## 27 November 2011

### VIII Physics Some Natural Phenomena Solved Questions(Notes)

Q1. Why does a plastic comb rubbed with dry hair attract tiny pieces of paper?

Ans: Plastic comb gets electrically charged due to rubbing & therefore it attracts tiny pieces of paper (which are neutral). As charged body can attracts an uncharged body.

Q2. Which of the following cannot be charged by friction, if held by hand?
a) a plastic scale b) a copper rod      c) an inflated balloon  d) a woolen cloth. and Why?

Ans: Copper rod.

Except copper, the other three are insulators whereas copper is a conducting object. As soon as it gets charged by rubbing with another material, the electric charge produced on its surface flow through our hand & body into the earth. And it remains uncharged.

Q3. What kind of electric charge is acquired?
a) by a glass rod rubbed with silk cloth?   b) by a plastic comb rubbed with dry hair?

Ans: a) positive charge.  b) Negative charge.

Q4. A negatively charged object attracts another charged object kept close to it. What is the nature of charge on the other object?

Ans: Positive Or Neutral (uncharged).

Q5. A negatively charged object repels another charged object kept close to it. What is the nature of charge on the other object?

Ans: Negative charge.

Q6. Mention three ways by which a body can be charged.

Ans: Three ways are:

a) Charging by rubbing: Charging of an object by rubbing it with another object is called charging by rubbing.
NOTE: i) When two bodies are charged by rubbing, they acquire equal & opposite charges.

ii) The body which loses electrons acquires positive charge whereas the body which gains electrons acquires negative charge.

b) Charging by conduction: Charging a neutral body by bringing it in contact with a charged body is called charging by conduction.

c) Charging by induction: Charging a neutral body by bringing it near a charged body is called charging by induction.

Q7. What is an electroscope? Explain its construction.

Ans: An electroscope is a device for detecting, measuring & finding the nature of a charge.An electroscope consists of a large jar. A metal rod is fitted into the mouth of the jar with the help of the cork. At the lower end of the metal rod a pair of thin leaves of gold or aluminium is suspended.

Q8. What are the uses of an electroscope?

Ans: An electroscope can be used for following purposes:
a) To detect & measure the charge on a body.
b) To determine the nature of charge on a body.

Q9. How would you use an electroscope to find out whether an object is charged or not?

Ans: Touch the body to be tested with the metal disc of an electroscope. If the leaves of an electroscope open up (diverge), the body is charged. If the leaves remain unaffected, the body has no charge.

Note: The extent of divergence (opening apart) of the leaves is a measure of the charge on the body. A body carrying higher charge will cause greater opening up of the leaves.

Q10. How would you use an electroscope to determine the nature of charge of a charged body?

Ans: Charge the electroscope with a known charge, say with negative charge, by touching a negatively charged ebonite rod to the metal disc of the electroscope. The leaves of the electroscope open up (diverge).
Now touch the body to be tested with the metal disc of the charged electroscope.
• If the divergence of the leaves increases, the body has similar charge that is the given body is also negatively charged.
•  If the divergence of the leaves decreases, the body has unlike charge that is the given body is positively charged.
Q11. What will you observe when the metal cap of an electroscope is touched with a plastic comb rubbed in dry hair? Give reason for your answer.

Ans: After rubbing, plastic comb acquires negative charge. Now when it is touched with the metal cap of an electroscope then both the metal cap & the leaves acquire negative charge due to conduction. Because of negative charge on both the leaves, divergence of leaves takes place.

Q12. What happens when we touch the metal cap of a charged electroscope with our finger? What is this process known as?

Ans: The leaves of an electroscope collapse as soon as we touch the metal cap with hand because the leaves of the charged electroscope lose charge to the earth through our body (in other words leaves are discharged). This process is known as EARTHING.
NOTE: The process of transferring of charge from a charged object to the earth is called Earthing.

Q13. What is the nature of charge a) on the metal cap b) on the leaves of an uncharged electroscope
when a negatively charged body is brought in contact with its metal cap?

Ans: a) Negative b) Negative

Q14. What is the nature of charge a) on the metal cap b) on the leaves of an uncharged electroscope
when a negatively charged body is brought near its metal cap (not in contact withmetal cap).

Ans: a) Positive         b) Negative

Q15. Touch the disc of an electroscope first with glass rod rubbed with silk & then with ebonite rod rubbed with fur. What do you observe & why?

Ans: After rubbing, glass rod acquires positive charge. Now when it is touched with the metal cap of an electroscope then both the metal cap & the leaves acquire positive charge due to conduction. Because of positive charge on both the leaves, divergence of leaves takes place. Electroscope is now positively charged.

After rubbing with fur, ebonite rod acquire negative charge & when this negative rod is touched with the metal cap of the above positively charged electroscope then collapsing of leaves takes place as this negative charge starts neutralizing the positive charge already present on the leaves.

Q16. Touch the disc of electroscope with an ebonite rod rubbed with fur. Now bring a glass rod rubbed with silk close to the disc of this electroscope. What do you observe?

Ans: After rubbing, ebonite rod acquires negative charge. Now when it is touched with the metal cap of an electroscope then both the metal cap & the leaves acquire negative charge due to conduction. Because of negative charge on both the leaves, divergence of leaves takes place. After rubbing with silk, glass rod acquire positive charge & when this positive rod  is brought near the metal cap of the above negatively charged electroscope then due to induction positive charge gets induced in the leaves as a result collapsing of leaves takes place.

18.  What is seismograph?

Ans:Tremors or vibrations caused by the earthquakes which travel in the form of waves within the earth or along the earth's surface, are called seismic waves. Seismograph is an instrument which records these waves.

19. List two places in India which are most threatened by earthquake.

Ans: Two places in India which are most threatened by earthquake are 1. Kashmir  2. Rann of kutch.

20. What are tectonic plates?

Ans: The earth's lithosphere is fragmented into many pieces. Each fragment is called a plate, also called tectonic plate. These plates are in continuous motion i.e. they float over hot magma.

21.  What is a lightning conductor?

Ans: Lightning conductor is a device used to protect buildings from the damaging effects of lightning. It runs from the top to the bottom, along the outer wall of the buildings or any other object, which is to be protected. If lightning strikes the buildings or any other objects, then the lightning conductor provides an easy and direct path for the lighning bolt to pass to the ground without effecting them.

22.  What is earthing?

Ans:  The process of transferring of charge from a charged object to the earth is called earthing. For our safety, most of the electrical appliances and the mains of the house are connected to earth, so that we can be prevented from getting an electric shock.

23. We can easily charge non-metals like rubber, woollen clothes, plastics, etc. whereas we cannot charge a copper rod by rubbing easily. Why?

Ans: When the metallic rods like copper rod are rubbed, charges does not build on their surface because charges can escape through metals as they are conductors whereas when non-metals like rubber, woollen clothes, plastics, etc. are rubbed, charges will build up on their surface because charges are not conducted through them as they are insulators.

24.  Explain the process of an electric discharge?

Ans:  During the development of thunderstorm, air currents move in the upward direction and the water droplets move in the downward direction. These movements causes the seperation of charges. Usually, the negative charges accumulate at the lower part of the clouds and the positive charges are accumulated at its upper part. The positive charges are also accumulated at the ground also. When the accumulation of charges becomes large, a high potential difference is set up between lower part of clouds and earth, which is sufficient to break the insulation of air. As a result, negative and positive charges meet, producing streaks of bright light and sound. This process is called an electric discharge.

25. Draw the diagram of an instrument, which can be used to detect the charge on a body. How it can be charged through conduction?

Ans: An electroscope is used to detect the charge on a body. A plastic comp is taken and it is rubbed on hair. Now, the plastic comb gets charged. The comb is touched with the electroscope plate. The static charges which are developed on the comb travels down the conducting wire and reach the two leaves of aluminium foil. Similar charges are acquired by both the leaves and as a result, they repel each other. Thus, the method of charging an uncharged body by bringing another charged body directly in contact is called charging by conduction. Hence, by this way, an electroscope can be charged through conduction.

26. Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?

Ans:  The following precautions should be taken :-

1. Find a clear spot, away from buildings, trees, poles and electric poles, signboards and overhead power lines anddrop to the ground.

2. Do not use elevators if they are available at some place outside your house.

3. If you are in a car or a bus, do not come out anddrive slowly to a clear spot. Stay inside a cartill the tremors stop.

27. Suppose you are at your home and an earthquake strikes. What precaution would you take to protect yourself?
Ans: The precautions that should be taken are :-

1. Take shelter under a table and stay there only, till the shaking stops.

2. Stay away from the objects which are tall and heavy, that may fall on you.

3. If you are on bed, do not get up and remain there only andprotect your head with pillow.

28.  What is earthing? Why earthing is provided in buildings?

Ans: The process of transferring of charge from a charged object to the earth is called earthing. Earthing is provided in buildings to protect them from electrical shocks due to any leakage of electrical current. For our safety, most of the electrical appliances and the mains of the house are connected to earth, so that we can be prevented from getting an electric shock.

29.  A crackling sound is heard while taking off sweater during winters. Explain ?

Ans:As we know that electrical charges that are generated through friction are static, i.e they do not move by themselves and Motion of charges constitutes an electric current. When we take off our sweater there is a motion between the charges on the sweater and our body that produces electric current,which produces a crackling sound. Infact we can see a spark if we take off the sweater in the dark.

30. What is lightning. Explain the experiment conducted by Benjamin Franklin that showed sparks shared some similarity with lightning ?

Ans: Lightning is an atmospheric discharge of electricity, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms. In June 1752, Benjamin Franklin raised a kite, accompanied by his son williams as an assistant. On his end of the string he attached a key, and he tied it to a post with a silk thread .
After some time he noticed that small pieces of strings were beginning to stand apart like the hair on the back of a scared dog. He then brought his hand close to the key and received a tingle of an electric shock from the key.As the rain came down an the string became soaked the electricity began to conduct freely throug the key.
Further solved questions:

## 26 November 2011

### Class VII Assignment of Lesson- 10 Lesson REFRACTION AND DISPERSION OF LIGHT

Q.1. Define refraction of light.
Q.2. What is the cause of refraction of light?
Q.3. Define dispersion of light. Name the colour that is least deviated.
Q.4. See the picture given below and draw the path of refracted ray.
Q.5.Give any two examples of refraction of light, which we observe in everyday life.

Q.6.What would be the speed of light in water if its refractive index is 4/3? Given speed of light is 3 ilometers per second.
Q.7. A coin, kept in a cup of water appears ‘raised up’. Why?
Q.8. What is meant by optical density? Why does light bend when it travels through two optically different mediums?
Q.9. When and how is rainbow formed?
Q.10. What are the rules for refraction?
Q.11. A magnifying glass produces a hole on the black paper when sunlight is focused on it. Why?
Q.12. If the refractive index of the medium is high, will the bending of the light be more or less?
Q.13. Distinguish between a convex and a concave lens.
Q.14. Why does light split into several colours as it passes through the prism?
Q.15. Define:- (a) Focal length (b) principal axis (c) radii of curvature with the help of a diagram.
Q.16. What will happen if you hold a torch vertically above the water surface, so that the ray of light is perpendicular to the water surface? What will happen to the speed of light as it travels from air to water?
Q.17. Draw a diagram to show the path of light through a rectangular glass slab.
Q.18. The speed of light in water is 225,000km/s. Calculate the refractive index of water. Given that speed of light in air is 300,000km/s.
Q.19.An object is placed at the following distances from a convex
lens of focal length 10 cm----- (a) 8cm (b) 15cm (c) 20cm (d)25cm . Which position of the object will produce;-- (i) a diminished real image (ii) a magnified real image (iii) a magnified virtual image (iv) an image of the same size as the object.
Q.20. State some of the application of lenses.
Q.21. What is a lens? What are its different types?
Q.22. Why does a pond appears less deep than it actually is? Explain with the help of a diagram.
Q.23. Define refractive index of a medium. How does the bending of the ray light depend upon refractive index of the medium?
Q.24. Who was the first to obtain the spectrum of sunlight?
Q.25. Write the rules for the formation of images by lenses.

### 8th physics light reflection refraction eyes defect and dispersion

The ray of light which falls on the mirror surface is called incident ray.
The point at which the incident ray strikes the mirror is called the point of incidence.
The ray of light which is sent back by the mirror is called the reflected ray.
The ‘normal’ is a line drawn at right angles to the mirror surface at the point of incidence.
The angle between incident ray and normal is called the angle of incidence.
The angle between reflected ray and normal is called the angle of reflection.Laws of reflection of light:
Ø  The incident ray, the reflected ray, and the normal {at the point of incidence}, all lie in the same plane.
Ø  The angle of reflection is always equal to the angle of incidence.
Regular Reflection and Diffuse Reflection of Light
Regular Reflection

In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction.
• Regular reflection of light occurs from smooth surfaces like that of a plain mirror.
•  Images are formed by regular reflection of light.
Diffused Reflection

In Diffuse reflection, a parallel beam of incident light is reflected in different directions.
Ø  The diffuse reflection of light takes place from rough surfaces
Ø  A sheet of paper produces diffuse reflection of light. No image is formed in diffuse reflection of light.
Characteristics of image formed by a plane mirror
Ø  The image formed by a plane mirror is virtual & Erect.
Ø  The image formed in a plane mirror is the same distance behind the mirror as the object is in front of it.
Ø  The image formed in a plane mirror is of the same size as the object.
Ø  The image in a plane mirror is laterally inverted.
Periscope :  A Periscope is a device through which a person can se the objects that are out of the direct line of sight. For example, by using a periscope, we can see the objects on the other side of a high wall which cannot be seen by us directly. Periscope has two plane mirrors arranged parallel to one another. Each plane mirror, however, makes an angle of 45with the side of the tube.
Some of the uses of periscopes are given below:
Ø  Periscope is used to see over the heads of a crowd
Ø  A Periscope is used by soldiers sitting in bunker  to observe the enemy activities outside {over the ground}.

A Periscope is used by a navy officer sitting in a submarine to see ships over the surface of water in the sea.

Multiple Images: When two plane mirrors are kept inclined at an angle, they can form multiple images of an object. If two plane mirrors are inclined at an angle q, then the number of images formed in them is given by the formula:   No. of images formed = (3600/ q)  -  1

Kaleidoscope : The kaleidoscope is an instrument which produce multiple reflections of coloured glass pieces {or coloured plastic pieces} and create beautiful patterns.

Construction: The Kaleidoscope consists of three long and narrow strips of plane mirrors inclined at 600  to one another forming a hollow prism, and fitted into a cardboard tube. One end of the cardboard tube is closed by an opaque disc {cardboard disc} having a small hole at its centre. The other end of cardboard tube is closed with circular discs of  glass. The inner disc being of transparent glass {clear glass } and the outer disc of ground glass {translucent glass}. A number of small pieces of different coloured glass {or plastic } and having different shapes are kept between the two glass discs {which can move around freely in the space between the two glass discs}.  When we hold the kaleidoscope tube towards light and look inside it through the small hole, we see beautiful patterns of coloured glass.

Patterns formed by Kaleidoscope
Dispersion of light
The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light. The seven colours of the spectrum of white light are: violet, indigo, blue, green, yellow, orange and Red.

The Human Eye
• The main parts of the human eye are : Cornea, Iris, Pupil, Ciliary muscles, Eye lens {which is a flexible convex lens}, Retina and Optic nerve.
• Cornea is made of a transparent substance
• The light coming from an object enters into the eye hrough cornea. Functionof cornea is to protect the eye.
•  Iris is the colored part of the eye. The iris has a hole at its centre which is called pupil. Pupil appears like a dark spot in the centre of iris because no light is reflected from it. Iris controls the amount of light entering the eye by adjusting the size of pupil.
• The eye-lens is a convex lens made of a transparent and flexible material.
• The eye-lens is held in position by ciliary muscles. The function of ciliary muscle is to hold the lens & changes its size according to the need.
• The retina is a screen on which the image is formed in the eye.
• The optic nerve carries the image formed on retina to the brain in the form of electrical signals.
Rods and cones

•  Rods are the rod-shaped cells present in the retina of an eye which are sensitive to dim light.
• Cones are the cone-shaped cells present in the retina of an eye which are sensitive to bright light. Cones also cause the sensation of colour of objects in our eyes.

Blind spot
At the junction of optic nerve and retina in the eye, there are no light sensitive cells {no rods or cones }due to which no vision is possible at spot. This is called blind spot.
Persistence of vision
The image of an object seen by our eyes persists {or remains}on the retina for about 1/16th of a second even after the object has disappeared from our view. The abilily of an eye to continue to see the image of an object for a very shot duration even after the object has disappeared from view, is called persistence of vision.
Defects of the Eye:
a) Myopia (short-sightedness) : In this defect the person is able to see the near by objects but not able to see the far-off objects.
This defect can be corrected by using spectacles containing Concave lens.
b) Hypermetropia (Far-sightedness) : In this defect the person is able to see the far-off objects but not able to see the near by objects.
This defect can be corrected by using spectacles containing Convex lens.c) Cataract : Cataract develops when the eye lens of person becomes cloudy (or even opaque) due to the formation of membrane over it. Cataract decreases the vision of the eye gradually. It can even lead to total loss of vision of the eye.
The opaque lens is removed from the eye of the person by surgical operation & a new artificial lens is inserted in its place.
Refraction of Light
The deviation in the path of light when it passes from one medium to another medium of different density is called refraction.
The twinkling of stars is due to atmospheric refraction of starlight. Since the atmosphere bends starlight towards the normal the apparent position of the star is slightly different from its actual position. Hence the star appears slightly higher than its actual position.
Refraction and Reflection of Light
IO incident ray        OR' reflected ray  OR refracted ray
The diagram shows how the light gets refracted when it is traveling from one optical medium to another. Like reflection, refraction of light takes place according to certain laws.
Terms which are commonly used to explain the phenomenon of refraction.
Incident Ray The ray of light striking the surface of separation of the mediums through which it is traveling is known as the incident ray.
Point of Incidence The point at which the incident ray strikes the surface of separation of the two mediums is called the point of incidence.
Normal The perpendicular drawn to the surface of separation at the point of incidence is called the normal.
Refracted Ray The ray of light which travels into the second medium, when the incident ray strikes the surface of separation between the mediums 1 and 2, is called the refracted ray.
Angle of Incidence (i) The angle which the incident ray makes with the normal at the point of incidence, is called angle of incidence.
Angle of Refraction (r) The angle which the refracted ray makes with the normal at the point of incidence, is called angle of refraction.
Cause of Refraction A ray of light refracts or deviates from its original path as it passes from one optical medium to another because the speed of light changes.
Laws of Refraction
• The incident ray, the refracted ray and the normal to the surface at the point of incidence all lie in one plane.
• For any two given pair of mediums, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. The above law is called Snell's law after the scientist Willebrod Snellius who first formulated it
Thus, Sin i /Sin r = a constant = m
Where m is the refractive index of the second medium with respect to the first medium.
We know that the phenomenon of refraction is taking place because the speed of light changes when it is traveling from one optical medium to another.   Thus we can define refractive index in terms of the speed of light in the two media.   The refractive index of glass with respect to air is given by the relation
m =  speed of light in air/speed of light in glass
If the medium 1 is air or vacuum, the refractive index of medium 2 is referred to as the absolute refractive index.
Critical angle is that angle of incidence for which a ray of light while moving from a denser to a rarer medium just grazes over the surface of separation of the two media (that is, angle of refraction = 90o).
Total Internal Reflection
If the angle of incidence of a ray of light traveling from a denser medium to a rarer medium is greater than the critical angle for the two media, then the ray is reflected into the denser medium and this phenomenon is described as total internal reflection.
The conditions to be satisfied for total internal reflection to take place are
• The ray of light must travel from a denser medium to a rarer medium.
• The angle of incidence must be greater than the critical angle for those two mediums.
The refractive index of a medium depends on the following factors:
1. the nature of the medium 2. the color or wave length of the incident light
Points to remember
1.) There can be only one reflected ray for a given single incident ray falling on a plane mirror.
2.) A ray of light which is incident normally on a mirror is reflected back along the same path because the angle of incidence for such a ray of light is 0o the angle of reflection is also 00.
3.) Diffuse reflection is caused by the roughness in the reflecting surface of an object.
4.) Laws of reflection are valid for both regular reflection & irregular reflection.
5.) If a person is standing at a distance of 1 metre in front of a plane mirror, then distance between the person & his image will be 2 metres.
6.) In an image formed by plane mirror, the left side of the object appears on the right side in the image whereas the right side of the object appears on the left side in the image.
This change of sides of an object & its mirror image is called Lateral inversion.
7.) As the angle between the two plane mirrors decreases, the number of images formed increases.
When the angle between the two plane mirrors becomes 0 deg ie. When the two mirrors are parallel to each other then infinite number of images is formed.
8.) An interesting feature of a kaleidoscope is that we can never see the same patterns again. Every time a new pattern is formed. Kaleidoscope are used by designers of wall papers & fabrics, as well as by artists to get ideas for new patterns.
9.) Rainbow in the sky is natural phenomenon showing the dispersion of sunlight. Rainbow is produced by the dispersion of sunlight by the tiny rain drops suspended in the atmosphere (which act as tiny prisms made of water).
10.) The adjustment of the size of pupil takes some time. That’ when we go from a bright light to a darkened cinema hall, at firstwe cannot see our surroundings clearly.
11.) The range of vision of a normal human eye is from infinity to about 25 centimeters.

### 8th physics Chemical effects of electric current Assignments

Q1. Do liquids also conduct electricity? Name two liquids which conduct electricity & two liquids which do not conduct electricity.
Q2. Name a device which glows even when a weak electric current passes through it.
Q3. Write full form of LED.
Q4. Which effect of electric current is utilized for detecting the flow of current through a solution:
a) When a torch bulb is used?
b) When a compass is used?
Q5. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.
Q6. When electric current is passed through acidified water, then hydrogen & oxygen are formed. What type of effect of current is illustrated by this statement?
Q7. State some of the characteristics of chemical changes brought about by the chemical effect of electric current.
Q8. Name the metal which is usually electroplated on car parts such as bumpers & bicycle handlebars made of steel.
Q9. An iron key is to be electroplated with copper. Draw a labeled sketch of the experimental set up used for this purpose.
Q10. Name two metals which are usually electroplated on cheaper metals for making jewellery (or ornaments).
Q11. You are required to do electroplating of copper on an iron object. Name the positive electrode (anode) & the electrolyte you will use for this purpose.
Q12. Write a short note on electroplating.
Q13. Why does compass needle show more deflection when you test sea water, compared to ordinary water?
Q15. Name a liquid which when poured in the beaker may cause magnetic needle to deflect. Name the possibilities if a bulb in tester does not glow (it is for sure that liquid is conducting).

Q16. What happens to the needle of a magnetic compass kept nearby when electric current is switched on in a wire? Why does this happen?
Q17 Iron is used in bridges & automobile to provide strength. Which metal is electroplated on iron to protect it from corrosion?
Q18. We should never handle electrical appliances with wet hands or while standing on a wet floor. Why?
Q19. When the free ends of a conductivity tester (made by using a battery connected to a wire wound around a compass) are dipped into the following solution, then in which  solution or solutions the compass needle shows deflection. Solutions are: Lemon juice, Vinegar, Tap water, Vegetable oil, milk, Honey.
Q20. Why does an electric bulb glow when a current passes through it?
Read more topics to Excel in exam

### 8th physics Some Natural Phenomena Assignments

Q1. What type of electric charge is acquired by a rubber balloon when rubbed with a woolen cloth?

Q2. A positively charged object repels another charged object kept close to it. What is the nature of charge on the other object?

Q3. A glass rod is rubbed with a silk cloth & a rubber balloon is rubbed with a woolen cloth.
a) Which two objects acquire negative charge? b) Which two objects acquire positive charge?

Q4. Explain why, a glass rod can be charged by rubbing when held by hand but an iron rod cannot be charged by rubbing, if held by hand?
Q5. Consider three charged bodies P, Q & R. If P & Q repel each other & P attracts R. What is the nature of force between Q & R?
Q6. Name the device used to detect electric charge on a body. Draw its labeled diagram.

Q7. What will you observe when the metal cap of an electroscope is touched with a glass rod which has been rubbed with silk cloth? Give reason for your answer.
Q8. What are the uses of an electroscope? (3 points)
Q9. Why do the leaves of a gold leaf electroscope diverge when a charged body is brought in contact with its disc?
Q10. Do the leaves of an electroscope always diverge to the same extent when a charged body is brought in contact with its disc?
Q11. What will you observe when the metal cap of an electroscope is touched with:
a) A positively charged object?
b) A negatively charged object?
Q12. What happens to the charge on an electroscope when its disc is connected to that of an uncharged electroscope with the help of a conducting wire? What happens to the leaves of the two electroscopes?
Q13. What would you expect in the following cases?
a) A glass rod rubbed with a piece of silk is brought near a negatively charged paper cylinder.
b) A negatively charged body is brought in contact with the disc of a positively charged electroscope.
c) Someone touches the disc of a negatively charged electroscope.
Q14. What will be the charge
a) on the metal cap
b) on the leaves of an electroscope, if you bring a positive rod near the end of its metal cap.
Q15. What will be the charge
a) on the metal cap &
b) on the leaves of an electroscope, if you bring a positive rod in contact with its metal cap.
Q16. Explain why a charged body loses its charge when we touch it with our hand.

### 8th physics Light Reflection Refraction and Eyes Assignments

1. Draw a diagram to show the reflection of light from a plane mirror. Label the following on the diagram:
(a)  Plane mirror (b)  Incident ray (c) Reflected ray (d) Point of incidence (e) Normal (f) Angle of incidence  (e) Angle of reflection.
2. How many reflected rays can there be for a given single incident ray falling on a plane mirror?
3. State the Laws of Reflection of Light.
4. An incident ray makes an angle of 75 deg.
with the surface of a plane mirror. What will be the angle of reflection?
5  A ray of light is incident normally {perpendicularly} on a plane mirror. Where will this ray of light go after reflection from the mirror ?
6 What is the difference between regular reflection and diffuse reflection of light ?
7 State whether the following statement is true or false :  Diffuse reflection means the failure of the laws of reflection of light.
8  Draw a labeled diagram showing how a plane mirror forms an image of a point object  placed in front of it.
9  If an object is placed at a distance of 7.5 cm from a plane mirror, how far would it be from its image ?
10  You see your image in a plane mirror? State two characteristics of the image so formed.
11  Name the phenomenon responsible for the following effect:
When we sit in front of a plane mirror and write with our right hand, it appears in the mirror that we are writing with the left hand.
12  Fill in the following blanks:
a.If you touch your------ ear with right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with -------
13 How are the two plane mirrors in a periscope arranged?
a.  With respect to one another?
b.  With respect to sides of the tube?
14    Explain how, a hair dresser makes you see hair at the back of your head after the hair cut is complete.
15    How many images of an object will be formed when the object is placed between two plane mirrors which are inclined at the following angles to one another?
{a}120 deg {b} 45deg c}  180 deg {d}  60 deg {e} 90 deg
16 .What will be the number of images formed when an object is placed between two parallel plane mirrors facing each other?
17.Describe the construction of a kaleidoscope.
18.How many plane mirror strips are there in a kaleidoscope? How are they arranged?
19.State one use of kaleidoscope.
20  What is meant by “dispersion of light “?  Name a natural phenomenon which is caused by the dispersion of sunlight in the sky.
21.  What are the functions of the following parts of the eye?
(a) Iris (b) Ciliary muscles (c) Retina (d) optic nerve.
22 Name the cells on the retina of an eye:
(a) which are sensitive to bright light.
(b) which are sensitive to dim light.
(c)  which produces sensation of colours.
23.  Explain why, we cannot see our surroundings clearly when we enter a darkened cinema hall from bright sunshine but our vision improves after sometime.
24. What happens to the size of the pupil of our eye:
(a)  in dim light?        (b) in bright light?
25. What is the range of vision of a normal human eye?
26. Name the phenomenon which enables us to see movies in cinema hall.
27. What is meant by ‘persistence of vision’?
28 What are the various defects of eye? Explain them in brief? Also suggest the corrective measures of the defects.

### CLASS VIII physics Chapter Sound Assignments

Q.1  How does a sound making object differ from one that is silent?
Q.2  How does sound from a sound producing body travel through air to reach our ears?
Q.3  Why are the voices of men, Women and children different?
Q.4  Why a sound cannot be heard on the moon?
Q.5  How do astronauts talk to one another on the surface of moon and Why?
Q.6  If you want to hear a train approaching from far away, why is it more convenient to put the ear to the track?
Q.7  State one observation from everyday life which shows that sound travels much more slowly than light?
Q.8  What is the scientific name for the following? The number of vibrations made per second.
Q.9   Why do we not hear the screams of a bat?
Q.10  Which of the following frequency of sound can be heard by a dog but not by a man?
(a) 50,000 hertz (b)  15,000 hertz
Q.11  Explain how, noise pollution {or excessive loud noise} is harmful to human beings.
Q.12  State the various measures which can be taken to control {or reduce} noise pollution in our surroundings.
Q.13  Give two causes of noise pollution from the homes.
Q.14   Sound of different pitch can be produced using a flute. Explain, how?

### Class VIII Physics Force of Friction Assignment

1 Explain Why, it is easier to drag a mat on floor when nobody is sitting on it but much more difficult to drag the same mat when a person is sitting on it.
2 What are the factors affecting friction? Explain with examples.
3 What is the cause of friction? Explain with the help of labeled diagram.
4 Which type of surfaces produce {a} least friction, and {b} too much friction?
5 What is the direction of force of friction acting on a moving object?
6 A car is moving towards North. What will be the direction of force of friction acting on this car due to surface of road?
7 When a pencil cell is released from a certain point on an inclined wooden board, it travels a distance of 35 cm on floor A before it comes to rest. When the same pencil cell is released from the same point on the same inclined board, it travels a distance of 20 cm on floor B before coming to rest. Which floor, A or B, offers  greater friction? Give reason for your answer.
8 What kind of friction comes into play:
(a) When a block of wood kept on table moves slowly?
(b) When a block of wood kept on table just tends to move ( or slip)
(c) When a block of wood kept on cylindrical iron rods moves?
9 Out of sliding friction, static friction & rolling friction:
(a) Which one is the smallest? (b) Which one is the largest?
10 Explain why, sliding friction is less than static friction.
11 What is meant by ‘rolling friction’?
12 Why does a man slip when he steps on a banana peel thrown on the road?
13 Explain why:
(a) a pencil will write on paper but not on glass
(b) The handles of motor cycle are covered with a rubber sheet with spikes.(c) The soles of our shoes wear out gradually.
(d) Tyres of car wear out gradually.
14 Why does a matchstick light when we strike it on a rough surface?
15 Why do brake pads of bicycles have to be replaced quite often?
16 What prevents you from slipping every time you take a step forward?
17 What happens when you rub your hands vigorously for a few seconds? Why does this happen?
18 What enables us to fix a nail in a wall & knot to be tied?
19 How does bicycle stop when its brakes are applied?
20 What makes the steps of foot over-bridges at Railway stations to wear out slowly?
22 Why do gymnasts apply a coarse substance to their hands?
23 Why do kabaddi players rub their hands with dry soil?
24 Why are grooves provided in the soles of shoes?
26 Explain why, oil or grease is applied to those parts of machine which are in motion.
27 Fill in the blanks:
(a) Sprinkling of powder on the carom board ………………. Friction.
(b) Ball bearings reduce friction because they …………………rather than slide.
28 Two men tried to push a heavy box & could succeed. Finally wheels were fitted to the box & now a single man could move it. Justify.
29 What is the special name of frictional force exerted by fluids (like air or water)?
30 What are the factors that affect the fluid friction?
31 What is the name of ‘special shape’ which is given to objects moving through fluids to reduce drag?
32 Why are cars, airplanes & rockets streamlined?

### 8th physics Chemical effects of electric current Solved Questions

 CBSE VIII(8th)CHEMICAL EFFECT OF ELECTRIC CURRENT Physics Questions with their Answers Q 1. Which of the following liquids conduct electricity and which do not conduct electricity?Lemon juice, Milk, Vinegar, Salt solution, Distilled water, Honey, Sea water, Rain water. Ans: Conducting           : Lemon juice, Vinegar, Salt solution, sea water, Rain water        Non Conducting    : Milk, Distilled water, Honey. Q 2. What is advantage of using LED over bulb in testing the electrical conductivity of liquids? Ans: When electric current flows through a bulb then due to heating effect of current,the  filament of the bulb gets heated up to a high temperature ; it starts glowing. Now for a liquids having low electrical conductivity, the current flowing through the circuit isvery weak due to which the filament does not get heated sufficiently & hence the bulb does not glow. Therefore LED is used in place of bulb because LED glows even when weak electric current flows in the circuit. Q 3. Which effect of electric current is utilized for detecting the flow of current through a solution:   a) When a torch bulb is used?       b) When a magnetic compass is used? Ans: a) Heating effect      b) Magnetic effect. Q 4. Distilled water does not conduct electricity. What substances can be added to distilled water in small amounts to make it a good conductor of electricity? Ans: Salt, acid & base. Q5. In case of a fire, before the fireman uses the water hoses to throw water to douse fire, they shut off the electricity supply for the area. Explain why this is done? Ans: To prevent electrocution of fireman because ordinary water is conductor of electricity. Q6. When the free ends of a conductivity tester (made by using a battery connected to a wire wound around a compass) are dipped into a solution, the magnetic needle shows deflection. Can you give the reason of this deflection? Ans: Electric current flowing through the wire produces a magnetic field around it. And this magnetic field of electric current acts on the magnetic needle of compass; deflects it. Q7. What effects does an electric current produce when flowing through a conducting solution? Ans: When an electric current flows through the conducting solution, it causes a chemical reaction (or chemical change).These chemical reactions may produce following effects:i) Bubbles of gas/es may be formed on the electrodes.ii) Deposits of metals may form on electrodes.iii) Change in colour of solution may occur. Q8. When electric current is passed through acidified water then what is produced at(a) Positive carbon electrode (anode)?          (b) Negative carbon electrode (cathode)? Ans: a) oxygen gas b) hydrogen gas. Q9. Which effect of electric current is utilized when a thin layer of chromium metal is deposited on an iron tap? What is this process known as? Ans: Chemical effect of current is utilized. The process is known as electroplating. Q10. What is meant by electroplating? What is the purpose of electroplating? Ans: The process of depositing a layer of any desired metal on another material, by means of electricity, is called electroplating. Electroplating is donei) for protection against corrosion (or rusting).ii) for decorative purposes. Q11. Which properties of chromium metal make it suitable for electroplating it on car bumpers, bath taps & bicycle handle, etc., made of iron? Ans It has shiny appearance. It does not corrode easily. It resists scratches. Q12. Which metal is electroplated on iron for making ‘cans’ used for storing food & Why? Ans: Tin.  Tin metal has shiny appearance, it does not corrode & it is non poisonous. Tin is less reactive than iron. Due to tin plating over the surface of iron, the food does not come in contact with iron and  is protected from getting spoilt. Q13. On what factors the chemical effect produced by an electric current depends? Ans: The chemical effect produced by an electric current depends on the nature of conducting solution (through which it is passed), & on the nature of electrodes used for passing the electric current. Q14. In the process of purification of copper metal, a thin plate of pure copper and  a thick rod of impure copper are used as electrodes & a metal salt solution is used as an electrolyte:a) Which electrode is connected to the positive terminal of the battery?b) Which electrode is connected to the negative terminal of the battery?c) Which metal salt solution is taken as electrolyte? Ans: a) Thick rod of impure copper. b) Thin rod of pure copper. c) Copper sulphate solution. Q15. Write down the important points which should be remembered while electroplating? Ans: i) The metal on which electroplating is to be done should be cathode i.e., the negative electrode.ii) The metal to be deposited should be anode i.e., the positive electrode.iii) A water soluble salt of the ‘metal to be deposited’ is taken as the electrolyte. Post Related to this chapter

## 24 November 2011

### Physics Assignment-I Chapter:Gravitation

JSUNIL TUTORIAL,SAMASTIPUR
Class 9th (SA-II)
Physics Assignment-I (For month of November)
Chapter:Gravitation
Q1. A block weighing 1.0 kg is in the shape of a cube of length 10 cm. It is kept on a horizontal table. Find the pressure on the portion of the table where the block is kept. (ans. 1000Pa)

Q2. Find the thrust acting on the human body due to atmospheric pressure. Take the surface area of a man of middle size to be 1.5m2 and atmospheric pressure (1atm) =1.013×105 Pa. (ans.15.2 ton wt)

Q3. Calculate the mass of a body whose volume is 2 m3 and density 0.52 g/cm3. (ans. 1040 kg)

Q4. A dining hall has dimensions 50m × 15m × 3.5m. Calculate the mass of air in the hall. Given, density of air =1.30kg/m3. (ans. 3412.5 kg)

Q5. A thread of mercury of 10.2 g is in a tube of uniform cross section 0.1cm3. Calculate the length of thread. The density of mercury is 13.6g/cm3. (ans. 7.5cm)

Q6. A cubical block of water is dipped completely in water. Each edge of the block is 1cm in length. Find the buoyant force acting on the block. (ans. 10-2N)

Q7. A body of mass 2.0 kg and density 8000 kg/m3 is completely dipped in a liquid of density 800 kg/m3. Find the force of buoyancy on it. (ans. 2N)

Q8. A piece of iron of density 7.8 × 103kg/m3 and volume 100 cm3 is totally immersed in water. Calculate (a) the weight of the iron piece in air (b) the upthrust and (c) apparent weight in water. (ans. (a) 7.8N (b) 1N (c) 6.8 N)

Q9. A solid body of mass 150g and volume 250cm3 is put in water. Will the body float or sink.

Q10. A solid of density 5000kg/m3 weights 0.5 kg in air. It is completely immersed in water of density 1000kg/m3.
(a)   Calculate the apparent weight of solid in water.(ans. 0.4 kg)
(b)   What will be its apparent weight if water is replaced by a liquid of density 8000kg/m3? (ans. 0)

Q11. The mass of a block made of certain material is 13.5 kg and its volume is 15 × 10-3 m3. Will the block float or sink in water.  Give reason for your answer.

Q12. (a) What is the density of air in NTP?                    (b)What is the unit of relative density?

Q13. (a) When does a body sinks in a fluid?
(b)Why does a balloon filled with hydrogen gas rise up against gravity?
Q14. (a) Which has greater density: 1 kg of iron or 2 kg of iron?
(b)If a hollow sphere and a solid sphere are both made of the same amount of iron, which sphere has greater average density?

Q15. (a).  A body weighs 10 N in air and 8 N when fully immersed in water. How much is the buoyant force acting on the body?
(a) Why are the buoys making the channel in a river are hollow spheres?

Q16. State one important effect produced by the buoyant force exerted by water.

Q17. Where does a solid weigh more- in air or in a liquid?

Q18. Name two factors on which the buoyant force depends?

Q19. What is the relationship between the buoyant force on an object and the liquid displaced by it?

Q20. The relative density of mercury is 13.6. What does this statement mean?

Q21. The density of turpentine oil is 840 kg/m3. What will be its relative density?

Q22. Explain why big boulders can be moved easily by floods.

Q23. Why is a slight blow on a cork of bottle fully filled with a liquid sufficient to break the bottle?

Q24. Why is it easier to walk on soft sand with a flat shoe than a pencil-heeled shoe?

Q25. Lead has greater density than iron and both are denser than water. Is the buoyant force on a lead pencil greater than, less than or equal to the buoyant force on an iron object of same volume?

Q26.  Why do you feel lighter when you swim?

Q27. Why is a bucket of water lighter when in water than when it is taken out of water?

Q28. Why it is easier to swim in sea water tan in river water?

Q29. Two different objects are completely immersed in water and undergo same loss in weight. Is it necessary that the weight of these objects in air be also the same?

Q30. If two equal weights of unequal volumes are balanced in air, what will happen when these are completely dipped in water?