Lightning and Safety

Lightning and Safety

Lightning is caused due to the heavy electric discharge from the clouds to the earth. If lightning is not controlled, it can cause heavy damage to life and property. Hence, protective measures are of utmost importance during lightning strikes. Interiors of buildings are safe places to protect ourselves from lightning strikes. Taking shelter in interiors and not staying in open areas is one of the best measures you can take. You should not take shelter under trees when in the open, or go to an open area if outside. If you are in a vehicle, you should not come out of it; you should also close the doors and windows of the vehicle.

Tall buildings are provided with lightning rods. A lightning rod a metal rod that is erected at the top of a building. It is connected to a thick metallic plate, made of a good conducting material like copper and buried under the earth, through a thick copper wire. If lightning is likely to strike the building, it is attracted by the lightning rod, and the electrical discharge is transferred through the wire to the ground. Thus, it keeps the building safe.

Introduction and Lightning

Introduction and Lightning

Lightning is a natural phenomenon that has fascinated people for ages. Several people thought and researched about the cause of lightning and its process. Benjamin Franklin discovered that there is an electric discharge between clouds that produces a spark, and it is the electric spark between the clouds and the earth that appears as lightning. His famous kite experiment proved this fact. The occurrence of lightning is as follows. The formation of clouds involves friction between water particles in the atmosphere. The friction charges the particles. Among the positive and negative charges, the negative charge accumulates at the bottom of the cloud and the positive charges in its top. As the accumulation of the charge increases, the cloud will create a positive charge on the ground nearby. As the amount of charge increases, the negative charge on the cloud tends to make a path towards the ground, and it results in a narrow streak of electrical discharge, which we call lightning.

An electroscope is a device that detects the type of charge on a body. Like charges repel and unlike charges attract each other.  This is used in an electroscope. An electroscope consists of a glass jar fitted with a cork lid and a metallic wire passing through it. There are two metallic strips at the bottom of the wire. The upper end of the wire is connected to a metal disc. A body that is positively charged is touched to the metal disc, so that the charge is transferred to the metal strips through the wire, and they diverge from each other on gaining a like charge. Now, if a negatively charged object is brought into contact with the disc, the strips converge towards each other, indicating the unlike charge on the body. Similarly, if a positively charged body is brought in contact with the metal disc, the divergence of the metal strips increases, indicating the like charge on the body. If the metal disc of the electroscope is touched with the hand, it loses its charge to the ground by transfer of charge through the human body. This is called earthing.

Chemical Effect of Electric Current

Chemical Effect of Electric Current

The most common application of the chemical effect of electric current is electroplating. In this process, there exists a liquid, usually called the electrolyte, through which current passes. Two electrodes, connected to the terminals of a battery with a switch in between, are inserted in the liquid. The electrode that is connected to the positive terminal of the battery is called the “anode,” and the other connected to the negative terminal is called the “cathode”. Electroplating is done in industries to have an anti-reactive coating on the parts of machines so that they do not react with the raw material, to have an anti-corrosive coating for the machines so that they do not get corroded, and a heat-resistive coating for parts like boilers to resist the heat produced by the machinery.

Gold plating is one of the most common applications of electroplating in ornament-making. In electroplating, the solution gets dissociated into its respective ions. The positive ions are called cations, and the negative ions are called anions. The current in the solution is due to the flow of these ions. The anions move towards the anode, and the cations move towards the cathode. The cationsare converted into atoms at the cathode and form a layer on it, which we call electroplating. To compensate for the loss of ions in the solution, the atoms of the anode dissociate into the solution, forming the ions. The process of electroplating is used for plating parts of vehicles with nickel and chromium, which protects them from corrosion.

Conductivity of Liquids

Conductivity of Liquids

We can use a tester to check if electricity is flowing through a wire in a circuit. If the bulb in the tester glows, it indicates that current is flowing through the wire; else, current does not exist. Electrical conductivity of liquids can be explained by a simple activity of taking a liquid, like lemon juice, in a container, inserting electrodes in it, connecting the two electrodes to the terminals of a battery with a bulb between them. The bulb glows, indicating that lemon juice is a conductor of electricity. Several liquids can be checked for electrical conductivity on in the same way.

We find that liquids like lemon juice, liquid soap, rain water, salt solution, etc. conduct electricity, whereas liquids like distilled water, oil, etc. do not conduct electricity. The materials that conduct electricity are conductors and those that do not conduct are called insulators. When electricity is passed through a conducting solution, the molecules of the solution dissociate into ions, which cause electrical conduction through the liquid. This process is due to the chemical effect of electric current.

Noise and Music

Noise and Music

Any intolerable and irritating sound is called noise. The word noise comes from the Latin word nausea, meaning seasickness.
Music refers to any sound that is pleasant to the ear. Sound produced by musical instruments is pleasing to the ear. But if the intensity of the sound exceeds a certain limit, then it becomes intolerable and is noise.
Undesirable sounds and disturbances cause noise pollution.
Noise pollution may cause high blood pressure, panic attacks and lack of sleep among those exposed to it. To reduce noise pollution, trees should be planted along roads and in residential areas, factories should not be set up in residential areas, vehicles should not blow horns around schools, hospitals and residential area, TVs and music systems should not be played at high volume.
Continuous exposure to loud noise may cause temporary or even permanent hearing impairment.
Sign language is used by the hearing disabled to communicate.

Characteristics of sound

Characteristics of sound

Sound is a form of energy produced by a vibrating body. Sound requires a medium for its propagation. Sound does not propagate in vacuum. Sound is produced due to the vibration of an object. The to and fro or back and forth motion of an object is called vibration. To produce pleasant sounds, a number of musical instruments have been developed. Some instruments produce sound due to the vibration of membranes, some due to the vibration of strings, and some others produce sound due to the vibration of an air column. The maximum displacement of a vibrating particle from its mean or equilibrium position is called its amplitude. The time taken by the vibrating particle for one full vibration or oscillation is called the time period of vibration. The number of vibrations per second is called the frequency. Frequency is measured hertz (Hz). Sound produced by any means has the following characteristics, namely, loudness, pitch or shrillness, and quality or timbre.

The loudness of sound depends on its amplitude. The loudness of sound is proportional to the square of the amplitude. A roar of a lion is louder than a  woman’s voice. The pitch of sound depends on its frequency. If frequency is more, then the pitch or shrillness is more. The pitch of a woman’s voice is more and it is shriller then a man’s voice. The loudness of sound is measured in Decibel (db). If loudness exceeds 80 db, then the sound becomes physically painful. Not all sound produced by vibrating bodies is audible. The human ear can only recognise sounds of frequencies in the range of 20 Hz and 20,000 Hz. This range of frequency is called audible sound. Some animals like dogs and snakes can hear sounds of frequencies greater than 20,000 Hz. Sounds of frequencies less than 20 Hz are called infrasonic sounds, while sounds of frequencies greater than 20,000 Hz are called ultrasonic sounds.

Production and Propagation of sound

Production and Propagation of sound

Sound is a form of energy that causes the sensation of hearing. Sound needs a medium to travel. Sound travels through gases, liquids and solids. The speed of sound is the maximum in solids, less in liquids and the least in gases. Sound cannot travel through vacuum. In humans, sound is produced by the voice box or the Larynx. Vocal cords in the larynx vibrate and produce sound. We hear sound with our ears. The ear drum has a vibrating membrane like a stretched rubber sheet. When a sound note reaches the ear, the ear drum vibrates, and the vibrations get converted into signals that are carried to the brain to get a sensation of hearing.

To produce pleasant sounds, a number of musical instruments have been developed. Some instruments produce sound due to the vibration of membranes, some due to the vibration of strings, and some others produce sound due to the vibration of an air column. The to and fro or back and forth motion of an object is called vibration. The sitar, veena, violin, guitar and ektara are some stringed instruments. The tabla, cymbals, ghatam, kartal and manjira are some instruments that work on the vibration of a membrane, and instruments like the flute and the trumpet produce sound due to the vibration of an air column present in them.

Minimising Friction

Minimising Friction

Friction is a necessary evil. Without friction, we cannot walk, hold articles, the brakes of automobiles do not work, and the power of a motor cannot be transferred to a grinding machine. However, friction is undesirable in certain areas and we need to reduce it. Friction between the sliding surfaces of two objects can be reduced by making the surfaces in contact smooth by polishing the surfaces. Sliding friction between the moving parts of vehicles can be reduced by using oil, grease or graphite. In electrical plugs, graphite is used to reduce friction between the plug pins and the corresponding socket. In certain machines, like the drill used by a dentist, a layer of air is used as a lubricant to reduce friction between the moving parts.

The interlocking of irregularities on the surfaces of two bodies in contact, which cause friction, is overcome to great extent by the use of lubricants, the material used to reduce friction, and friction is reduced. Rolling friction is less than sliding friction. Hence, sliding friction is replaced by rolling friction by using rollers, like ball bearings between the hub and the axles in the moving parts of machines and vehicles. When an object moves through a fluid, the friction offered is called fluid friction. It is also called drag. Fluid friction depends on the shape of the object and the nature of the fluid. Aeroplanes, boats, fishes and birds which move through fluids have bodies of special shape so as to reduce the friction due to fluid.

Friction and its Types

Friction and its Types

Friction is a force that opposes the relative motion between two surfaces of objects in contact. The force of friction always acts in a direction opposite to that of the applied force. Friction is due to irregularities on the surfaces of the objects in contact. Friction depends on the smoothness of the surfaces in contact. The force of friction depends on the nature of the surfaces in contact. The force of friction increases if the surfaces are pressed harder. Frictional force that comes into action before the start of the motion of an object is called static friction.  When an object slides over another surface, the frictional force that comes into action is sliding friction.

When a roller rolls over a surface, the frictional force that comes into action is rolling friction. Rolling friction is less than sliding friction, while sliding friction is less than static friction. Friction due to gases and liquids is called fluid friction, and is also called drag.  Friction is a necessary evil. There are instances in daily life where friction is a necessity. For example, without friction, we cannot hold objects in our hands; we cannot walk and cannot light a match stick. Examples where friction has to be minimised and not desirable is the friction between machinery parts, which causes wear and tear.

Pressure

Pressure

The thrust acting per unit surface area is pressure. The force acting on an object perpendicular to its surface is thrust. Pressure is measured in newton/metre2, which is equal to 1 pascal (Pa). Liquids and gases exert pressure on the walls of the vessel in which they are carried. Air exerts pressure on all objects. The pressure exerted by air is called atmospheric pressure. A rubber sucker sticks to the surface of an object as atmospheric pressure acts on it. In order to fix nails to walls, sharp nails are used, as the area of contact of the nail with the wall is small and more pressure acts on it for a given force.
A suction pump sucks out all the air from a closed container, creating a vacuum in it.

Force

Force Force is a kind of push or a pull on an object.

Force is a kind of push or a pull on an object. The interaction of one object with another results in a force between the two objects. If a force acts on an object in the direction of its motion, then the object moves faster. If a force acts on an object opposite to the direction of its motion, then the objects slows down. Force may also change the direction of motion of an object. The effect of force changes with the direction of its application. Forces acting on an object in the same direction add to each other, while forces acting opposite to each other results in the difference of the forces. The strength of force is called its magnitude. The international unit used to measure force is newton. A force can change the shape and size of an object. There are many types of forces.

The force resulting from the action of muscles is called muscular force. We make use of muscular force of animals like bullocks, horses and camels get our activities done. The point where force is applied on an object is called the point of contact. Friction is another type of contact force that always acts opposite to the direction of motion of the objects. Magnetic force and electro-static force are non-contact forces as they act on an object from a distance. The force of gravity is also a non-contact and an attractive force exerted by the earth on objects, due to which objects fall to the ground. Gravitational force is an attractive force between any two objects in the universe, and is a non-contact force.

Tests of Divisibility

Tests of Divisibility

In your previous class you have learnt about various types of numbers and the properties that they satisfy. You have also learnt the tests of divisibility with 10, 5, 2, 3, 6, 4, 8, 9 and 11. In this lesson we will learn why the numbers are divisible by 10, 5, 2, 3, 6, 4, 8, 9 and 11.
A number is said to be divisible by another number, when the remainder is zero.

• A number is divisible by 10, if its ones digit is 0.
• A number is divisible by 5, if its ones digit is 0 or 5.

• A number is divisible by 2, if its ones digit is 0, 2, 4, 6 or 8.
• If a number is divisible by 10, then the number is also divisible by 2 and 5.

• A number is divisible by 9, if the sum of its digits is divisible by 9.
• A number is divisible by 3, if the sum of its digits is divisible by 3.
• If a number is divisible by 9, then the number is also divisible by 3.
• If a number is divisible by 3, then it may not necessarily be divisible by 9.

Playing With Numbers

Playing With Numbers

In your previous class you have studied about various types of numbers such as Natural numbers, Whole numbers, Integers, Rational numbers and the various properties such as closure, associative, commutative and distributive. The number in the general form can be written as for example 26 = 2x10 +6. The general form of a 2-digit number  is . The sum of a 2-digit number and the number obtained by interchanging its digits is always divisible by 11. The difference between a 2-digit number and the number obtained by interchanging its digits is always divisible by 9.

The general form of a 3-digit number  is . The difference between a 3-digit number and a number obtained by reversing its digits is always divisible by 99.

Linear Graphs

Linear Graphs

A linear graph is different from a line graph. In a linear graph, the set of points on the graph line up in an orderly manner.  A linear graph is a graph that consists of a single straight line segment.

Cartesian System: when you draw a graph, you start with two straight lines at right angles to each other. The horizontal line is called the x-axis of the graph. The vertical line is called the y-axis of the graph. The x-axis and the y-axis together are called the axes of the graph. The intersection point of the axes is called the origin. Generally, the origin is denoted by the letter O.The position of a point that we draw on the graph is indicated by a pair of numbers within brackets – these are called the coordinates for that point.


The graph of a proportional relationship results in a linear graph. A proportional relationship is expressed as an equation containing two variables. An independent variable can have any value. A dependent variable’s value is determined by the independent variable.

Graphs

Graphs

A graph is a visual display of some data. In most cases, the data exists in the form of a table. In many cases, the table itself is displayed. However, the same data, displayed as a graph, often creates more impact.  A graph also makes it easier to understand, or interpret, the data that is being presented. When you interpret a graph, you can compare values and spot trends in the data. There are many different types of graphs.
A bar graph is shown on the screen.  A bar graph is useful for comparing values over time. A bar graph consists of a series of rectangles or bars.
A bar graph is also useful for comparing values between groups of items. Such a graph is called a double bar graph.
A bar graph in which there is no gap between the bars is called a histogram. A histogram is useful for showing the distribution of data over intervals.



A pie graph or a pie chart is useful for showing the proportion of each data value to the whole.

A line graph usually shows a visual comparison between two quantities using a series of line segments. The line segments are joined end-to-end. The line graph, thus, allows us to observe trends easily. This is a major advantage of the line graph.

Division of Algebraic expressions

Division of Algebraic expressions

We have learnt addition, subtraction and multiplication of algebraic expressions. In this lesson we will learn how to divide algebraic expressions.
In case of numbers Division is the inverse operation of multiplication but the same is applicable for the division of algebraic expressions also. To divide a monomial by a monomial, first express the numerator and the denominator in their irreducible form, and then cancel the common factors. To divide a polynomial by a monomial, either divide each term of the numerator by the denominator or factorise the numerator by the common factor method. To divide a polynomial by a polynomial, first factorise the numerator and the denominator by using the appropriate method and then cancel the common factors.

Factors of Algebraic expressions

Factors of Algebraic expressions

We have already learnt prime factor form wherein a number is written as product of prime factors. Similarly we can factorise an algebraic expression.
The irreducible factor of an algebraic term is a factor of the term that cannot be further factorised. An algebraic expression written as the product of its irreducible factors is called the irreducible form of the term.
Expressing an algebraic expression as the product of its factors is called the factorisation of the expression. This is the factor form of the expression.
The factors of an algebraic expression may be numbers or algebraic expression. The basic methods to factorise an algebraic expression are:

• Identifying the common factors
• Regrouping the terms
• Using algebraic identities

The basic identities used to factorise an algebraic expressions are:

Variations

Variations

In our daily life we come across many situations wherein if there are two quantities, if one increase the other increase or vice-versa.  If one quantity decreases other quantity decreases.
For any two quantities x and y, if the value of y increases with an increase in the value of x such that x/y remains constant, then the two quantities x and y are said to be in direct proportion. Two quantities x and y in direct proportion are written
as x ∝ y.
Two quantities x and y are in direct proportion, then the ratio between the corresponding values of x and y is constant. Thus x₁/y₁ = x₂/y₂ = x₃/y₃ and so on.


For any two quantities x and y, if the value of y decreases with an increase in the value of x such that xy remains constant, the two quantities x and y are said to be in inverse proportion.
Two quantities x and y in inverse proportion are written as x ∝ 1/y. We can also say that if two quantities x and y are in inverse proportion, then the product of corresponding values of x and y is constant, thus, x₁y₁ = x₂y₂ = x₃y₃ and so on.

Exponents

Exponents

A number that represents how many times the number is multiplied by itself is known as exponent.
A given numbers can be expressed in standard as well as usual form. Any number expressed as a decimal number between 1.0 and 10.0, including 1.0 multiplied by a power of 10, is said to be in its Standard Form.
  Where, m is a positive integer and  is called the multiplicative inverse of .
Negative exponents can be used to express very small numbers.

For any non-zero integers a and b we have,
Laws of exponents:
1.      
2.     
3.     
4.      
5.    
6.    
Here m and n are integers.