Current Affairs 8th Class

*     Magnetic Force   A magnet is a material that attracts certain other types of the material towards itself. Such materials are called the magnetic material. For example, the materials like iron, nickel etc are attracted by the magnet. The force that attracts the magnetic material is called magnetic force. The magnet has two poles, north pole and south pole. Like poles repel each other and unlike poles attract each other. If we break a bar magnet into several pieces, each piece will behave like an independent magnet. We cannot separate the poles of the magnet.       common.jpg            Rojer takes a rod and connects its one end with the electric current. He finds the rod behaves like a magnetic and exerts attractive force on magnetic material, such as, iron and nickel. The force which is being exerted by the rod is known as_________. (a) Magnetic force (b) Electrostatic force (c) Electromagnetic force (d) Nuclear force (e) None of these   Answer: (c) Explanation The magnetic force that generates due to the electric current is called the electromagnetic force.            Our earth behaves like a huge bar magnetic. All the object falling from a certain height is attracted towards the centre of the earth. The magnetic behavior of the earth is solely due to the certain elements present in the core of the earth. The elements due to which earth behave like a huge bar magnetic is ___? (a) (Ni, Fe)  (b) (Ni, Co)     (c) (Fe, Co)     (d) (Zn, Fe) (e) None of these   Answer: (a) Explanation The element, which is present in the core of the earth and due to which it behaves like a magnet, is Ni and Fe.     *            Electrostatic Force An electrically charged object exerts force on an uncharged object. Such force is called the electrostatic force. For example, if you rub a plastic comb with your hair and bring the comb near the tiny piece of the paper, the piece of paper will either stick to the comb, or will start moving due the electrostatic force. Similarly, an ebonite rod when rubbed with the woolen cloth, it acquires the negative charge and attracts the tiny piece of paper.               common.jpg                 When a plastic comb is rubbed with hair, it acquires some charges due to which it attracts tiny pieces of paper. The types of charge, which the comb acquires form the hair is ____. (a) Positive  (b) Negative (c) α charge (d) β charge (e) more...

*       Introduction   Light is a form of energy, which produces the sensation of vision. The sense ofsight is one of the important senses that the God has given to the humanbeings. The object, such as, the Sun, the stars, lamps, even lightning bugs,gives off light. They are called luminous bodies (from the Latin word lumen,meaning light). Other objects, such as, trees, grass, keyboard etc. are theexamples of non luminous objects. They are visible only when they receivelight from some luminous sources and reflect it to our eyes. Whether a body is luminous or non luminous, depends much on the nature ofmaterial of which it is made up of. By changing the nature of material, we canmake many familiar objects, both luminous ornon luminous, at will. Thefilament, or fine wire inside an electric light bulb is non luminous, unless it isheated by an electric current passing through it. We can take a piece of ironand make it glow red, yellow, or white by heating it in burning coals or over agas flame. When solid and liquid, such as, melted metals are heated totemperatures above 800 degrees Celsius (about 1500 degrees Fahrenheit), theyturn to be a source of light. Such materials are called incandescent bodies. Wecan, in fact, see any object only when ray of light falls on it and gets reflected..

*       Force   Force is required to move or stop an object from its current state. If a car is running on the road, driver applies a force on the break to stop the car. Again, force is also required on the accelerator to move the car form its rest. In both the conditions, the force is required. The unit for the measurement of applied force is Newton and it is represented by the first letter of its name, N. Let a body P is in rest at point A, a force of 10 N is applied by a body J, and another force of 20 N is applied by a body K in the same direction as shown in the picture below. Direction of possible displacement by the body P on resulting force       The net or resulting applied force on the body is the sum of both the applied forces = Applied force by J + Applied force by K = 10 N + 20 N = 30 N and the body P moves in the direction the applied forces. If the body J applies the same amount of force in the same direction but the direction of applied force by the body K is changed, and the body K applies the same amount of force in opposite direction as shown in the figure below.     The net or resulting force on the body P = Applied force of 20 N by the body K -Applied force of 10N by the body J = 20 N-10 N = 10 N and the body P moves in the direction of applied force by the body K (greater applied force).   *         Types of Force Let us know about the different types of forces in detail.   *         Gravitational Force Everything is attracted by the Earth. The force with which the earth's pull the things towards itself is called gravitational force. If an object is thrown upwards, it comes down on the earth due to the gravitational force.   *           Magnetic Force The force applied by a magnets on other magnetic materials is called the magnetic force. When an iron piece is placed near a magnet, it attracts the iron due to its magnetic force. Two magnets can attract each other or repel each other. In both the conditions, the force which works between them is called magnetic force.   Look at the following picture of magnetic field around a magnet:     *           Muscular Force We can pull an object due to the force applied by the muscles of our body. more...

*     Sound   Sound plays an important role in our lives. Sound waves help us to communicate with each other. Sound is produced due to the vibration of an object. If we strike a steel plate, sound comes out until the vibration of the steel plate stops. Therefore, sound is produced when a body strikes on another body.     *            Voice box in Human Body The upper part of wind pipe has voice box which produces sound in the human body. Two vocal cords across, the voice box or larynx, leave a narrow slit between them for the passage of air. Sound is produced when lungs force air through the slit and the vocal cords vibrate.   Look at the following picture of voice box (Larynx) in the human body:       Sound cannot travel in vacuum, it requires medium to travel. The medium can be anything. For example, air, water, solid, liquid, etc. The intensity of sound is called loudness. The speed of sound in a medium depends on the property of the medium. Dense medium allows the sound to travel fast. Whereas, light medium allows the sound to travel at low speed. Sound travels faster in a water body in comparison to air, because the density of air is less than the density of water. The speed of sound depends on the temperature of the medium also. It increases on increasing the temperature. The speed of sound in air at 20°C is 343 metre per second whereas the speed of sound in air at 0°Cis 331 meter per second.  
Sr. No. Medium Speed in meter per second
1. Air 343
2. Water 1493
Learning Objectives
  • Introduction
  • Force
  • Effects of force
  • Types of force
  • Thrust and Pressure
  • Pressure in fluids
  • Factors affecting pressure in fluids
  • Atmospheric pressure
  •   INTRODUCTION It is our common experience that to move a stationary table, we need to push it or pull it. To stop a moving ball we have to apply some force opposite to its direction of motion. If the object is quite heavy it may not be set into motion by a force. A heavy object in motion may not be brought to rest on applying force on it. Thus based on these observations we can now define force.   FORCE Force is defined as an external agent (push or pull) that changes or tends to change the state of rest or 01 uniform motion of a body.   Do You Know Acceleration due to gravity on the surface of the earth \[(9.8m{{s}^{-2}})\] is 6 times the acceleration due to gravity on the surface of moon    FORCE Force is defined as an external agent (push or pull) that changes or tends to change the state of rest or of uniform motion of a body. Mathematical definition of force - Whenever force acts on a moving body it changes the speed of the body. Thus force produces acceleration (Positive or negative). Greater the force, greater is the acceleration. Also more the mass of the body, more is the force required to produce acceleration in it. Hence force is directly proportional to the mass and acceleration of the body.   \[\text{Force = mass}\times \text{acceleration}\] \[\text{F}\,\text{= ma}\]   Thus, mathematically forces defined as the product of mass and acceleration. When a body falls under the force of gravity. Then, the force exerted on the earth is equal to the weight of that body i.e. \[W=m\times g.\] Where, g = acceleration due to gravity\[g=9.8\text{ }m/se{{c}^{2}}\] on the surface of the earth at mean sea level.   Units of force The S.I. unit of force is newton (N) 1 newton force moves a body of mass 1 kg with an acceleration of \[1m/{{s}^{2}}.\]C.G.S. unit of force is dyne. 1 dyne force moves a body of mass 1 g with an acceleration of \[1\,cm{{s}^{-2}},\]   Relationship between newton and dyne Using F = ma \[1N=1\,kg\times 1m/{{s}^{2}}\] \[=1000g\times 100\,cm/{{s}^{2}}\] \[=100000gcm/{{s}^{2}}\] Since 1 dyne = 1\[g\,cm/{{s}^{2}}\] \[\therefore 1\,N\,={{10}^{5}}\,\text{dynes}\]   Do You Know It is possible to have motion in the absence of force and to have force in absence of motion. more...

    Learning Objectives
  • Introduction
  • Sources of light - Natural and Artificial
  • Luminious and Non-luminous objects
  • Transparent, translucent and opaque objects
  • Rectilinear propagation of light
  • Pinhole camera
  • Shadows and Images
  • Solar and Luna eclipses
  • Reflection of light
  •   INTRODUCTION We see the colourful and beautiful world around us. Light helps us to see things around us. We cannot see light but light creates sensation of vision. We detect light with our eyes. Light is a form of energy.   SOURCES OF LIGHT - NATURAL AND ARTIFICIAL The objects that give out light are called sources of light. There are two types of source of light
  • Natural sources: Sun and stars are natural sources of light.
  • Man-made (or artificial sources): Electric bulb, glowing tube-light, fire, burning candle etc. are all man-made sources of light.
  •   Do You Know Sometimes luminous objects give off light at different wavelength which is invisible. These materials are called fluorescent materials.   LUMINOUS AND NON-LUMINOUS OBJECTS The objects which emit their own light are called luminous objects. For e.g. Sun, stars, torch, fire, flame of a candle are luminous objects. We can see luminous objects due to the light emitted by them. The objects which do not emit their own light are called non-luminous objects. For e.g. Earth, table, chair, flowers etc., are non-luminous objects. Non-luminous objects are visible only if the light falling on them from luminous objects.   TRANSPARENT, TRANSLUCENT AND OPAQUE OBJECTS An object which allows most of the light pass through it is called a transparent object. For e.g. Air, glass, water, etc. An object which allows only a part of the light to pass through it is called translucent object. For e.g., Milky white plastic, frosted glass etc. An object which does not allow light to pass through it is called opaque object. For e.g., wood, cement, metal sheet etc.   Do You Know Ultraviolet rays can go through translucent objects. Due to this a person behind this object can get a sunburn on a Sunny day. Speed of light is different in different media. Speed of light is maximum in vacuum i.e., \[3\times {{10}^{8}}m/s.\]   RECTILINEAR PROPAGATION OF LIGHT Light travel in a straight line. It is known as rectilinear propagation of light.   If you see the flame through the holes at same level, the flame is visible but if one of the cardboards is displaced such that holes are not in straight line the flame more...

    Learning Objective
    • To understand the term percentage and value of percentage.
    • To understand the terms cost price, selling price, profit and loss.
    • To learn how to calculate profit, loss, profit percent, loss percent, selling price cost price.
    • To understand the terms simple interest and amount.
    • To learn how to calculate simple interest and amount.
      PERCENTAGE     The term percent means "for every hundred". A fraction whose denominator is 100 is called percentage and the numerator of the fraction is called the rate percent. Thus, when we say a man made a profit of 20 percent we mean to say that he gained Rs 20 for every hundred rupees he invested in the business, i.e., 20/100 rupees for each Rupee. The abbreviation of percent is p.c. and it is generally denoted by %.   VALUE OF PERCENTAGE Value of percentage always depends on the quantity to which it refers consider the statement: “65% of the students in this class are boys". From the context, it is understood that boys form 65% of the total number of students in the class. To know the value of 65%, the value of the total number of student should be mown. If the total number of students is 200, then, the number of boys \[=\frac{200\times 65}{100}=130;\] It can also be written as \[(200)\times (0.65)=130\]. Note that the expressions 6%, 63%, 72%, 155% etc. do not have any value intrinsic to themselves. Their values depend on the quantities to which they refer.   COMPARING PERCENTAGES Which of the three 25%, 5% and 125% is the largest? If should be remembered that no comparison can be made about the above percentages, because they do not have intrinsic values. If 25% refers to 25% of 10,000 then its value is 0.25 x 10,000 = 2,500 and if 75% of 100, its value is 0.75 x 100 =75. And so we can conclude that 25% of 10,000 > 75% of 100.                                       Note: Percentages can be compared only when the quantities they refer to are known.                    IMPORTANT RESULTS 1. To express a percentage as a fraction divide it by \[100\Rightarrow \] a percentage = 1/100. Example: Express the following as fraction (a) 25%                 (b) \[33\frac{1}{3}%\] Sol. (a) \[25%\,=\frac{25}{10}\left[ \text{since}\,\text{ }\!\!%\!\!\text{ }\,\text{means}\,\frac{1}{100} \right]=\frac{1}{4}\] (b)  \[33\frac{1}{3}%\,=\frac{100}{3}%\,=\frac{100}{3\times 100}=\frac{1}{3}\] 2. To express a fraction as a percent multiply it by \[100\,\frac{a}{b}\,=\left[ \left( \frac{a}{b}\times 100 \right) \right]\,%\] Example: Express \[\frac{1}{8}\] as a percentage. Sol. \[\frac{1}{8}=\frac{1}{8}\times 100%\] \[=\frac{100}{8}%=\frac{25}{2}%\,=12\frac{1}{2}%\] 3. To express percentage as a decimal we more...

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