## Why is a Metal Plate “Colder” Than a Plastic Plate?

No, it isn’t!

### What is Cold?

According to the dictionary, a body at a relatively lower temperature, especially when it is compared to the temperature of a human body is described as a colder one. So, any object below the normal human body temperature – about 37 degrees Celsius – is a cold thing. But wait a minute!

When you touch an object, what does it tell you about the temperature of the object? Can you really judge if it is a cold one or a hot one? Unfortunately, our bodies aren’t thermometers, we are not so smart when it comes to judging the temperature. Consider the following case.

A book and a steel plate kept in the same environment for a long time attain the same temperature eventually (it is called thermal equilibrium). This can be checked by using a thermometer on both the objects. But, when people are asked to touch a metal plate and a book, they find the former to be much cooler. You can try this out yourself by touching different materials around you. You’ll see how some things ‘feel colder’ while the others feel warmer. A YouTube channel Vertasium conducted a social experiment to record this on camera. See the video below:

### There is no cold – only heat

So, in the video, ice melts faster, if kept on steel plate than on a plastic plate, even when the steel plate ‘feels colder’. Common sense dictates that the colder thing is supposed to sustain the ice block for a longer time, just like your refrigerator does. So why does the opposite happen?

A better way to understand this ‘contradiction’ (not really a contradiction) can be this:

According to thermodynamics, simply put, everything has heat in it. So, even a cold ice block has some amount of heat stored in it (say, around 273.15 Kelvin or 0 degree Celsius). When one object comes in contact with other object, it loses or gains heat till their temperatures get equal or till they attain ‘thermal equilibrium’. Which object loses heat and which one gains it, is decided by their relative temperatures. In case of ice and steel, ice has a lower temperature than steel (assuming it isn’t already freezing out there). Therefore, here, ice gains heat from steel till they attain the same temperature and ice melts.

Side note: The ice is also in contact with a relatively ‘hotter’ atmosphere. Hence, it gains heat from there also. In this case, we are only concerned about the steel and ice interaction.

### Why does it melt faster on steel?

There is a particular property which depends on the kind of material and is called thermal conductivity. This is the parameter which decides which objects lose heat quicker and which ones do it slower.

Here, for instance, steel has a higher thermal conductivity than plastic. Hence, the steel plate gives away heat to the ice block faster than a plastic block does. As a result, ice melts faster on a steel plate than on a plastic one.

Incidentally, this effect can also be used to explain why one plate feels colder than the other, in our hands. Think of it like this, the ice is replaced by our hand. So, a steel plate, due to its better thermal conductivity, draws heat faster from our hand than a plastic plate. This makes us feel that the steel plate is colder than the plastic one.

As checked by a thermometer, both the plates have the same temperature, our bodies are only fooled into believing that the thing we feel is temperature; it isn’t. None of the plates is actually colder than the other (according to the dictionary – see first paragraph). We don’t feel the temperature. What we feel is actually the rate of heat being drawn away from our hand. Faster an object draws heat, the colder it feels.

## How Loud Can it Get?

###### by Anupum Pant

Wives and moms can scream really well. But is it loud enough to inflict physical pain? Can sounds get louder than a nuclear bomb? How much damage can a loud sound cause? How about mass extinction? Read on to find out the answers.

### What is sound?

Sound, as most of us know is a longitudinal, mechanical wave. That means, it is just a series of pressure changes [compressions and rarefactions] in a particular medium. So, the property of sound is as good as the medium it uses to travel. For instance, sound cannot travel in a vacuum due to the absence of any medium, but it can travel much faster in solids than in air. That is the reason you can’t hear someone talking in space (yes, movies that show loud explosions in space, lie). Also, the faster speed of sound in steel rails is exactly the reason why, you can tell a train is approaching, if you stick your ears to the rails (do not try this on electric rails).

Two of the fundamental parameters that describe a sound wave in numbers are pitch and amplitude. Pitch is measured in hertz – we’ll talk about it some other day. But, the amplitude of a sound wave determines how powerful it is; greater the amplitude, louder the sound. The loudness of sound is measured in Decibels (abbreviated dB).

Like most other linear scales, Decibel isn’t as easy to understand. A 10 point rise in the dB scale can be visualized as a 10 times increase in the loudness. Adding dB levels of different sound sources also doesn’t really work, the calculation is much more complicated; the resultant loudness depends on the coherency of the source [See this decibel addition applet]. Also, the perceived loudness is obviously lesser as you go away from the sound. Normally, a decibel scale ranging from 0 dB to 130 dB is enough for measuring the loudness of most things. But, things can get louder…much louder.

To get an idea of the decibel scale: 10 dB is 10 times more powerful than 0 dB, not 10 points greater. Similarly, 20 dB is 100 times more powerful and 140 dB is 100,000,000,000,000 times more powerful than a o dB sound.

0 dB is the loudness of near silence (a mosquito 10 feet away), while 120 dB is the loudness of a loud car horn heard from 1 meter away. Humans can hear sounds starting from 0 dB. But it can be quieter than 0 dB [the world’s quietest room]. It measures record setting -9 dB and can literally drive you crazy. In fact, the longest someone stayed in that room was for 45 minutes.
On the upper side of dB spectrum, a whisper is around 15 dB, conversations range from 40 – 60 dB and a jet engine measures 130 dB on the decibel scale. Like I said before, the perceived loudness depends on your ear’s distance from the source, so the loudness of a lawnmower can range anything from 90 dB to as much as 110 dB if you stand 3 feet away from it. [see the Decibel chart]

90 Decibels or a sound as loud as only a raised voice can cause gradual hearing loss (Refer to the hearing safety chart here). While 140 dB can cause physical pain. After 150 dB (firecracker) sounds can be felt in the form of shock waves. The pressure difference they cause in the medium can actually be felt by your body.

### Beyond Decibels

Since the loudness depends on the medium, the maximum loudness a medium can propagate is dependent on its density. Our atmosphere can do nothing more than 190 dB, that, by the way, is enough to make you deaf or cause death. Sounds in water can get louder. A pistol shrimp is able to create a 200+ dB sound at 97 km/h to stun or kill its prey by snapping claws really fast. This is a very short lived pulse which doesn’t carry enough energy to do us any harm.

For events like the Saturn V launch, volcanic explosion, nuclear bomb explosion, earthquakes, star-quakes the concept of sound doesn’t really apply anymore. They are measured in terms of the shock wave they produce using the Richter scale. On this scale, 9 means total destruction (8.2 was measured during the explosion of the largest bomb ever, Tsar Bomba). An earthquake or earthly event measuring 10 has never been observed.

However, in the universe beyond earth, the starquake on the magnetar SGR 1806-20 registered 22.8 to 32 on the Richter scale. The magnetar released more energy in one-tenth of a second than our sun has released in 100,000 years. An event which thankfully took place 50,000 light-years away from earth. Had it been even 10 light-years away, the energy released would have wiped off life on earth. [read this BBC article for more information on this event]