21. Why can you not get sunburned behind a glass?

To completely answer your question, we need to know a little about the sun & a little about the properties of glass.

Let start with the sun. We all know that the sun is the cause of the sunburn but we should be aware that light coming from the sun, even though it looks yellow, contains many colors (many wavelength). In fact the sun emits a spectrum of color (wavelength) & we see it yellow simply because it emits more yellow than red & blue for example. Now we have to also note that, along with red, yellow & blue, the sun emits radiation that is not visible & part of it is called ultraviolet (UV). We cannot see this color because our eyes are not made to detect it but the sun emits plenty of UV. In fact this UV light is so strong & energetic that it burns our skin hence the sunburn. Now about the glass.

Glass is transparent because it lets all the colors go through it but what we can not see is that it acts as a filter for UV light. Most of UV radiation is stopped by glass & this is why you will not get sunburns behind a glass. The glass simply filters out the UV radiation that is responsible for the sunburns & protect your skin from these energetic & somewhat harmful radiation. Sunblock lotions also work exactly the same way, they simply filter out UV light.

 

 

22. When a mercury thermometer is immersed in a hot liquid, the mercury level goes down at first and then goes up. Why?

 

They are the thermal expansion and the thermal conductivity of the materials that compose the thermometer.

Glass has a relatively low thermal conductivity, as well as low expansivity, but mercury is a good conductor and has a good expansivity as well.

As energy flows into the thermometer the glass warms first and starts to expand and its volume increases. Because of the low thermal conductivity of the glass, heat can not reach mercury. So, at the start, mercury does not expand initially. So its volume doesn’t change initially and the height of the mercury column actually decreases due to the larger capillary volume of the glass. Eventually when heat does reach mercury, it expands to a high degree, easily exceeding the expansion of the glass. So the level of mercury then starts to rise.

 

23. How do people keep warm in an igloo? What's the underlying Physics behind it?

The principle behind an igloo is hidden in the material it's constructed out of. Igloos are normally built from compressed snow, which is sawn into blocks, and then these blocks are stacked around a hole, which is dug out after the blocks have been set. Solid ice is a poor insulator, when compared to compressed snow. The snow has many many more air pockets per cubic foot, and is also lighter.

Also, igloos do not have flat bottoms. The inside of the igloo is terraced, the uppermost level being where the people sleep, the middle is where the fire is and the work takes place, and the bottom level actually is a "cold sump". The principle is that all the coldest air from inside the igloo runs downward off the terraces and collects in the bottom, thus allowing the upper portions to stay warmer.

The entrance for the igloo is usually at the bottom, and includes at least one right angle, which keeps the high winds from blowing straight into the igloo and chilling the residents or blowing out the fire. They also all have a small hole on the top that keeps the smoke from building up inside the igloo. All of these factors take advantage of underlying physics, and the temperature inside an igloo is likely to be 20 degrees or so, while the outside temperature in northern regions can drop down to -50 degree Fahrenheit during the daytime. 20 degrees may not be what some consider to be comfortable, but a 70 degree difference is certainly welcome somewhere so cold.

 

24. Can you boil water without heat? Why and how does it occur?

The short answer is: Yes.

Now to the details. Boiling is the spontaneous change of water from the liquid phase to the gas phase. However, as is true for all materials, temperature is not the only parameter that determines the phase; there is also pressure that affects its phase. For example, if the pressure is low, molecules can rise up and spread (i.e. vaporize) even without sufficient heat. This means their boiling point is lowered on decreasing the pressure. However, if the outside pressure is increased, molecules feel difficult in rising and spreading. So they have to be given higher energy if they are to rise, i.e. they have to be heated to a higher purpose for the purpose. Therefore their boiling point is raised on increasing the pressure.

The familiar 100 °C (212°F) is the boiling point of water at 1 atmospheric pressure (i.e., the mean pressure at sea level).

So, if you want to boil water, you have two options:

1. Add heat until the temperature reaches the boiling point at the fixed pressure.

2. Reduce pressure (e.g. by vacuuming the container water is in) until the boiling point at that pressure drops below the fixed temperature.

So, if you would rather not heat your water, you can lower the pressure around it. Then it will boil, but of course it does not mean that it is "hot"!

 

25. Why does ice melt faster in water (at room temperature) than in air?

Keep in mind that temperature is the average kinetic energy of the particles in a substance, whereas heat is the total energy of all the particles. The air and water may have particles moving the same speeds, but the water has more heat because there are more particles.

Picture it this way. The ice melts as fast moving particles slam into the slow moving ice particles. As in any collision, some of the energy is transferred to the ice particle, and with its new energy it can break out of the crystal and flow as a liquid water molecule.

To make the ice melt faster, you can use hotter (faster moving) particles to slam into it. This is why the ice melts faster on a hot day than a cold one. Alternatively, you can just use more collisions. The water is much more dense than the air, with many more particles per cubic millimeter. Thus even though the water molecules have the same kinetic energy as the air particles, there are many more of them hitting the ice each second, compared to air and so the ice melts faster.

 

26. Will a lake freeze faster on a windy day than a calm day?

Evaporative cooling could cause the overall temperature of a lake to cool faster on a windy day than on a still one of the same temperature.

Any liquid has a thin layer of vapor over its surface, including water. This layer of gas is made up of a transient population of molecules with enough kinetic energy to separate from the rest of the molecules in the liquid and enter the gas phase. At any given temperature, the molecules in this layer will reach an equilibrium pressure, with a higher pressure for higher temperatures due to the higher average energy of the particles.

A windy day can pull can strip away some of the molecules in this vapor layer, thereby reducing the overall average energy of all particles. This is because the particles in the layer is made up of the molecules with a higher average energy than those in the liquid state, and so removing them lowers the overall average. Thus wind will speed energy loss from the lake. This is essentially the same reason you will feel cooler on a breezy day compared to a still one.

 

27. How does a thermos flask retain heat?

A thermos flask has mirrored walls inside that reflect the escaping heat back inside. It also has a vacuum partition between the walls, which helps stops heat from escaping by conduction.

 

28. Why does milk expand when it is cold?

Milk is made mainly of water, which has the curious property of expanding when it freezes [Remember, the density of water is around 1 gm/cm³ (1000 kg/m³), whereas the density of ice is 0.92 gm/cm³ (920 kg/m³)]. This is because the molecules in water rearrange themselves into a crystal structure, which just happens to take up more space than the normal, liquid structure of water. So milk, which is mainly water, also expands when frozen.

 

29. Why does a kettle go quiet just before it boils?

As water is heated, dissolved gases in the water start to come out of solution. The process goes on increasing as the temperature of water increases, As it approaches the boiling point, all the dissolved gases have been released so there is no more bubbling. This is the exact time when the kettle goes quite.

But as the water then begins to boil, the convection currents in the water become so violent that water becomes noisy again.

 

30. Why does the boiling point of a liquid rise under high pressure? (Why do liquids boil at higher temperatures on increasing pressure?)

Boiling means sending liquid molecules from within the periphery of liquids to the outside through their boundary, i.e. the surface. This definitely requires some energy since the molecules have to be lifted and work has to be done against gravity. This energy is provided by the heat provided by different sources from below.

Now the degree of difficulty in sending the molecules off the surface depends on the gas or air pressure outside the surface. If the pressure there is high, molecules can not rise easily. If they are to be lifted i.e. in other words, if they have to be boiled, they have to be given more energy so that they can overcome this extra pressure. This is only possible when they are heated to a higher temperature (higher temperature means higher energy).

It means if they are to be boiled they have to be heated to a higher temperature, i.e. the temperature at which they boil, the boiling point will rise.

 

31. Why does the boiling point of a liquid fall on reducing pressure? (Why do liquids boil at lower temperatures on decreasing the pressure?)

Boiling means sending liquid molecules from within the periphery of liquids to the outside through their boundary, i.e. the surface. This definitely requires some energy since the molecules have to be lifted and work has to be done against gravity. This energy is provided by the heat provided by different sources from below.

Now the degree of difficulty in sending the molecules off the surface depends on the gas or air pressure outside the surface. If the pressure there is low, molecules can not rise easily. However if the outside pressure is low, the molecules can rise much more easily, without much effort. If they are to be lifted against high pressure, i.e. if they have to be boiled at high pressure, they have to be given more energy so that they can overcome this extra pressure.

In case of low outside pressure, the case is just opposite. They rise and escape even before the boiling point is reached. It means if they are to be boiled in such case, heating to a lower temperature is enough. So, the temperature at which they boil, i.e. the boiling point will be less.

 

32. Why is cooking difficult at higher altitudes? (Why does food remain raw at higher altitudes, even though they seem to give vapor?)

Cooking of food is a process in which certain chemical processes occur in the organic materials of food so that they become easier to digest and good to test. A 'cooked' food item does show certain characteristics. Such processes require certain temperature, which varies very much according to the nature of food itself.

Now to the details. Boiling is the spontaneous change of water from the liquid phase to the gas phase. However, as is true for all materials, temperature is not the only parameter that determines the phase; there is also pressure that affects its phase. For example, if the pressure is low, molecules can rise up and spread (i.e. vaporize) even without sufficient heat. This means their boiling point is lowered on decreasing the pressure. However, if the outside pressure is increased, molecules feel difficult in rising and spreading. So they have to be given higher energy if they are to rise, i.e. they have to be heated to a higher purpose for the purpose. Therefore their boiling point is raised on increasing the pressure.

However, when the outside pressure is less, the reverse happens. Water boils at a lower temperature than usual. Because of this reduction at higher altitudes, the outside atmospheric pressure is low, so the food might boil, but the boiled food materials are still at lower temperature, i.e. they have not yet reached the required temperature and they have not received the required amount of heat energy for initiating the chemical and biological processes for changing the status of the food substance. So though they may vaporize or show usual signs of a cooked food, they still remain raw inside.

 

33. A clock that has brass pendulum beats correctly at 30⁰C. Will it run faster or slower if the temperature a. decreases b. increases?

As the time period T of a pendulum clock having the length of pendulum l and acceleration due to gravity g is given as

From above relation, the time period varies directly with the square root of the length of pendulum and the length also varies directly with temperature. If the temperature is decreased then length of pendulum decreases due to contraction which decreases the time period but if the temperature is increased then length of pendulum increases due to expansion which increases the time period of pendulum. Hence, the clock runs faster when the temperature is decreased and runs slower when the temperature is increased.

 

34. A hole of 1cm is drilled through a piece of steel at 20⁰C. What happens to the diameter of the hole as the steel is heated to 100⁰C ?

If the steel is heated to 100⁰C then the diameter of the hole increases due to thermal expansion. When the object is given thermal energy, it undergoes expansion - not contraction. Hence, increase in temperature will increase the diameter of the hole i.e. expansion of hole occurs.

 

35. Why are glass windows possible to be cracked in very cold regions?

In very cold regions, the glass windows contracts more during cold night and expands less during the day which results in the continuous expansion and contraction process that results in the cracking of the glass windows. The another reason for glass windows to cracked is due to the temperature difference between the two sides of the window. As the outside of window is very cold due to cold temperature and inside of window is almost at the higher temperature needed for human beings for survive in the cold region. This temperature difference causes the molecules of window to contract and expand continuously at a time. This process continues for long period causing the cracking of glass windows in very cold regions.

 

36. Small space is left between two rails on a railway track. Explain why?

The rails are heated either due to increase in temperature during summer days or due to friction of the wheels and rails. If the rails are heated then they expands irregularly resulting in the bending of rails, resulting in accidents. In order to allow expansion of rails and to prevent the bending of rails, small space is left between two rails on a railway track.

 

37. Why do clocks go slower in summer and faster in winter?

As the time period T of a pendulum clock having the length of pendulum l and acceleration due to gravity g is given as

From above relation, the time period varies directly with the square root of the length of pendulum and the length also varies directly with temperature. In summer, temperature rises and length of pendulum increases due to expansion which increases the time period but in winter temperature falls and length of pendulum decreases due to contraction which decreases the time period of pendulum. So, clocks go slower in summer and faster in winter.

 

38. Explain why steel tyres are heated before slipping them into the wheel?

Steel tyres are generally used as a protection to the rubber of the wheels. They have to be tight and long lasting if their are to perform their job properly. When steel tyre is heated then it expands and it can be easily slipped into the wheel as the diameter of the tyre increases and when tyre cools then it contracts and grip the wheel tightly. Hence, steel tyres are heated before slipping them into the wheel.

 

39. A beaker is full of water at 4 ⁰C. What will happen if the temperature of the water (I) decreases (II) increases? (ignore the expansion of vessel)

A beaker contains full of water at 4⁰C. As the volume of water at 4⁰C is minimum so, if the temperature of water is decreased to 0⁰C then its volume increases as the density decreases so, the volume of water rises in the beaker resulting in overflow of water.

When the temperature of water is increased beyond 4⁰C, water expands and again overflows.

 

40. Why are telephones wires taut in winter and slag in summer?

During summer, due to the high temperature the expansion of wire occurs so it becomes slag but in the winter due to the cold temperature wires contracts and become taut. Hence, telephones wires are taut in summer and slag in summer.