Wikipedia:Reference desk/Archives/Science/2021 October 21

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October 21

Why Lights go dimmer even current increases?

Voltage drops occur when loads are increased and also an increase in current occurs. Lights that are on the same circuit to appear dimmer. Why they go dimmer ?Rizosome (talk) 04:42, 21 October 2021 (UTC)

What is the context in which you found these statements? Voltage drop occurs across any load. The statement does not make much sense; it is similar to "Gravity occurs when weights are increased". What is the circuit? The current may increase in one branch while decreasing in another branch. Does the current increase in the branch with the light source?  --Lambiam 06:48, 21 October 2021 (UTC)

It says here: As load current increases, the voltage drop in the wiring increases and the voltage delivered to the system drops. Rizosome (talk) 17:36, 21 October 2021 (UTC)

Take a circuit with two components in parallel. One component has a variable resistance, say an electric heater with an on–off switch. The other is a light bulb. Label the two end points of this parallel circuit A and B. This circuit is connected in series with a constant resistor (representing the resistance of the wiring) with end points B and C. The two ends of the combined circuit, A and C, are connected to a source with a constant voltage. There are two paths for electricity to flow: between A and B either via the variable resistance or via the light bulb, and then between B and C via the resistor. Assume the variable resistance is initially infinite (the heater is switched off). All current flows through the light bulb. Now assume the heater is switched on, so the resistance in its branch goes from infinite to fairly low. The combined resistance of the parallel part of the circuit, and therefore that of the whole circuit, then drops considerably. So now a much higher current flows between A and C, and therefore via the resistor between B and C. The voltage between B and C therefore increases, and that between A and B drops by the same amount. Therefore the current through the light bulb decreases. But in this new situation there is a strong current through the parallel branch with the heater. The sum of these two currents is higher than before.  --Lambiam 20:05, 21 October 2021 (UTC)

Can you give me circuit diagram of this bulb and electric heater? Rizosome (talk) 05:12, 22 October 2021 (UTC)

See here. For simplicity I have indicated the constant-voltage power source with the symbol for a battery, but the story is the same for an AC source as for DC.  --Lambiam 13:13, 22 October 2021 (UTC)

Do cetaceans hear in the air

Cetaceans have peculiar auditory system evolved specifically for underwater hearing. Does it mean that they are completely deaf when out of water (for example, when stranded ashore or when they are transported in hammocks)? Эйхер (talk) 16:40, 21 October 2021 (UTC)

NOT A RELIABLE SOURCE, but this says that they can. --Jayron32 16:47, 21 October 2021 (UTC)

It is well known that cetaceans being in water are able to hear sounds produced above the water. For example, dolphin trainers routinely use whistles for cueing dolphins. But it is usually assumed (or at least, assumed by some trainers) that in this case the sound passes from the air into the water. I mean the case when the animal is completely out of the water. For example, do dolphins, when transported by plane in hammocks, suffer, like terrestrial anumals (including humans), from the noise, or they rather suffer from dead silence? Эйхер (talk) 05:08, 22 October 2021 (UTC)

The acoustic properties of a dolphin aren't too dissimilar to those of water. If the sound can pass from air to water, it should be able to pass directly from air to dolphin. Most of the sound will be reflected though. The whole purpose of the middle ear in terrestrial animals like humans is to transfer the sound from gas to liquid without too much reflection. PiusImpavidus (talk) 08:24, 22 October 2021 (UTC)

The problem is that, as far as I can understand (being not a biologist), cetaceans have degenerate middle ear with disfunctional ossicles and rely instead on bony hearing, that is enhanced in them through some sophisticated adaptations. Эйхер (talk) 13:09, 22 October 2021 (UTC)

Industries that need electrical power just for part of the day?

Are there any industries that need reliable electricity, but can tolerate having it for only part of the day [not just day-night!] because it can shut down and up with little difficulty? I am thinking things like Chloralkali process. JoJo Eumerus mobile (main talk) 16:47, 21 October 2021 (UTC)

"Industry" is very broad. We got along just fine before electricity came along. Industries that don't rely on electricity include Commercial fishing, most types of Agriculture, Haulage, Forestry, the Military, most forms of Transport, many divisions of the Construction Industry, and Tourism.--Shantavira|^{feed me} 08:34, 22 October 2021 (UTC)

Well, yes, to some extend. With the rise of intermittend, clean energy sources like wind and solar, we need such industries that can adapt their usage to the available electricity. Some industries can rapidly adjust their power use and production rate to available power. Completely shutting down may be a bit much. Aluminium smelters are one example. When electricity gets expensive, they shut down some of their electrolysis cells, just supplying them with enough power to keep the contents molten. Not using the factory at full capacity at all times takes money, but they recuperate this with on average cheaper electricity. Clean hydrogen production is another one. Storing energy in hydrogen isn't very efficient, but if you only make hydrogen when energy is free, it may pay off. PiusImpavidus (talk) 08:36, 22 October 2021 (UTC)

@PiusImpavidus: Aye, I was wondering about aluminium smeltering and electrolysis. Out of curiosity, do you have any sources that discuss these aspects? Jo-Jo Eumerus (talk) 10:13, 23 October 2021 (UTC)

@Jo-Jo Eumerus:Nothing but what I read in the newspapers. PiusImpavidus (talk) 08:01, 27 October 2021 (UTC)

Doctor House and Bipolarism

In the episode Failure_to_Communicate, we saw a person, who has a bipolar disorder. The amazing thing is that the patient is manic during the day and sleeps off his depression at night. Is this account plausible? A bipolar disorder that changed within one day?--2A02:908:426:D280:D82B:8A4C:3F13:8022 (talk) 18:38, 21 October 2021 (UTC)

I have not seen the episode, but our article states that the patient is thought to have been using amphetamines during the day and sleeping pills at night. Amphetamines are a dopaminergic stimulant of the central nervous system that can induce euphoria similar to a manic state, so it seems plausible to me that in the logic of the plot the patient took both forms of self-medication, day and night, to counter symptoms of depression.  --Lambiam 10:37, 22 October 2021 (UTC)

Thermodynamics question

Suppose that the inside of a house is initially at the same temperature as the outside. A heat pump is then used to raise the inside temperature by some given amount. This uses x amount of supplied energy to drive the heat pump. A heat engine is now employed to extract work from the inside-outside temperature difference, and this produces y amount of mechanical energy. Is the minimum possible x equal to the maximum possible y? Please note that this question is about theoretical limits under laws of thermodynamics, not about what can be achieved practically. 109.147.111.142 (talk) 20:59, 21 October 2021 (UTC)

No. Impossible. One of the consequences of the Second law of thermodynamics is that the processes you described will never occur with 100% efficiency - some of the energy supplied to the pump and the engine will be unavailable and will leak into the surroundings in the form of heat at a low temperature. Dolphin (t) 21:09, 21 October 2021 (UTC)

The fact that not all the heat energy inside the house can be recovered in usable form is incorporated into the question as an essential part of it. 2A00:23C8:7B08:6A00:BC7D:139D:88FE:38A5 (talk) 21:23, 21 October 2021 (UTC)

See Carnot's theorem and Carnot cycle. Эйхер (talk) 21:16, 21 October 2021 (UTC)

I am aware of those articles. It is beyond me to figure out the answer to my question from this. 2A00:23C8:7B08:6A00:BC7D:139D:88FE:38A5 (talk) 21:32, 21 October 2021 (UTC)

Assume your heat pump is the best theoretically possible. It runs at Carnot efficiency. The heat pump does not increase entropy, so it's reversible. So you can run your heat pump in reverse without decreasing entropy, and you get exactly the heat engine you use to extract work from the temperature difference. So, indeed, the minimum work you need to run the heat pump equals the maximum work you can extract with the heat engine. PiusImpavidus (talk) 08:43, 22 October 2021 (UTC)

• (edit conflict) Yes (almost). Per Coefficient_of_performance#Theoretical_performance_limits: At maximum theoretical efficiency, [the ratio of transferred heat to work for a heat pump] is equal to the reciprocal of the ideal efficiency for a heat engine (thus the product of the two efficiencies is close to 1). The paragraph also gives a short derivation (the "it can be shown that..." part is a consequence of Entropy#Reversible_process).

Of course, that is only with a perfect heat pump and a perfect heat engine, so the actual performance will not reach that. The table at Heat_pump#Performance indicates that actual heat pumps reach about 25% of the maximum efficiency. I am not sure what the situation is for a plausible heat engine in the context of the question, but surely it must be worse than power plants that from my memory reach 1/2 to 2/3 of Carnot efficiency. The total yield on one cycle (y/x in the terms of the question) would be around 10-20% with those numbers, quite far from 100%. Tigraan^{Click here for my talk page ("private" contact)} 09:23, 22 October 2021 (UTC)

Let me clarify one important point implied in the question by selecting a few representative temperatures. Let's assume the temperature outside and inside is a uniform 10°C. The heat pump produces air at 30°C and this air enters the house, raising the inside temperature by "some given amount". Let's assume that given amount is 10°C so the temperature inside the house rises to 20°C. Some responders to this question have assumed that the heat engine would have access to air at 30°C. However, the question appears to be based on the heat engine only having access to air from within the house which is at a significantly lower temperature than 30°C. This indicates another reason why the proposal is impossible. Dolphin (t) 10:57, 22 October 2021 (UTC)

Well, if the system experiences no heat losses, then the air temperature inside becomes asymptotically close to 30°C, and given sufficient time, will be immeasurably close to 30°C; which is to say given an arbitrarily long length of time, the inside temperature will reach our target temperature to within the limits of our measurements. --Jayron32 11:17, 22 October 2021 (UTC)

When you blow 30°C air into a 10°C room, the mixing is irreversible, so entropy increases and the efficiency must be less than Carnot efficiency. For maximum efficiency, you take air from the room, make it infinitesimally warmer with heat pumped from outside and blow it back into the room. The Carnot efficiency is the limit, which you can approach arbitrarily closely, but in reality, you rarely get anywhere near. But then, OP asked about the theoretical limits, not what can be achieved practically. PiusImpavidus (talk) 14:48, 22 October 2021 (UTC)

• Thanks a lot for the replies. Here in the UK, heat pumps are very prominent in the news right now. While I have read numerous explanations about how these work on a practical and even theoretical basis, I have found no explanation anywhere that helps one gain any intuitive understanding of how on earth it is possible to get more energy out of a heat pump than one puts in when the outside temperature is colder than the inside. Per my comment at Talk:Heat_pump#Suggestion_for_improvement, it seems rather like pumping water uphill and finding that one has gained more energy than one has supplied. If the answer to my original (self-devised) question is yes, then, for me, this is the first glimmer of understanding about how heat pump >100% "efficiency" may be possible. In my scenario, not all the heat can be recovered from the house as usable energy, which I understand, and so if the process is (theoretically) reversible then it must imply that the house can be heated with less energy than the resulting heat energy in the house. However, I think that this observation may not be very helpful for most people. Anyway, if anyone has any bright ideas how to address my comment at Talk:Heat_pump#Suggestion_for_improvement, I think it would be to the benefit of that article. 2A00:23C8:7B08:6A00:F55B:27D8:6291:2C56 (talk) 21:59, 22 October 2021 (UTC)

I think the answer to your question is that the heat pump in that scenario is using a different heat reservoir than you're imagining. A Ground source heat pump can be a source of heat (or cool!) even in situations when the outside air is not the right temperature.

If you go far enough below the frost line, the ground temperature is basically the year-round average temperature. In many places, that's ideal for a heat-pump. ApLundell (talk) 04:45, 23 October 2021 (UTC)

That doesn't seem to be answering the question. Air source heat pumps are getting increasingly common, both for heating internal air but also for heating water in large part due to climate change concerns. In countries with mild temperatures (e.g. UK [1], France [2] [3], Germany [4], Australia [5] [6] [7], NZ [8] although our articles notes they are useful even in the Yukon on Canada with temperatures down to -30 °C with the right heat pumps), they do use less electricity than a simple electrical resistive heater, generally one of the key reasons they are getting more popular.

To be clear, electric resistive heaters directly convert electricity into heat at close to 100% efficiency (of the electricity which reaches them not from the PoV of production) so the fact heatpumps generally use less electricity is I think the source of the OP's confusion as it is for a lot of people. And they are nearly always used when the outside temperature is cooler than the outside or reverse if being used to cool the air inside i.e. to increase the temperature difference. As water heaters, the temperature differential is generally even greater. Ground source hear pumps would generally use even less electricity to produce the same heating (or cooling) effect but the high price of installation and maintenance means in a lot of countries especially those with mild temperatures and for installation in existing buildings, they're still rare or at least far less common than air-source ones.

There is some concern of their increase electricity use. One is because for internal air heaters, they will be used in reverse direction i.e. to cool the air during summer whereas in the past most people would have just used fans and ventilation so this leads to an increase electricity use during summer even in cases where their adoption leads to a decrease during winter. The other because in many countries they are not being used to replace electric heaters but instead some form of combustion heater. But for the latter, the desire is that the country will produce enough of their electricity by renewables or other forms of generation that don't contribute significantly to increased emissions of greenhouse gases.

The first part of an answer to the question seems to be the OP is misunderstanding what a heat pump is doing. Despite the fact you are moving more heat than you could produce via a simple resistive heater, from a cooler outside to a warmer inside, it's not accurate to say you are producing more energy then you put into the system. Probably next part is to remember that even air source heatpumps are moving heat from or to effectively a massive reservoir as it's not just the air outside the structure that's the source of the heat.

Nil Einne (talk) 07:56, 23 October 2021 (UTC)

"confusion" is not a fair word to use in respect of my post, nor is there any "misunderstanding" as to what the heat pump is doing. "get more energy out of a heat pump than one puts in" is just somewhat loosely worded: I don't mean more energy overall, in the whole Universe. I am well aware that the extra is moved from the outside. The difficulty is in understanding how this is possible when the outside temperature is colder. I don't understand how the outside being a "massive reservoir" helps to explain this. Common responses to questions about this are inability on the part of the responder to understand the question, and/or mistaken assumption that the person asking the question has made some blindingly simple error of understanding about what is going on. I wish we could get past these stumbling blocks and onto the actual point. In response to the earlier reply, whether the outside source is air, ground or water is completely irrelevant. 2A00:23C8:7B08:6A00:AA:B98:8E44:3013 (talk) 09:45, 23 October 2021 (UTC)

Many people, including me, have wondered how a heat pump works exactly, and how it is possible to move x joules of thermal energy from a lower temperature to a higher temperature without having to expend the same amount (x joules) of electrical energy, little realising that most of us already own a heat pump called a refrigerator! A refrigerator takes x joules of heat from the cold storage space and pumps it out into the kitchen where it slightly raises the temperature of the air inside the kitchen. (That small rise in temperature of the air in the house is a nuisance in the summer but slightly beneficial in winter.) A conventional heat pump does the same thing except that instead of taking heat from a small enclosed cold storage space it takes it from the environment. Both a refrigerator and a conventional heat pump raise the temperature of their working fluids to a bit above room temperature in order to discharge heat into the interior of the house, thereby raising the temperature inside the house. Dolphin (t) 12:52, 23 October 2021 (UTC)

As far as the oft-repeated refrigerator analogy is concerned, please see my comment at Talk:Heat_pump#Suggestion_for_improvement. 2A00:23C8:7B08:6A00:15E5:2AA5:F4D3:2978 (talk) 19:04, 23 October 2021 (UTC)

A heat pump is not like “a refrigerator in reverse”. Thermodynamically it is exactly like a refrigerator. I will think about your request at Heat pump and make a suggestion. Dolphin (t) 21:25, 23 October 2021 (UTC)

Reverse effect. Reverse desired outcome. If you are to think about it, please be sure that you actually understand the question. Two replies in this thread have been on topic, those of PiusImpavidus and Tigraan. Everything else is just a useless waste of bandwidth. There are too many people at this forum eager to jump in with totally irrelevant and useless contributions, not even understanding the question, let alone the answer. 109.147.111.142 (talk) 22:02, 23 October 2021 (UTC)

At the Wikipedia Reference Desks, everyone is welcome to ask a question, and everyone is welcome to offer an answer. It is unlikely that all answers will be consistent; it is unlikely that all will be correct. There is no central coordination of answers, or censorship on the grounds of technical correctness. If you don’t like this approach, or you want to nominate who can and cannot offer an answer, you are on the wrong website. Dolphin (t) 23:44, 23 October 2021 (UTC)

We do have Reference Desk guidelines, one of which is: Do not offer answers on topics on which you are not qualified, so perhaps not everyone is equally welcome to offer an answer. Unfortunately, the Dunning–Kruger effect can manifest itself here too.  --Lambiam 11:04, 24 October 2021 (UTC)

In the article Heat pump I have inserted a new section Principle of operation. Hopefully readers who are new to the concept of the Heat pump will find it helpful. Dolphin (t) 12:46, 27 October 2021 (UTC)