How do erasers remove pencil marks from paper?

When a person writes, graphite particles wear off the pencil tip.

Erasers are used to remove the graphite marks made while writing on paper with a pencil. Erasers are generally made of 'sticky' substances like plastic, rubber, vinyl, gum, etc

When a person writes, graphite particles wear off the pencil tip and get interlocked with paper molecules. The soft action of the eraser causes the graphite particles to stick to it instead of the paper, as graphite has a stronger attraction to the eraser molecules than to paper. The 'dust left over after rubbing consists of worn rubber clumps mixed with graphite particles. Some types of erasers tend to remove the top layer of paper along with the graphite particles and rubbing the same spot repeatedly can make holes in the paper.

Did you know graphite has a stronger attraction to the eraser molecules than to paper?

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IF FRESH GRAPES ARE PUT IN A THICK SUGAR SYRUP THEY SHRIVEL UP IN A FEW HOURS. WHY?

If two solutions of different concentrations are separated by a semi-permeable membrane (that is, a membrane that allows some substances but not others to pass through), the water from the weaker solution will start moving into the stronger solution. This is the process of osmosis.

 The flow will continue till the solutions on both sides have the same concentration. When grapes are put in the thick sugar syrup, the skin of the grapes act as a semi-permeable membrane and water from the grapes moves into the sugar syrup. The grapes shrivel up as a result.

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WHY DOES AN EXPANDING RUBBER BAND BECOME WARMER?

Try it with a rubber band. Stretch a rubber band and quickly hold it against your cheek. It feels warmer. Usually when things expand, a gas or liquid, for example, they become cooler.

When a liquid (or gas) expands, it takes the energy required for expansion from the liquid itself. So the liquid becomes cooler.

When a rubber band is stretched, it is not using up its internal energy. Energy is being supplied to it from the outside (by the one who is stretching it). So its internal energy increases and its temperature rises, making it slightly warmer to the touch.

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WHY DO SOME SURFACES GET WET?

Find out from this experiment

If water is poured on a glass slab, the glass becomes wet; but if mercury is poured on the same glass surface it does not become wet. Why?

Whether a surface gets wet or not depends on

 1. The force of attraction between the molecules of the liquid and the molecules of the surface with which it is in contact, and

2. The cohesive force between the molecules of the liquid.

In the case of mercury, its molecules hold on tightly to each other. The molecules of the glass surface cannot pull them away. So the surface does not get wet.

In the case of water, the adhesive force is greater than the cohesive force between the water molecules. So the water clings to the glass surface, wetting it.

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WHEN THE CEILING FAN GETS DIRTY, WHY DOES MORE DIRT COLLECT ON THE EDGES OF THE BLADES?

It's time to clean your ceiling fan. But you'll find it's tough to clean the edges of the blades...

Static electricity develops on the blades of the fan when they are in motion, due to friction with the air. The static electricity attracts dust particles. As the edges of the blades are the first to come into contact with the surrounding air, more dust collects on them than on the other parts of the blades.

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WHEN WE SHAKE SOFT DRINK VERY FAST THEN THAT COMES OUT WITH THE PRESSURE FROM THE BOTTLE. WHY IS THIS HAPPEN?

Do you enjoy an aerated soft drink? Have you noticed that the content fizzes and gushes out when you open the cap?

If I take a soft drink bottle from the fridge and open it after shaking it vigorously, the liquid spurts out. Why does it come out so forcefully after shaking?

If the liquid rushes out when a softdrink bottle is opened, it is because carbon dioxide is escaping from the water in the drink.

Carbon dioxide is introduced into the water at high pressure and cold temperatures. Besides causing the liquid to bubble when opened, a little of the carbon dioxide reacts with water to form carbonic acid which gives the drink its refreshing tangy taste.

If you shake a bottle vigorously before opening it, some of the gas in the head of the bottle above the liquid will get dissolved in the liquid. This gas is more than the water in the drink can normally dissolve and so when the bottle is opened the sudden release of pressure will cause the gas to rush out. This will bring the liquid out in a gush.

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WHY ARE CAR TYRES FILLED WITH AIR? CAN’T THEY BE FILLED WITH WATER ?

Air is compressible, water is not. When the tyre hits a bump or drops into a depression on the road, the impact pushes the air in the tyre into a smaller space. Thus the shock of the impact is absorbed by the cushion of air in the tyre, and is not passed on to the body of the car and consequently to the passengers.

Water cannot get compressed in this way. If a water-filled tyre were to hit a bump on the road, the water would retain its rigidity. As a result, the shock of impact would be passed on to the body of the car, jolting the passengers.

Secondly, water-filled tyres would increase the weight of the wheels and the vehicle would have to overcome greater rolling resistance. This would increase the load on the engine.

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WHY DOES A CYCLIST BEND INWARDS WHILE TAKING A TURN?

If you attach a stone to a piece of string and whirl it around, you will feel the string tighten and the stone being pulled away. If the string is not strong, it will break and the stone will be hurled away in a straight line. This is the power of centrifugal force, the force exerted on a body in circular motion

When a cyclist takes a turn in a curved motion he is subjected to centrifugal force which pushes him to the edge of the curve. To maintain his balance he has to lean inwards while turning.

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WHY CAN'T WE SMELL THE PERFUME WE'RE WEARING?

When we get a smell, it is due to the brain, and not the nose. The nose collects scent molecules through odour receptors and information about these molecules passes on to the brain. The brain decodes this information and besides identifying the molecules, decides whether the smell is an indication of a threat or is harmless. If harmless, it ignores the smell after two or three breaths. So if you dab on perfume you'll get a whiff or two and then you won't smell it again, though others will.

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Why does a shoe shine with shoe polish?

The surface of the shoe has tiny hollows and valleys. Light falling on an irregular surface gets scattered in all directions. Hence it does not shine. When we polish our shoes, the cream that we apply fills up the hollows. This makes the surface smooth causing the light falling on it to be reflected more evenly. As a result, the surface shines. . Shoe polish is not a cleaning product: it is suited for clean and dry shoes. A vigorous rubbing action to apply the polish evenly on the boot, followed by further buffing with a clean dry cloth or brush.

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How does an ice skater move easily on ice?

The skater glides on a thin film of water that acts as a lubricant- enabling her to glide smoothly at high speed. Pressure on any body generates heat. This heat, in turn, raises the temperature at the point of pressure. The frozen mass melts at that point. Pressure is generated by the ice skates due to the skaters weight, creating a thin film of water between the ice and the skates. The ice refreezes as soon as the skater has passed. So, in reality, it is the frictionless surface of water that makes skating so effortless.

What you need:

 A thin strip of wood that is about 34 cm wide, a little more than a centimeter thick and about 60 cm long; a length of thin copper wire; a large cube of ice; a big plastic bottle; and two chairs.

What you do :

  • Place the wooden strip across the back of two chairs.
  • Make a loop measuring about 30 cm in diameter from the copper wire.
  • Pass the loop over the middle of wooden strip.
  • Now place the ice cube on the strip so that the loop (see illustration), so that it hangs down.
  • The weight of bottle pulls down on the loop , applying great pressure to the ice cube.
  • Gradually the loop cuts its way through to the bottom of the cube.

What happens:• The cube does not split into two because the ice refreezes behind the wire when the pressure is off.

• This is also why the ice does not crack after the skater has gone over it.

 Unlike other liquids, water doesn't contract when it cools. It expands! However, water contracts like other liquids when cooled up to 40 C, but if it is cooled below this temperature it starts expanding! This is why water pipes in cold places burst in winter.

Solid water or ice is less dense than liquid water and hence it floats. When water freezes in lakes and ponds, it freezes only at the surface. Below the layer of ice, the water remains in its liquid form, enabling fish and other creatures to survive.

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How do microwaves cook food?



Microwave ovens are quick. No one can argue with that. Cooking and reheating food which in normal ovens might take twenty or thirty minutes can be done in less than a tenth of that time in a microwave oven. With speed like that it’s no wonder they’ve become the latest kitchen craze.



When you think about it, not a lot has changed in cooking since the days when our primitive ancestors cooked up mammoth steaks in caves. They used fire to cook their food. And even the most up-to-date ovens still apply the same principle. They may not use fire, but the food is cooked by exposing it to outside heat.



Microwave cooking literally turns this inside out. Instead of cooking or heating food from the outside in, the food is cooked all the way through at the same time.



The electromagnetic waves that do the cooking or heating bombard the food at a fantastic rate. This stirs up the molecules in the food, creating heat right through it at the flick of a switch. The whole process is fantastically quick. Instead of waiting hungrily for something hot to eat, with a microwave oven it takes the same times to cook, eat and wash up a meal as it does to cook it in a conventional oven.



 



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Can you tell how computers work?



All computers work in basically the same way. They follow a set of instructions called a program that enables them to do calculations on information fed into them.



This process produces a result that is used in some way. The great advantage of computers over other machines is that the program can be changed, so that a computer can be given a wide variety of tasks to perform.



Computers consist of four main units – an input unit, a central processing unit is at the centre of operations and generally consists of a microchip located in the computer case. It controls the operations of all other units, which may be part of the computer or connected to it.



The input unit is used to feed information or data into the computer. It is usually a keyboard, but it may also be a light pen that interacts with a computer screen, or simpler devices such as a joystick, a mouse or a bar-code reader. The keyboard is also used to write programs.



The central processing unit first passes the information to the internal memory, where it is held temporarily. The program is also held in the memory, and the processing unit follows the program to produce the results. These go to the output unit, which is usually a video screen or printer, or they may be sent along telephone lines to other computers.



The computer also has an external memory unit such as disc drive that takes programs and data from the internal memory and records them for use at a later date.



 



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How computers are used in industry?



The electronic computer is used in many fields of activity and is extremely valuable in doing complicated work accurately and quickly. It has removed much of the drudgery from such routine tasks as telephone se wonderful machines work? We can see in the simple example of checking the stocks held by a warehouse.



In large scale industries it costs a great deal of money to keep a large number of goods in store. Nevertheless a company must always know how many goods it has at a given time in case it runs out of any item. So there must always be a reserve level below which stocks must not go. When that level is reached the company orders more goods to be delivered.



One way of keeping a check is to use a punched-card system. Each article which is delivered to the warehouse has its own card punched with required information which may relate to style, colour, price, size or other relevant details, and this is fed into computer.



When the article is sold and leaves the warehouse the computer is fed with this information too. At any time the computer can show exactly how many of those articles are in stock and if the stocks have to be replenished. The computer does this job with great speed and accuracy and can give an account of exactly how many articles of many different types are in stock.



The initial effect of computers is as an efficient means of performing complicated or routine tasks. In the long term, however, they will make new and different activities possible for instance, education and many occupations will be greatly affected as methods of storing and retrieving vast quantities of information are further developed.



 



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