooden spoon gives a higher reading than the air near it
   A student who thinks wood is warm in itself picks this. Wood does not supply energy: after a night in the refrigerator the wooden spoon is in thermal equilibrium with the air there and is at 4°C.
4. DThe steel pan gives a higher reading than the steel spoon
   A student who thinks a larger object, holding more energy, is hotter picks this. The pan and the spoon are both in thermal equilibrium with the same air, so the average kinetic energy of their particles, and their temperature, is the same.

CED 6.3.A.3 · Read this in Fix

1. ACollisions between block and water particles continue, with no net transfer of energy Correct
   By 300 s the two curves have merged at 30°C, so the block and the water are in thermal equilibrium: their particles have the same average kinetic energy. The particles still collide, but energy passes equally in both directions on average, so there is no net transfer and the temperatures stay constant.
2. BCollisions between block and water particles have ended, and no energy is being transferred
   A student who thinks nothing happens at the particle level once the temperatures stop changing picks this. The particles keep moving and colliding; the transfers in the two directions balance, so there is no net transfer.
3. CThe block and the water now contain equal total amounts of thermal energy
   A student who thinks energy is transferred until the two bodies hold equal amounts picks this. The graph shows equal temperatures, which means equal average kinetic energies of the particles; the total energy of each body depends also on its amount and on what it is made of.
4. DThe particles in the block and the molecules in the water have equal average speeds
   A student who thinks equal temperatures mean equal particle speeds picks this. Equal temperatures mean equal average kinetic energies; metal atoms and water molecules have different masses, so their average speeds differ.

CED 6.3.A.3 · Read this in Fix