Question Video: Plastic Cube Thermal Radiation Physics

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

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Video Transcript

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

In this example, we have a plastic block that’s being exposed to sunlight. We’re also told that even though the block is in sunlight, its temperature is decreasing. Based on this information, we want to choose which of these five answer options is correct. Considering this plastic block, we could say that there are two processes going on. One is that it is absorbing infrared radiation and, therefore, having energy transferred to it. By itself, this would tend to make the temperature of the block increase rather than decrease. But there’s another process going on, infrared radiation of the block. This is a means by which energy is transferred away from the plastic block. These two processes are in a sense competing. One transfers energy to the block, and one transfers energy away from it.

From the fact that the overall temperature of the block is decreasing, we can tell which of these two processes is dominant. As the block emits infrared radiation, more energy must be transferred away from it than is transferred to it as the block absorbs sunlight. Reviewing our answer options, this corresponds to option (D).

Considering the other answer choices, note that option (A) is not an accurate statement. Absorbing sunlight will transfer energy to the block. Option (B) is also not accurate. Infrared radiation absorption and emission do have an effect on temperature. Indeed, these are the two competing processes in the case of our plastic block that lead to a temperature decrease in the block. Option (C) describes the scenario where the block loses exactly as much energy by cooling as the energy of the infrared radiation that it absorbs. If this were true though, it would lead to a constant temperature of the plastic block. We know, however, that really the temperature of the block is decreasing. Lastly, option (E) says that the plastic absorbs more energy from the infrared radiation than the energy it loses by cooling. If this were going on, it would mean the temperature of the block would increase rather than decrease.

For our final answer, we choose option (D). The plastic loses more energy by cooling than the energy of the infrared radiation that it absorbs.

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