Current Affairs 7th Class

Structures and Functions of the Living Body   All animals and plants have different organs to perform various functions. Each part of the body of an animal or plant is different in structure. The organs, however, function in coordination with one another.   SPECIFIC PARTS OF PLANTS

• Plants have two main systems: (i) the root system (ii) the shoot system

  The Root System The root system grows mainly underground. Root systems are of two types: (i) Tap root system (ii) Fibrous root system (i) Tap root system: It is a main root and grows vertically down into the soil. The tap root gives out branches. For example, pea, neem, mango. (ii) Fibrous root system: Some plants do not have main root. They have many fibre-like roots. These are called fibrous roots. These roots spread out in the soil and give firm support to the plant. For example, wheat, grass, maize and millet. Advantages of the root system: As roots grow normally underground, they fix the plant to the ground. They absorb the mineral salts and water from the ground, which are needed for the plant to grow. Roots also help hold the soil together. They save the soil from being blown off or washed away.   The Shoot System The shoot system grows above the ground. It consists of the main stem, its branches and leaves. (i)   The stem: The stem holds the plant upright. The stem is the strongest part of a tree and is known as the trunk. Most trunks are observed with bark. The bark protects the inner part of the tree. The stems-of some plants are weak. They cannot stand erect. Stems carry water from the roots to the leaves and flowers. They also carry food from the leaves to other parts of the plant. They hold the leaves in such a way that the leaves get plenty of light from the sun. (ii) The leaves: Leaves are important parts of plants. They manufacture food for the plants. They are green because they contain a green pigment. This pigment is called chlorophyll. To manufacture food, the green leaves need sunlight, air and water. The process of making food in the presence of sunlight is called photosynthesis. (iii) Flowers and fruits: In a flower, the green leaf-like parts in the outermost circle are called sepals. Towards the centre of a flower many little stalks with swollen tops are present and they called stamens. The swollen tops are called anthers. They contain a powdery substance called pollen. The stamen is the male part of a flower. In the centre of the flower, there is a flesh-shaped organ called the carpel. The carpel is the female part of a flower. The little swollen portion at the base is called the ovary. The ovary contains egg-like structures called ovules. Pollen are transferred to the carpel in a process called pollination. This is done by insects, wind and water. Eventually, the ovules of the flower turn into more...

Elements, Compounds and Mixtures  

• Matter can be classified into elements, compounds and mixtures. These, in turn, are made of small particles such as molecules and atoms.
• Elements: When a substance is made of only one kind of atoms, it is called an element. For example, copper, gold, silver, iron, are all elements. Similarly, oxygen is a gaseous element made of oxygen molecules and each oxygen molecule is made of two oxygen atoms. Hydrogen, nitrogen, chlorine are other examples of gaseous elements.
• Compounds: When two or more atoms combine in a fixed ratio, they produce compounds. They can be broken down into the elements from which they are made. Water is an example of a compound. Each water molecule is made of two atoms of hydrogen and one atom of oxygen. Thus on analysing water, we find two different types of atoms, those of hydrogen and oxygen. Sugar is yet another example of a compound. A molecule of sugar is made of 12 atoms of carbon, 22 atoms of hydrogen and 11 atoms of oxygen. Each compound contains its constituent elements in a fixed proportion.

Note: Properties of a compound are different from the properties of its constituent elements. For example, hydrogen bums, oxygen supports burning and both are gases but water is a liquid and it puts off fire.

• Mixtures: Many substances we come across in our daily lives are neither pure elements nor compounds; they are mixtures. A mixture could contain several elements and compounds. For example, air is a mixture of several gases. It contains elements such as nitrogen, oxygen and compounds such as carbon dioxide and water vapour. It also contains some other gases and dust particles.

  SYMBOLS AND FORMULAE

• Till date, more than hundred chemical elements are known to man. All the matter in the entire universe is made from them. These elements include metals like zinc, copper, silver, gold, iron, sodium, potassium. And non-metals like carbon, silicon, oxygen, hydrogen, sulphur. Nitrogen and iodine. Most of them occur on the Earth in various amounts. Ninety per cent by mass of the Earth's crust is composed of only five elements—oxygen, silicon, aluminium, iron and calcium.
• When referring to these elements and their many compounds, it would be very inconvenient to use their full names all the time. Thus scientists use symbols and groups of symbols as "chemical shorthand" to represent elements, compounds and chemical reactions conveniently and accurately.
• Each element is symbolised by a single letter or two letters of the English alphabet. In most cases, the first letter of the name of an element is taken as a symbol and is written in capitals. For example, H stands for hydrogen, S for sulphur, 0 for oxygen and C for carbon.
• In some cases, two or more elements have names beginning with the same letter. To avoid confusion, more letters from an element's name are added to its symbol. For example, C1 is used more...

Heat Energy  

• Energy is the ability to do work. When an object has the ability to do work, it is said that it has energy.
  • Heat is a form of energy and has the ability to do work.
  • Heat energy can also be converted to other forms. For example:
• When charcoal (coal of wood) is burnt, it emits light. In this way, heat energy produces light.
• The heat energy in a firecracker produces both sound and light.
• In a hot air balloon, the hot gases are lighter than the surrounding air. They rise and are capable of lifting heavy masses. Here heat energy is used to produce mechanical energy.
• A steam engine converts heat energy into mechanical energy and powers the train.
• Other forms of energy can also be converted to heat energy. For example,
• The heat (heat energy) produced from the mechanical energy by rubbing palms vigorously against each other can be felt easily.
• When a candle bums in air, chemical energy is converted into heat energy.
• In an electric bulb, electrical energy is converted into light and heat.

  EFFECTS OF HEAT

• When an object is heated, many changes take place. For example,

• The object may expand in size.
  • It may change its state (ice changes into water on heating).
  • Heat can also speed up chemical reactions.
  • Heat can even kill.
• In fact, we boil milk and drinking water to kill harmful bacteria. So, heat may kill germs.

  HOTNESS AND TEMPERATURE

• The hotness or coldness of an object can be measured from the temperature of that object, i.e., the degree of hotness of an object is called its temperature.
• We can compare the temperature of two objects and decide which is higher by using our sense of touch. But we do this only if their heat is bearable to us.
• Even if the heat of the two objects is bearable, touching them might not give a very reliable estimate of their temperatures. Similarly, we cannot rely on our sense of touch to measure the temperature of a sick man.

  AMOUNT OF HEAT

• We can measure the amount of heat from the change in temperature that it produces in an object.
  • The unit of heat is called Celsius.
  • One calorie is the amount of heat that can raise the temperature of 1 gram of water by\[1{}^\circ C\] . This means that when a calorie of heat is supplied to 1 gram of water, its temperature will rise by\[1{}^\circ C\].
• Sometimes, we also use kilocalorie to measure the amount of heat. One kilocalorie is equal to 1000 calories. That is, 1 kilocalorie can raise the temperature of 1 kilogram of water by\[1{}^\circ C\] .
  • The energy content of food is measured in kilocalories.
• The modem and generally accepted unit of heat energy is no longer calorie, more...

Transfer of Heat  

• When hot objects are kept away from a source of heat, they cool down. For example, when milk is removed from a burning stove, it starts cooling down. This is because the hot milk releases part of its heat energy to the surrounding air and cools down. The surrounding air absorbs the heat from it and heats up. Of course, the increase in temperature of the air is so less that it can hardly be detected. But, if we dip a spoon in the milk, we can quickly detect the rise in the temperature of the spoon. This is called transfer of heat.
• Heat from a hot body is transferred to a cold body in three different ways: (i) conduction (ii) convection (iii) radiation

(i) Conduction: In a solid, the molecules are closely packed. When one end of the solid is heated, the molecules at that end absorb the heat energy and begin to vibrate rapidly. They, in turn, cause their neighbouring molecules to vibrate. This process continues along the rod and energy is transferred from the hotter part to the colder part. Such a transfer of energy between different parts of an object or from one object to another in contact, is called conduction. There are two important conditions for heat to be conducted from one object to another. These are: (a) The two objects should be in contact (b) Their temperatures should be different When we place the spoon in the hot milk, all these conditions are satisfied. Note: Heat flows only from a hotter object to a colder one. (ii) Convection: The process of hotter fluid moving and transferring heat to the colder surroundings is called convection. For example, on heating water in a container, water at the bottom gets heated first. This warm water, being lighter than the surrounding cool water, rises and the colder water moves down, gets heated and rises again. This process continues until all the water in the container is heated. The heat is transferred from the hot water at the bottom of the container to the cooler at the top by the actual movement of the water molecules. This is called convection. (iii) Radiation: This mode of transfer of heat does not need any medium between the two bodies. The hot body emits heat rays to its surroundings just as a bulb or a candle emits lights rays. Therefore, this method of heat transfer is called radiation. We receive heat from the Sun by radiation. The Sun radiates heat rays in all directions which travel all the way to all the planets in the solar system. The amount of heat absorbed by a body depends on the distance between it and the source of the radiation. The farther the two are, the smaller the amount of heat is transferred. The planets farther away from the Sun receive less heat radiation from the Sun than the Earth does. They are colder than the Earth. Note: Besides more...

Light and Shadows   LIGHT

• Light is needed to see things. We may shave eyesight, but we cannot see anything without light. We need a source of light to make objects visible to us.

  SOURCE OF LIGHT

• An object that gives out light is called a source of light. These sources of light can be categorised in two ways:

(i) Natural sources of light: Sun, moon, stars, glow-worm are some examples of natural sources of light. (ii) Artifical sources of light: Candle, oil lamp, gas lamp, torch, electric bulb are some example of artificial sources of light.

• Some light sources are brighter than the others. For example, a candle is brighter than a firefly.
• The modem internationally accepted unit of brightness of a source is lumen (1 watt is equal to about 700 lumen).
• Light from a source such as a candle or an electric bulb or the Sun spreads in all directions. It gets dimmer as v/e move away from it. The brightness on given surface is measured in units of "lumen per unit area".
• Sources of light can be categorised as hot and cold sources also. For example, an electric bulb is a hot source while a tubelight is a cold source of light. The reason for this difference is the method light is produced them. For example, the electric bulb has a wire inside it called the filament. Unless the filament is heated to a high temperature, it will not glow. The filament is heated by passing an electric current through it. A hot filament which glows is said to be incandescent. A tubelight does not use a filament but a different mechanism to glow. Hence, it is a cold source.
• The only objects in the sky which are natural sources of light are the Sun and the stars. The Moon is only a cold non-luminous mirror for the Sun's light. It is a reflector of the Sun's light.

  HOW LIGHT TRAVELS

• Light travels in a straight line. To find out how light travels, let us perform an activity. Take a lit candle and place it on a table. Take a rubber tube and look through it at the flame. First, stretch the tube straight and look. The flame can be seen. Now bend the tube and look through. The flame is not visible. This implies that light always travels in a straight line.

  SHADOWS

• Some materials allow light to pass through them completely, some allow only partially light to pass through them and some do not allow the passage of light all. Based on this, materials are categorised in three ways:

(i)   Transparent materials (ii)   Translucent materials (iii) Opaque materials (i) Transparent materials: Those materials that allow light to pass through them are called transparent materials. For example, we can look at a lighted candle through a clear glass or some more...

Mirrors and Reflection of Light  

• An object acts in three ways to the light that falls on it. It may allow the light to pass through, it may completely cut off the light from passing by absorbing the light, or, the object may scatter the light incident on it.

  REFLECTION OF LIGHT  

• The light beam (1) above the mirror consists of unstopped light rays. Light beam (2) defines the length of the shadow. Rays (3), (4), (5) and (6) are stopped by the mirror.
• Instead of passing through, the mirror surface bounces light rays like a smooth wall bounces a ball.
• The light rays falling on a mirror are scattered back. This scattering back is called reflection.
• The reflection of light from a smooth and an uneven surface is different, as the smooth surface gives a clear image due to regular reflection. On the other hand, the reflected rays from an uneven surface are scattered in all directions. This is why a mirror that has lost its smoothness does not give a clear image.

  IMAGE FORMED BY A PLANE MIRROR

• Our image in a plane mirror looks different from us. The left appears right, and the right appears left in the mirror. This phenomenon is called "lateral inversion". We, therefore, say that the image formed by a plane mirror is laterally inverted.
• On looking at the image of all the letters from A to Z in a plane mirror, the images of some letters appear to be the same as the original letters. H and 0 are two examples.
• The image formed by a plane mirror is of the same size as the object.
• The image is also erect. It means the top of the object does not become the bottom part of the image and bottom vice-versa.
• Another feature of the image formed in a plane mirror is that if we move close to the mirror, our image also moves closer. Similarly, if we move away from the mirror, our image also moves away. Thus, the distance of the image behind the mirror is equal to the distance of the object in front of the mirror.

The image formed of an object by a plane mirror is "virtual". It means that we cannot obtain the image formed in a mirror on a screen placed anywhere behind the mirror. An image which can be obtained on a screen is called "real image".   SPHERICAL MIRRORS

• The plane, convex and concave mirrors act very differently from each other. We use all three mirrors in our daily life. Plane mirrors form virtual images. Convex and concave surfaces are segments of a hollow sphere like a tennis ball. The outside surface is convex while the surface facing inside is concave.
• Concave mirrors are used in designing headlights of cars, train more...

Sound  

• We hear many sounds around us. For example, the sound of buses and cars running on the street, the chirping of birds on the trees, the rustling of leaves in the breeze, the sound of rain falling on the roof and the sound of thunder in the monsoon.
• In each case, the sound is produced when the object undergoes a rapid to-and-fro motion. Such motion is called vibration or oscillation. In some cases, the vibrations are easily visible to our naked eyes such as the rustling of tree leaves. But in some cases, they are so small that we feel them with our palms. For example, we can feel the vibrations of a transistor, radio or television by placing our hands on the speaker. If we grab the vibrating tree leaves, their vibrations stop and so does the sound.

  VIBRATIONS OR OSCILLATIONS

• A vibration is a repeated to-and-fro motion. This motion is also called an oscillation.
• The distance an object travels from its central position to an extreme position is called the amplitude of the oscillation.

 

• When the object goes from one extreme position "B" to the other extreme position "C" and then back to "B", we say that it Mid-position of object completes one oscillation. The rime taken to complete one oscillation is called "time period".
• The number of oscillations per second is called frequency of the oscillation. Frequency is measured in hertz. If an object makes 10 oscillations in a second, we say that the frequency is 10 hertz (10 Hz).

  LOUDNESS AND PITCH

• Any vibration or oscillation has an amplitude and a frequency. The amplitude tells us how far the object travels from its central position. The frequency tells us how fast it repeats its position oscillatory motion about its central position.
• Sound is produced when objects vibrate. Due to vibrations of the object, the molecules of air close to it also start vibrating with the same frequency. The motion of these molecules causes the neighbouring molecules to vibrate similarly and so on. Soon all the air molecules in the vicinity begin to imitate the vibrating object and start oscillating. If we place our ear in the vicinity of vibrating air molecules, we can feel the vibrations as sounds.
• The loudness of a sound depends on the amplitude of vibration. When the amplitude of vibrating air molecules is large, we say that the sound is loud.
• The frequency of vibration gives a sound its shrillness or pitch. If the frequency of vibration is high, we say that the sound has a high pitch.
• Our ears cannot hear a sound if its amplitude is too small, i.e., they do not respond if the frequency of oscillation is less than 20 hertz nor can they hear this sound if its frequency is greater than 20,000 hertz.
  • Sounds of frequency greater more...

Motion and Time   Synopsis  

• Change in the position of a particle with respect to time and a fixed place (observer) is known as motion.

 

• The rate of change of position in unit time interval is known as speed. Speed is also the distance covered by an object in unit time. Speed is relative and the same body can be said to be moving fast or slow when compared to different things.

   

• The S.I. unit of distance is metre (m) and that of time is second(s), therefore the unit of speed is metre per second (m/s or m s'1). The C.G.S. unit of speed is cm/s or cm s1.

 

• A body is said to be in non - uniform motion, if its speed or direction keeps changing.

 

• A body is said to be in uniform motion, if it moves with a constant speed in a straight line.

 

• Time is measured in seconds(s), minutes(min), hours(h), days, weeks, months, years and so on.

 

• Time is measured using clocks, watches, sundials, sand clocks, etc.

 

• Devices to measure time make use of some kind of periodic motion.

 

• The simple pendulum is one of the simplest instruments used to measure time. A simple pendulum consists of a heavy metallic ball (called the bob) which is suspended from a rigid surface by a thread.

 

• One complete ‘to’ and ‘fro’ motion is known as an oscillation.

 

• The time taken by the pendulum to complete one complete oscillation is known as the time period of the pendulum. The time period of a pendulum at a place depends only on the length of the pendulum and not on the weight or material of the bob.

 

• The symbols of all the units are represented by a single letter. To measure the speed of an object, the distance travelled by it and the time taken must be known. If the speed and time taken by the body is known, then the distance travelled can be determined.

 

• Total distance travelled = Speed Total time taken
• If the speed and the distance travelled is known, then the time taken can be found

   

• Different graphs are used to represent different kinds of data (or information).

 

• A speedometer measures speed while an odometer measures the distance covered.

 

  Image   Learning Objectives  

• Images
• Concept of Mirror Image
• Concept of Water Image

  Images   The reflection of an object as seen in a surface is called Image. Image of an object is similar to the object but it may be inverted or laterally inverted.   Concept of Mirror Image   Image of an object as seen in a mirror is called mirror image or reflection of an object. The left of the object appears right and right appears as left in the image. This phenomenon is called lateral inversion. For example, mirror image of K =                Some English alphabets have identical mirror images. These are - A, H, I, M, O, I, U, V, W, X, Y.   Example Select one alternative which exactly matches with the mirror images of the word in the question given below. EMANATE             (a) ETANAME                   (b)              (c) ENAMEAT                                      (d) EATEMAN                          (e) None of these Ans.     (b) Explanation: Mirror image of the letters E =, M = M, A = A, N =, T = T Therefore mirror image of the word EMANATE will be.   Select one alternative which exactly matches with the mirror image of the word in the question given below. 247593               (a) 395742                     (b)                   (c) 392457                     (d)                     (e) None of these Ans.     (b) Explanation: Mirror image of the digits 2 =, 4 =  7 = , 5 =, 9 = more...

  Number System   Learning Objectives  

• Number Line
• Fractions and Decimals
• Rational Number

    Number Line       Number line is a model that helps us visualise adding and subtracting of integers. A number line is a picture of a straight Sine on which every point is assumed to correspond to a real number and every real number to a point.   Absolute Value Absolute value means to think only about how far a number is from zero.                             For example “6” is 6 away from zero, but” -6” is also 6 away from zero. So the absolute value of 6 is 6 and the absolute value of- 6 is also 6.   Properties of addition of integer

• Commutative property:  \[\left( A + B \right) = \left( B + A \right)\].
• Associative property:  \[\operatorname{A} + \left( B + C \right) = \left( A + B \right) +C\].

• Additive Identity:  Additive identity is any number which when added to a number N will result in the same number, i.e., N + 0 = N. Hence, 0 is the additive identity.
• Additive Inverse:  Inverse identity is any number which when added to a number M will result in zero. Here, N + (-N) = 0. Hence, in general, additive inverse of N is -N.

  Properties of subtraction of integer  

• Commutative property does not exist for subtraction, i.e.,  \[2-3~\,\,\ne \,\,3-2\]
• Associative property also does not exist for subtraction, i.e.,  \[2-\left( 3 - 4 \right)~\ne \left( 2 - 3 \right)  4\]

• Identity property: Same as that of Additive identity. 0 + (-N) = -N, where N is positive.
• Inverse property: N is the inverse of a number -N where N is positive, since N - N = 0.

  Properties of multiplication of integers

• Closure property: Integers are closed under multiplication, i.e. for any two integers, a and b, ab is an integer.

     Example:  \[5~\times 6 = 30; -9~\times -3 = 27\]

• Commutative property: Division is NOT commutative for integers- For any two integers a and b

           Example: 3/6 = 1/2.

• Associative property: Multiplication is associative for integers. For any three integers, a, b and c, more...