Question Video: Describing an Object That Has Reached Equilibrium | Nagwa Question Video: Describing an Object That Has Reached Equilibrium | Nagwa

# Question Video: Describing an Object That Has Reached Equilibrium Physics • Third Year of Secondary School

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An object is placed close to a source of infrared radiation of constant intensity. Initially, the temperature of the object begins to increase. After a period of time, the temperature of the object stops increasing and instead becomes constant. Which of the following statements best explains why the temperature of the object becomes constant? [A] The object has stopped absorbing energy from the infrared source. [B] The object is emitting more energy per unit time than it is absorbing from the infrared source. [C] The amount of energy that the object absorbs from the infrared source per unit time has become equal to the amount of energy that the object loses per unit time through cooling. [D] The object has stopped emitting energy.

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### Video Transcript

An object is placed close to a source of infrared radiation of constant intensity. Initially, the temperature of the object begins to increase. After a period of time, the temperature of the object stops increasing and instead becomes constant. Which of the following statements best explains why the temperature of the object becomes constant? (A) The object has stopped absorbing energy from the infrared source. (B) The object is emitting more energy per unit time than it is absorbing from the infrared source. (C) The amount of energy that the object absorbs from the infrared source per unit time has become equal to the amount of energy the object loses per unit time through cooling. Or (D) the object has stopped emitting energy.

So, this question is telling us that an object, say this hat, is close to an infrared source. This source is emitting energy at a constant rate in the form of infrared radiation. We’re being told that the hat’s temperature initially increases but then stops increasing and becomes constant instead. Let’s recall that how much infrared radiation an object emits per unit time depends on the temperature of the object. The higher the temperature, the more infrared radiation it emits.

So if the hat was at a low temperature before being placed close to the infrared source, it would emit some small amount of infrared radiation per second, for example. Once it’s placed close to the infrared source though, it starts absorbing more infrared radiation than it emits every second. This causes the temperature of the hat to rise as it gains more energy than it loses. But this increase in temperature also causes the hat to start emitting more and more infrared radiation.

Therefore, the hat’s temperature still increases, but the rate of increase becomes slower and slower. Eventually, the hat emits exactly the same amount of infrared radiation as it absorbs from the source every second. At this point, the temperature of the hat has become constant.

Our question asks why the temperature of the object, our hat, becomes constant. Answer option (A) suggests that the object has stopped absorbing infrared radiation. But this isn’t true, as the source continually emits infrared radiation and the hat is still close to it. We can therefore eliminate this answer option.

Option (B) says that the object is emitting more energy per unit time than it is absorbing. But if this were the case, it would be losing energy over time and its temperature would be decreasing. Therefore, this is also incorrect.

Option (C) essentially says that the object absorbs and emits the same amounts of infrared radiation per unit time. This seems to match what we’ve just seen, and is a good candidate for the correct answer.

Option (D) says that the object has stopped emitting energy. But this would only be true if the object were at a temperature of absolute zero, as it would no longer have any energy to emit. And this is not what is being described in the question.

Therefore, we choose option (C) as our answer. The amount of energy that the object absorbs from the infrared source per unit time has become equal to the amount of energy that the object loses per unit time through cooling.

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