These sample papers have been taken from CBSE
Question 11: We wish to obtain an equal sized inverted image of a candle flame on a screen kept at distance of 4 m from the candle flame.
(a) Name the type of lens that should be used.
Answer: Convex Lens
(b) What should be the focal length of the lens and at what distance from the candle flame the lens be placed.
Answer: The focal length of the lens should be 2 m and candle flame should be at 4 m, i.e. twice the focal length. We know that when an object is placed at 2F1 of a convex lens, image is formed on 2F2. The image is of same size, real and inverted.
(c) Draw a labelled diagram to show the image formation in this case.
A 5 cm tall object is placed at a distance of 30 cm from a convex mirror of focal length 15 cm. Find the position, size and nature of the image formed.
Answer: Given, f = 15 cm, u = -30 cm, h1 = 5 cm
Image distance can be calculated as follows:
Or, `v=30 cm`
We know that `v/u=(h_2)/(h_1)`
Or, `h_2=5 cm`
Image is of same size, erect and virtual.
Question 12: Consider the following circuit:
What would be the readings of the ammeter and the voltmeter when key is closed? Give reason to justify your answers.
Answer: Let us find equivalent resistance for this combination of resistors
For series combination, equivalent resistance is given as follows:
Or, `R=5+8+12=25 Ω`
Using Ohm’s law we can find the current:
`V = IR`
Or, `I=(6)/(25)=0.24` ampere
Now, reading on voltmeter can be calculated as follows:
Or, `V=0.24xx12=2.88` V
What is electrical resistivity? Derive its SI unit. In a series electrical circuit comprising a resistor made up of a metallic wire, the ammeter reads 100 mA. If the length of the wire is doubled, how will the current in the circuit change? Justify your answer.
Answer: If l is the length and A is the area of cross section of a conductor, then resistance R is given by following:
Here, ρ is the constant of proportionality and is called electrical resistivity of the material of conductor.
Above equation can also be written as:
Since SI unit of resistance is Ω that of area is m2 and that of length is m hence,
We know that resistance varies directly as length of the conductor. So, doubling the length of the wire will result in resistance becoming double. This will reduce the current to half of earlier value, because current varies inversely as resistance.
Question 13: Calculate the total cost of running the following electrical devices in the month of September, if the rate of 1 unit of electricity is Rs. 6.00.
(i) Electric heater of 1000 W for 5 hours daily.
Answer: There are 30 days in September
So, monthly consumption `=30xx5 h\xx\1000 W=150000 Wh`
`=150 kWh=150` unit
Total cost `=6xx150=Rs. 900`
(ii) Electric refrigerator of 400 W for 10 hours daily.
Answer: Monthly consumption `=30xx10 h\xx\400 W=120000 Wh`
Total cost `=6xx120=Rs. 720`
Question 14: While teaching the chapter “Our Environment” the teacher stressed upon the harmful effects of burning of fossil fuels, plastic, paper etc. The students noticed the extensive use of plastic and polythene in daily life, which can be avoided and the surroundings can be kept clean. They decided to make their school “plastic and polythene” free and motivated each other for its minimum use.
(a) Why should the use of polythene and plastic be reduced in daily life?
Answer: Polythene and plastic are non-biodegradable and a significant portion of them cannot be recycled. This creates the problem of accumulation of plastic waste in our surroundings which is detrimental to the environment. So, we need to reduce the use of polythene and plastic in our daily life.
(b) In what ways the students would have avoided the use of plastic and polythene in their school?
Answer: The students could avoid the use of plastic and polythene in school in following ways:
(c) How the students would have motivated each other for the success of their decision?
Answer: Students can apply various ways and means to motivate each other for the success of their decision. This can be done through active dialogue, through suggestions and feedbacks. This can also be done by organizing some street plays to raise awareness level.
Question 15: What is “Sustainable Management of Natural Resources”? Why is it necessary? Which one out of reuse and recycle, would you practise in your daily life and why?
Answer: The practice of managing natural resources so that current generations needs should be met without jeopardizing the needs of the future generation is called Sustainable Management of Natural Resources. It is necessary in order to ensure that natural resources shall be available for many generations to come. I would prefer to reuse rather than recycle. Recycling required usage of some resources (fuel and some raw materials), but reuse does not involve usage of any other resource. So, ‘reuse’ is always better in terms of conservation of resources.
(a) List in tabular form any three chemical properties on the basis of which metals and non-metals are differentiated.
|Metals react with oxygen to make basic oxides.||Non-metals react with oxygen to make acidic oxides.|
|Some metals displace hydrogen from dilute acid||Non-metals do not react with dilute acids.|
|Metals make ionic compounds||Non-metals form covalent compounds.|
(b) State two ways to prevent the rusting of iron.
Answer: Painting and galvanization
Question 17: Explain giving justification for the trends in the following properties of elements, on moving from left to right in a period, in the Modern periodic Table.
(a) Variation of valency.
Answer: Valency first increases up to 4 on moving from left to right in a period and then decreases up to zero. This happens because number of valence electrons increases from 1 to 8. Valency is 1 for elements with valence electrons 1 or 7, is 2 for those with valence electrons 2 or 6, and so on.
(b) Change of atomic radius.
Answer: Atomic size and atomic radii decrease from left to right in a period. This happens because of decrease in electropositivity.
(c) Metallic to non-metallic character.
Answer: Metallic character decreases from left to right in a period. This happens because of decrease in electropositivity.
(d) Electronegative character.
Answer: With reduction in atomic size, a tendency to gain electrons increases across a period. Hence, electronegative character increases from left to right in a period.
(e) Nature of oxides.
Answer: Oxides become less basic and more acidic when we move from left to right in a period. This happens because of reduction in metallic character.
(a) What was the basis of Mendeleev’s classification of elements?
Answer: Atomic mass
(b) List two achievements of Mendeleev’s periodic tables.
Answer: Following are two achievements of Mendeleev’s periodic table:
(c) List any two observations which posed a challenge to Mandelẽev’s periodic law.
Answer: Following observations posed a challenge to Mendeleev’s periodic law:
(a) Write the reaction that occurs when glucose breaks down anaerobically in yeast.
(b) Write the mechanism by which fishes breathe in water.
Answer: Fishes breathe through gills. Gills absorb oxygen (dissolved in water) from water and then oxygen goes to the blood.
(c) Name the balloon likes structures present in lungs. List its two functions.
Answer: Balloon like structures in lungs are called alveoli. Two functions of alveoli are as follows:
(d) Name the respiratory pigment and write its role in human beings.
Answer: Haemoglobin is the respiratory pigment and it is responsible for carrying oxygen.
(a) Name the process and explain the type of nutrition found in green plants. List the raw materials required for this process. Give chemical equation for the mentioned process.
Answer: Plants follow autotrophic mode of nutrition and the process by which a green plant makes food is called photosynthesis. The raw materials required for photosynthesis are:
Chemical Equation for Photosynthesis:
(b) Write three events that occur during this process.
Answer: Three events during photosynthesis are as follows:
(a) What is variation? How is variation created in a population? How does the creation of variation in a species promote survival?
Answer: Differences in traits through successive generations are called variations. Variation is created due to some inaccuracies during DNA replication. Chances of variations are higher during sexual reproduction rather than during asexual reproduction. Variations can be either useful or harmful. Useful variations help an organism survive the onslaught of changed environment. Thus, most often useful variations are passed on to the next generation. Thus, variations help in survival of species.
(b) Explain how, offspring and parents of organisms reproducing sexually have the same number of chromosomes.
Answer: Gametes are produced through meiosis. We know that number of chromosomes becomes half after meiosis. Thus, gametes have haploid number of chromosomes. When male and female gametes fuse during fertilization, the number of chromosomes in zygote becomes diploid. This helps in maintaining the number of chromosomes in offspring of sexually producing organisms.
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