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.
