Question Video: Analyzing Energy Transfer for an Object near a Source of Infrared Radiation | Nagwa Question Video: Analyzing Energy Transfer for an Object near a Source of Infrared Radiation | Nagwa

Question Video: Analyzing Energy Transfer for an Object near a Source of Infrared Radiation Physics • Third Year of Secondary School

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An object is placed close to a source that emits infrared radiation at a constant intensity. Initially, the temperature of the object begins to increase. What is the reason for this? [A] In a given interval of time, the object absorbs the same amount of energy from the infrared radiation as it loses through cooling. [B] In a given interval of time, the object absorbs more energy from the infrared radiation than it loses through cooling. [C] In a given interval of time, the object absorbs less energy from the infrared radiation than it loses through cooling.

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Video Transcript

An object is placed close to a source that emits infrared radiation at a constant intensity. Initially, the temperature of the object begins to increase. What is the reason for this? (A) In a given interval of time, the object absorbs the same amount of energy from the infrared radiation as it loses through cooling. (B) In a given interval of time, the object absorbs more energy from the infrared radiation than it loses through cooling. (C) In a given interval of time, the object absorbs less energy from the infrared radiation than it loses through cooling.

In this example, we have a source of infrared radiation. And near to that source, we have some object. We’re told that the source emits infrared radiation at a constant intensity. Along with this, right after we place the object nearby the source, the object’s temperature begins to increase. We want to choose which of these three explanations gives the reason for this.

Each explanation describes two competing processes. In one process, energy is transferred to the object as it absorbs infrared radiation. In competition with that though is the fact that this object is naturally cooling off, emitting infrared radiation to its surroundings. Energy then is transferred to the object from its absorption of infrared radiation. And energy is transferred away from the object through cooling.

The key to choosing the right explanation is to recognize that the object’s temperature at this point is increasing. This means that, overall, more energy is being transferred to the object than is being transferred away from it. Answer option (B) gives the correct explanation. The object’s temperature begins to increase because, in a given interval of time, the object absorbs more energy from the infrared radiation than it loses through cooling.

Let’s look now at part two of this question.

Which of the following statements is true? (A) As the temperature of the object increases, the object emits more energy per unit time. The amount of energy absorbed by the object per unit time remains the same. (B) As the temperature of the object increases, the object absorbs more energy per unit time. The amount of energy emitted by the object per unit time remains the same. (C) As the temperature of the object increases, the amount of energy absorbed by the object per unit time decreases. The amount of energy emitted by the object per unit time increases.

Now, there’s one more answer option to look at. That’s option (D). Before we do though, let’s evaluate these three statements to see if any of them might be true. In all of these statements, we’re talking about the temperature of our object increasing. So we have our source of infrared radiation. And shortly after we put our object nearby, the temperature of the object goes up.

Answer option (A) says that as this happens, the object emits more energy per unit time, while the amount of energy the object absorbs per unit time remains the same. So the idea is that as this object’s temperature increases, more energy is transferred away from it through cooling per unit time. This generally is true for hot objects. The hotter an object is, that is, the higher its temperature, the more energy is transferred away from it per unit time through cooling.

At the same time, statement (A) is saying that the amount of energy the object absorbs per unit time remains the same. This is consistent with the fact that infrared radiation comes from our source at a constant intensity. Regardless of the object’s temperature, the object absorbs the same amount of infrared radiation per unit time. Option (A) then is looking like a promising candidate for our final answer.

Let’s look next at option (B). This says that as the object’s temperature increases, the object absorbs more energy per unit time. The trouble with this is that’s not consistent with the fact that the intensity of infrared radiation landing on the object is constant. Moreover, option (B) says that the amount of energy emitted by the object even as its temperature increases remains the same per unit time. This isn’t true either. A hotter object will tend to emit energy at a greater rate than a cooler one. For a couple of reasons then, we won’t choose answer option (B).

Looking next at option (C), this says that as the object temperature increases, the amount of energy absorbed by the object per unit time decreases. If this were true, we would expect either that the object becomes less able to absorb infrared radiation as its temperature goes up or that the intensity of radiation from our source is decreasing with time. Neither of these possibilities is true though. We expect instead that the amount of energy absorbed by the object per unit time to be constant.

So then this part of option (C)’s explanation is not correct. But notice that the part following it that says that the amount of energy emitted by the object per unit time increases is correct. This is the same as the part of our answer in option (A) that says the object emits more energy per unit time. Option (C) then is partly correct but not fully.

Knowing that we won’t choose options (B) or (C), let’s clear these away to make space for option (D), which we haven’t seen so far. This final answer option says this. As the temperature of the object increases, the amount of energy absorbed by the object per unit time increases. The amount of energy emitted per unit time by the object increases. Just like with answer choice (C), part of answer choice (D) is accurate. It is true that as the object’s temperature goes up, the amount of energy it emits per unit time increases.

But look at this other part of the statement. We’re also told in option (D) that the amount of energy absorbed by the object per unit time increases. This would mean that as the object’s temperature goes up, its ability to absorb infrared radiation also increases or that the rate of infrared radiation from the source is going up. Neither of these is true though. The amount of energy absorbed by the object per unit time does not increase. This confirms that option (A) is the only one of our statements which is completely correct. As the temperature of the object increases, the object emits more energy per unit time. The amount of energy absorbed by the object per unit time remains the same.

Let’s look now at the last part of our question.

After a period of time, the temperature of the object stops increasing and instead becomes constant. What is the reason for this? (A) The object has stopped emitting energy. (B) The object has stopped absorbing energy from the infrared source. (C) The object is emitting more energy per unit time than it is absorbing from the infrared source. (D) The amount of energy that the object absorbs from the infrared source per unit time is now equal to the amount of energy that the object loses per unit time through cooling.

Let’s think again of our source emitting infrared radiation and our object. The source gives off radiation at a constant intensity, and our object absorbs it at a constant rate. At the same time, energy is transferred away from our object through cooling. Even when the temperature of our object becomes constant, these two processes of radiation absorption and emission are ongoing. This means that answer options (A) and (B) can’t be correct. As long as the object continues to absorb radiation from the source, it will emit energy. And there’s no reason for it to stop absorbing infrared radiation from the source.

Answer option (C) claims that the object is emitting more energy per unit time than it is absorbing from the infrared source. If that were happening though, we would expect the temperature of the object to be going down. Instead, we’re told that that temperature is now constant. Even though option (C) is correct, that the object is still emitting energy and still absorbing it, the relative rates at which these happen is not described by option (C).

Lastly, option (D) says that the amount of energy the object absorbs from the source per unit time is now equal to the amount of energy the object loses per unit time through cooling. This option is describing a balance of energy transfer rates. The rate at which energy is transferred to the object through absorption is equal to the rate at which energy is transferred away from it. This is consistent with the object’s temperature neither increasing nor decreasing, but rather staying the same.

For our final answer, we choose option (D). The amount of energy that the object absorbs from the infrared source per unit time is now equal to the amount of energy that the object loses per unit time through cooling.

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