Video: Understanding How Heat Absorption and Emission Relates to Temperature Change

A metal can is placed in constant intensity sunlight and absorbs infrared radiation. During the can’s exposure to sunlight, the temperature of the metal increases. Which of the following statements is correct? [A] Infrared radiation absorption and emission have no effect on temperature. [B] The metal loses more energy by cooling than the energy of the infrared radiation that it absorbs. [C] The metal does not receive any energy from the sunlight. [D] The metal loses exactly as much energy by cooling as the energy of the infrared radiation that it absorbs. [E] The metal absorbs more energy from infrared radiation than the energy it loses by cooling.

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Video Transcript

A metal can is placed in constant-intensity sunlight and absorbs infrared radiation. During the can’s exposure to sunlight, the temperature of the metal increases. Which of the following statements is correct? (A) Infrared radiation absorption and emission have no effect on temperature. (B) The metal loses more energy by cooling than the energy of the infrared radiation that it absorbs. (C) The metal does not receive any energy from the sunlight. (D) The metal loses exactly as much energy by cooling as the energy of the infrared radiation that it absorbs. And, lastly, answer option (E) the metal absorbs more energy from infrared radiation than the energy it loses by cooling.

As we consider which of these five answer options correctly describes what’s taking place, we can recall that in this situation we have a metal can that’s in direct sunlight. And we’re told the can is absorbing some of the infrared radiation it receives. During this exposure, while the can is in the sunlight, the temperature of the can increases. It goes up. Knowing all this, we want to pick which of these answer options correctly describes what’s going on.

Starting off with option (A), this option says that infrared radiation absorption and emission have no effect on temperature. Considering this option, we were told in our problem statement that as the can absorbs infrared radiation, its temperature does go up. Absorbing infrared radiation does have some effect on the temperature of an object, and at the same time emitting this radiation does too. When an object gives off infrared radiation through emission, then it’s losing energy. And therefore, we would expect its temperature to go down. All this shows us that option (A) won’t be our choice.

Option (B) says that the metal loses more energy by cooling than the energy of the infrared radiation that it absorbs. This option tells us that as the metal can radiates or emits infrared radiation, it loses energy. And whatever the amount of energy lost through this cooling process option (B) tells us is greater than the amount of energy gained by absorbing the sunlight. Physically, it is possible for something like this to happen, for the total energy lost by an object through radiation to be greater than the amount of energy it receives by absorption.

But in this case, we know that that’s not going on. And the reason is the temperature of the metal can is going up. If the can was losing more energy than it was gaining, we would expect this temperature to go in the opposite direction, to decrease. Since it’s going up though, that must mean that the energy lost does not exceed the energy gained.

Option (C) tells us that the metal does not receive any energy from the sunlight. But once again, if the object didn’t receive any energy, then how would its temperature be going up? This could only happen if it was receiving energy from some unnamed source. But we can reasonably assume that no such source exists because we’re not told about it. So, this must mean that the temperature increase in the metal can is due to energy it receives from the sunlight. And indeed, our problem statement confirms this when it says that the can absorbs infrared radiation. By that means it does receive energy from the sunlight. So, we won’t choose option (C) as our answer either.

Option (D) says that the metal loses exactly as much energy by cooling as the energy of the infrared radiation that it absorbs. So, this option then says that the metal can indeed does receive energy from the sunlight and that it gives off just as much energy by cooling. If this were the case though, we would expect that the temperature of our object wouldn’t change at all because the net change in energy of the object is zero. That is, just as much energy came in from the sunlight, as went out due to radiative cooling. That’s what option (D) claims. Yet, if that were true, we would expect the temperature of our object to be constant. The fact that it heats up means that it must be receiving more energy than it is given off. Option (D) isn’t our correct description either.

This leaves us with our last choice option (E), which says that the metal absorbs more energy from infrared radiation than the energy it loses by cooling. This choice tells us that indeed the can does receive energy from the sunlight, and indeed, it does radiate some of that energy away. But that overall, it receives more energy than it emits. And that is consistent with the observation that the temperature of the metal can goes up. When that happens, we would expect more energy to be put in than to be taken out. And indeed, that’s what’s taking place. The correct description of what’s happening in this scenario is that the metal absorbs more energy from infrared radiation than the energy it loses by cooling.

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