→ Incident Ray: The ray of light which is directed towards the mirror is called incident ray.
→ Reflected Ray: The ray of light which bounces back after reflection is called reflected ray.
→ Normal: It is perpendicular to the reflecting surface at the point of incidence.
→ A plane mirror reflects the ray of light.
Laws of Reflection
(i) ∠i (Angle of incidence) = ∠r (Angle of reflection)
The angle of incident is equal to the angle of reflection
(ii) The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.
→ The laws of reflection are valid in regular as well as irregular or diffused reflections.
Types of Reflections
→ There are two types of reflection:
(i) Regular Reflections: Reflection from a smooth surface like that of a mirror is called regular reflection.
→ Smooth or polished surfaces gives regular reflection.
(ii) Irregular Reflections: When all the parallel rays reflected from a plane surface are not parallel, the reflection is known as diffused or irregular reflection.
→ Uneven of unpolished surfaces gives irregular reflection.
Image formation by a plane mirror
• Lateral inversion: Left part of the candle appears on the right and its right part appears on the left. This is known as lateral inversion.
→ Objects that give their own light are known as luminous objects
→ Objects that are visible because of reflected light are known as illuminated objects.
Multiple Reflections
→ Two mirrors kept parallel to each other forms multiple images of each other.
→ Kaleidoscope works on the principle of multiple reflections.
→ Periscope is an optical device used to see objects that are not along the line of sight.
Dispersion
→ Sunlight consists of several colours.
→ Splitting of white light into different colours is called dispersion.
• Number of image formed in multiple reflection = (360°/angle between two plane mirror) – 1
Human eye
→ The image forms on retina.
→ Iris controls the size of pupil.
→ Pupil controls the amount of light.
Blind spot
→ The junction of optic nerve and the retina is called blind spot.
→ No sensory cells are there. Therefore, no vision is possible.
→ Impression of an image persists for second in our brain.
→ Cone cells are sensitive to bright light and rod cells are sensitive to dim light.
→ The disappearance of the cross or the round mark shows that there is a point on the retina which cannot send messages to the brain when light falls on it.
Protection of Human Eye
• To protect your eyes, the following points should be remembered:
→ Avoid reading in dim light.
→ Wash your eyes at least four times a day with clean and cold water.
→ Wash your eyes quickly if dust particles or small insects enter your eye.
→ Visit an eye specialist regularly. Improper vision can cause stress, eyestrain, and even headaches.
→ While reading, maintain a distance of atleast 25 cm between your eyes and the book.
→ Do not rub your eyes. If redness in the eye persists, then consult an eye specialist immediately.
→ Avoid direct exposure to sunlight. Exposure to a large amount of light can harm your retina.
→ Vitamin A (raw carrots, broccoli, green vegetables, cod-liver oil, etc.) is necessary for good
vision.
→ Some persons cannot see since birth and some may loose their eye sight because of a disease.
→ Visually challenged persons try to identify things by touching and listening.
→ Braille system is helpful for visually challenged persons.
→ Many Indian languages can be read using the Braille system.
Chapter 15 Some Natural Phenomena notes class 8th science
A Natural Phenomenon is anything that occurs on its own in nature without any kind of human intervention.
For example, the weather of a place, fog, storms, winds, tides, volcanic eruptions and cyclones all can be categorized as natural phenomena.
Some natural phenomena can be destructive such as cyclones, thunderstorms, lightning and earthquakes.
However, there are some ways with which we can protect and prepare ourselves from these natural disasters.
Figure 1 Examples of Natural Phenomenon
Lightning
Lightning is an electric discharge or an electric spark that occurs in nature on a major scale. It is caused by the accumulation of charges in the clouds.
It can be deadly and cause the destruction of life and property.
In ancient times people were unaware of the cause of lightning and hence they were scared of it. Nowadays, scientists have evolved some precautions that can help us prepare and protect ourselves from this natural phenomenon.
Figure 2 Lightning
The sparks that Greeks knew about
The Greeks were already aware of the electric charges from 600 BC. They knew that when Amber (a type of resin or synthetic polymer) and fur are rubbed together, then amber can attract light objects like hair.
Similarly, woollen clothes or polyester clothes also attract hair and can produce a little spark or crackling sound when they are taken off from the body. This is because of electric charges.
Benjamin Franklin was an American scientist who in 1752 discovered and proved that lightning and spark produced from these clothes are all same things.
What are electric charges?
Figure 3 Electric Charges
We know that every atom comprises of subatomic particles such as electrons, protons and neutrons.
All these particles share a common property that they carry electric charges.
Electrons have a negative charge on them while protons have a positive charge.
We know that atoms carry a balanced charge however these charges may sometimes become out of order.
An object will be called electrically neutral if it is carrying a balanced proportion of positive and negative charges.
An object is called a charged object if there is an imbalance of electrons and protons in it.
Charging by rubbing
When we rub two objects with each other they get charged due to a transfer of electrons between them.
For example, if we rub a rubber balloon with animal fur, the balloon is made up of rubber attracts the electrons from the animal fur.
This results in rubber having an excess of electrons while fur having a shortage of electrons.
In the same way, if we rub a plastic comb with dry hair the comb acquires some charge.
Types of charges and their interaction
Figure 4 Charged objects
We know that charged objects may have a shortage or excess of electrons.
Objects having an excess of electrons are called negatively charged while an object having a shortage of electrons are called positively charged.
For instance, when a glass rod is rubbed with silk cloth it becomes positively charged while the silk cloth becomes negatively charged.
These charged objects are now capable of attracting other charged and uncharged objects.
Objects having the same kind of charges repel each other while objects with different kind of charges attract each other.
Figure 5 Interaction between Charges
What is an electrostatic force?
The force of attraction or repulsion experienced by charged objects is called electrostatic force.
Figure 6 Electrostatic Force
What is a static electric charge?
Static charge or static electricity is an electric charge which does not move. Static charges are a result when two objects are rubbed with each other. When two surfaces come in contact with each other repeatedly it results in the transfer of electrons from one material to another. The strength of an electric charge depends upon different factors such as:
The temperature and humidity
Properties of the surface such as its material
Figure 7 Static Charges when woollen sweater and balloon are rubbed together
In opposite to static charge, there is an electric current. The electric current results when the charges flow or move from one point to another. This electric current results in glowing of bulb or working of all the electrical appliances.
Figure 8 Electric Current
Transfer of charges
Charges can transfer from one object to another with the help of conduction and induction:
Conduction: when a charged object comes in contact with a conductor it results in the transfer of charges through the conductor.
Figure 9 Charging by Conduction
Induction: When a charged object is brought near a neutral object, it results in shifting in the position of the electrons in the other object.
Figure 10 Charging by Induction
The process of induction does not involve any physical contact between the charged and uncharged object while the process of conduction requires a physical contact between them.
How the transfer of electric charges leads to conservation of charge?
The net charge on a neutral object is zero. However, when an object receives some electrons from another object, the net negative charge on the first object is equal to the net positive charge of the second object. In this way, charges are conserved during transfer of charges.
Figure 11 (a) Amber and Cloth both have net zero charge (b) Amber and Cloth are rubbed together (C) Amber and Cloth together have net zero charge
What is an electroscope?
It is a device which can test if an object is charged or not. Abraham Bennet developed a gold leaf electroscope in 1787.
Structure of an electroscope
Generally, gold and silver are used to construct an electroscope because they are good conductors of electricity. Otherwise, copper and aluminium can also be used.
It consists of a glass jar having a vertical brass rod.
The rod is inserted into the jar through the cork.
The brass rod has a brass disc or horizontal rod attached to it.
From the other end, two leaves of gold are suspended.
Figure 12 Electroscope
Working of an electroscope
When a charged object touches the brass disc, electric charges get transferred from the brass rod to the gold leaves. As a result, the gold leaves move away from each other depicting the presence of charges.
Figure 13 Working of an electroscope
Discharging and Earthing
When a charged object loses its charges it is said to be discharged.
When a charged object transfers its charges to the earth it is called earthing. Generally, every building is provided with earthing to protect it from electrical shocks due to leakage of electric current.
Figure 14 How Earthing Protects us form Shock
Figure 15 How Can we get a Shock Without Earthing
Story of Lightning
Figure 16 Lightning
During a thunderstorm, the hot air moves upwards while the raindrops fall downloads.
This leads to a separation of charges in the atmosphere.
As a result, negative charges get accumulated in the lower ages of the clouds while positive charges accumulate at the upper edges.
The ground is accumulated with a positive charge all over.
These charges begin to multiply due to the increase in winds and rainfall.
Although the air is a poor conductor of electricity due to heavy charges it is unable to restrict the electric flow after some time.
The negative charges and positive charges meet which results in the production of electric sparks in the form of a streak of light accompanied by a sound.
The streak is called lightning and the whole phenomenon is called electric discharge. This electric discharge can occur between two or more clouds.
Why does lightning strikes tall buildings easily?
We know that lightning occurs as a streak of charges that fall toward the ground. Tall buildings and tall trees can easily conduct the charge towards the ground. The air gap between these buildings and lightning is short hence they are more susceptible to lightning. That does not mean that short objects would not be affected by lightning.
Lightning safety
One should not stay at an open place during lightning and thunderstorm. Hence as soon as we hear any alert about lightning or thunderstorm, we should rush to a safe place such as a house or a building. If somebody is there in a car or bus, they should stay inside and keep all the doors and windows closed. One should read inside the safe place until the storm lasts.
Do’s and don’ts for lightning safety
Outdoor Safety
One should not stay in an open place such as an open vehicle like a motorbike, tractor, or open fields, elevated places, or tall trees.
One should not carry an umbrella during the storm.
If a person is around a forest they should hide under short trees.
One should not get near to any poles on metal objects.
One should squat low on the ground instead of laying down.
Figure 17 Lightning Safety
Indoor Safety
Lightning is an electric discharge hence one should stay away from electrical wires telephone, cables and metal pipes during a thunderstorm.
One may use a cordless phone or a mobile phone in an emergency.
One should not come in contact with the running water hence one should avoid bathing.
One should unplug all the electrical appliances in the house, for example TV, computers or music systems. Electrical lights do not cause any harm and hence can be kept on.
Figure 18 Lightning Safety
Using a lightning conductor
In order to protect buildings from lightning, a lightning conductor device is used.
When the building is being constructed a metallic rod having height more than the building is placed in the walls of the building.
One end of the rod is in the air while the other end is buried inside the Earth.
This rod is a conductor and hence during lightning it allows the flow of electric charges to the ground.
Figure 19 Lightning Conductor
Working of a lightning conductor
The lightning conductor rod consists of pointed ends which are made up of copper wire.
These copper wires are brought down along the building and are attached to a metallic plate in the ground.
If lightning hits the building the copper wires carry these charges quickly to the ground.
Earthquakes
Some natural phenomena such as thunderstorms and cyclones can be predicted by meteorologists. However, there are certain natural phenomena that are uncertain and cannot be predicted accurately. One of them is an earthquake.
Earthquake is a natural phenomenon that occurs as shaking or trembling of the Earth’s surface.
It occurs or lasts for a very short span of time.
The main cause of earthquake is disturbances inside the crust of the earth.
Deep inside the earth, earthquakes occur all the time however they are not noticeable on the earth’s surface. Sometimes major earthquakes occur on the earth surface which can be destructive.
Earthquakes can lead to:
Loss of life
Loss of property such as buildings, dams and bridges
Floods
Landslides
Tsunamis
Figure 20 Destruction due to Earthquake
Cause of an earthquake
Earth’s surface is divided into several layers the crust, mantle, inner core and outer core.
Figure 21 Layers of Earth Surface
The outermost layer of the Earth is divided into several plates. These plates are always moving.
As they move past each other or collide, disturbances are caused in the earth’s crust. These disturbances are called earthquakes or tremors.
Figure 22 Plates on Earth’s Surface
Figure 23 Movement of India’s Earth Plates
Other causes of an earthquake can be:
Volcanic eruptions
When a meteor hits the Earth surface
The nuclear explosion under the Earth surface
Although the causes of an earthquake are clear the scientists cannot predict when the earthquake would occur.
What are seismic zones?
The movement of plates causes an earthquake. Hence the earthquakes are most likely to occur on their boundaries. The areas that lie on the boundaries of these plates are called weak zones, seismic zones or fault zones.
Figure 24 Seismic Zones in India
What is the Richter scale?
A scale which is used to determine the magnitude or strength of an earthquake is called the Richter scale.
Destructive earthquakes have a Richter scale magnitude of more than 7.
A Richter scale is not a linear scale, that is, a magnitude of Richter scale 6 does not imply that it is one and a half times more destructive than an earthquake with a magnitude of 4.
It rather means that an earthquake of a magnitude of 6 is 100 times more powerful than an earthquake with a magnitude of 4.
Figure 25 Richter Scale and its interpretation
What are seismic waves?
The earthquakes produce waves on the earth surface which are called seismic waves. These waves travel in all the directions on the Earth’s surface.
What is Focus of an earthquake?
The focus of an earthquake is a point inside the earth’s surface where the earthquake originates. It is also called hypocenter.
What is an Epicentre?
The epicentre is a region on the earth’s surface that lies just above the focus of the earthquake is called epicentre. The earthquake does not originate from the epicentre.
Figure 26 Map of Earthquake
What is a seismograph?
A seismograph is an instrument which can record the seismic waves.
It contains a metal rod or a pendulum which can vibrate as the earthquake occurs.
The metal rod is attached to a pen which records the waves on the paper.
Scientists study these waves and then construct a map of the earthquake.
This also helps them in determining the power of the earthquake.
Figure 27 Seismograph
Protection against an Earthquake
The buildings located in the seismic zones should be constructed in a way that they can handle major tremors. The following measures should be taken by people to make the houses of buildings quake-safe:
People should always consult architects and engineers before constructing a new building.
The roofs of the building should be kept light.
Timber or mud should be used in the construction of buildings rather than heavy materials.
The shelf and cupboard should be fixed to the walls so that they do not fall off easily.
Things such as photo frames, geysers, clocks and other wall hangings should be placed in a place that does not harm anybody if they fall off.
Buildings should have firefighting equipment as earthquakes may cause fires.
Figure 28 How buildings can be made Quake-Safe
How can people protect themselves from an earthquake?
Indoors safety measures
One should hide under a table until the earthquake stops.
One should not stay close to heavy and tall objects.
If one is in bed they should cover their head with a pillow instead of getting up.
Outdoor safety measures
One should find a place which has no tall buildings around or even trees.
If a person is inside a car or a bus they should drive slowly to a clear place and be inside until the earthquake stops.
A conductor is any material or substance that allows electricity to flow through it.
An insulator is any substance or material that prevents the flow of electricity through it.
Any substance can be called as a conductor of electricity if it allows movement of charges through it.
The electrons of the conductors can flow freely (they are delocalized) and hence can take electric current through them.
Insulators do not allow the flow of charges through them because their electrons are tightly packed with their particles.
Figure 2 Delocalized Electrons in a Conductor
Some materials can allow a little flow of electricity through them and are called poor conductors of electricity.
Some materials can allow the complete flow of electricity through them and are called good conductors of electricity.
Every material may conduct electricity in certain situations. For example, air is a bad conductor of electricity but in case of thunderstorms and lightning it carries electric charges through it. Hence, materials are always classifies as good and poor conductors of electricity rather than conductors and insulators.
Can liquids conduct electricity?
Not all liquids can conduct electricity. However, some of them can be regarded as good conductors of electricity while others as poor conductors of electricity.
Water containing salts and minerals dissolved in it always conduct electricity.
Distilled water which does not contain any salts cannot conduct electricity.
Any solution of acids or bases can also conduct electricity.
Figure 3 Liquids that are Poor and Good Conductors of Electricity
How do liquids conduct electricity?
Different substances when mixed in water and electricity is passed through them can break apart and form positive and negative particles or ions in the water.
These ions can pass the electric current through them.
The more is the number of ions in a liquid the better conductor it is of electricity.
That is why distilled water is a poor conductor of electricity but salt water is a good conductor of electricity.
However, many compounds do not form any ions on mixing them with water and therefore they are poor conductors of electricity such as sugar water, oil and alcohol.
Figure 4 Set up to detect if a liquid can conduct electricity
Why LED bulbs are more suitable for testing the electrical conductivity of liquids?
The electric current often causes heating effect due to which the filament of the bulb gets heated up and glows.
However, some liquids are capable of conducting electricity but they are weak conductors of electricity. Hence current passes through them but it is not that strong enough to heat up the filament. As a result, the filament would not light up in the case of such liquids.
However, the LED bulbs can detect the flow of even a small amount of electric current as well. Hence, LED bulbs are suitable for testing the electrical conductivity of liquids.
What is electrolysis?
The effect in which components of a compound get split due to passing an electric current through it is called electrolysis.
Figure 5 Electrolysis Process
What is an electrode?
An electrode is a conductor of electricity that can carry electric current into non-metals and other poor conductors of electricity.
What is an electrolyte?
A solution that breaks into its ions on passing electricity through it is called an electrolyte. Electrolytes are used in the process of electroplating.
What are an anode and cathode?
The positively charged electrode is called anode and the negatively charged electrode is called cathode.
What are anions and cations?
An anion is a negatively charged ion and a cation is a positively charged ion.
Effects of an electric current
Heating effect: electric current causes heating of the electrical equipment. For example, the filament of a bulb gets heated up due to electric current and therefore glows.
Mechanical effect: electric current can lead to generation of mechanical energy in appliances. For example, fans and motors work due to this effect.
Magnetic effect: electric current can give rise to the magnetic field of a substance.
Chemical effect: electric current can lead to the production of chemical energy or chemical reactions.
Chemical effects of electric current
We know that when an electric current passes through solution it ionizes and breaks down into ions. This is because of chemical reactions that take place when an electric current passes through a solution. Depending on the nature of the solution and the electrodes used, the following effects can be observed in the solution:
metallic deposits on the electrodes
change in the colour of the solution
a release of gas or production of bubbles in the solution
Applications of chemical effects of electric current
Electroplating
Electroplating is a process in which layer of metal is deposited on another material with the help of electricity.
Electroplating is used in many industries for depositing a layer of metal with desired characteristics on another metal.
Different metals used for electroplating are Nickel, Copper, Gold Silver, Tin, Brass, Zinc, Chromium and Platinum.
Process of electroplating
In order to conduct electroplating right electrodes and electrolytes must be chosen so that metal can deposit over a material.
For instance, if we want to deposit copper on a material we need an electrolyte that contains copper in it. Similarly, if we need gold on a material we need an electrolyte that contains gold in it.
Also, we should make sure that the electrode that we are choosing is completely clean.
The electrodes used are made up of different materials. One of the electrodes is of the same metal of which the electrolyte solution is. The second electrode needs to be the material on which we want to coat another metal.
For instance, in case we want to plate copper upon brass, one electrode should be of Copper and the other electrode should be of Brass and the electrolyte solution should be any salt which contains copper in it, for example, copper sulphate solution. Consider the diagram given below that describes the process of electroplating of copper.
Figure 6 Electroplating of copper on brass
Out of these two electrodes the copper electrode acts as the anode (positive electrode) and brass electrode acts as the cathode (negative electrode).
When electricity is passed through the solution, the copper sulphate breaks down into its ions.
The copper ions (they have a positive charge) get attracted by the brass electrode while the sulphur ions being negatively charged move towards the copper electrode.
As a result, copper starts depositing on the brass electrode.
The process of electroplating takes some time to complete.
The amount of time that it will take depends upon the strength of the current that is being passed through the circuit and also upon the concentration of the electrolyte.
As these two are increased the speed of the electroplating process also increases.
Applications of electroplating
Figure 7 Electroplated Objects
Medical equipment is made up of nickel which is harmful to the human body hence to avoid it from coming in contact with our body a coating of platinum or gold is applied on the surface of nickel.
Many kitchen equipments, bath taps, parts of cars etc. are covered with chromium coating. Chromium is an expensive metal hence the objects are created with the cheaper metal and chromium coating is provided. Thus, to bring a shining over the objects and prevent them from corrosion chromium coating is used.
Jewellery makers often make ornaments of less expensive metals and provide a coating of gold or silver upon them.
The tin cans that are used to store food are actually made up of iron and have a coating of tin on them. Iron can easily react with food and spoil it, however, tin prevents the food from getting reacted with iron and therefore helps in preventing it from getting spoiled easily.
Bridges and various parts of automobiles are made up of iron because it provides strength. However in order to prevent iron from getting rusted a coating of zinc is provided over it. This method is also called galvanization of iron.
Other applications of Chemical Effect of Electric Current
1. Extracting metals from their ores
Pure metals are extracted from metal ores with the process of electrolysis. Electricity is passed through the metal ores and they get broken down into an ionic lattice and thus the metal is obtained separately. For example, metals like aluminium, magnesium, potassium, sodium and calcium are obtained from their ores in this way.
Figure 8 Extraction of Aluminium
2. Purification of metals
The method of electrolysis is also used to purify a metal by separating it from the impurities. The impure metal is used as an anode which first dissolves in the electrolyte solution and then deposits on the cathode in the pure form. The impurities of the metal remain in the electrolyte solution only. Metals like aluminium, zinc and copper are purified in this way.
Figure 9 Purification of Copper using Electroplating
3. Production of compounds
The electrolysis method is used for the production of some compounds. For example, sodium hydrochlorite
4. Decomposition of compounds
The electrolysis method is also used to decompose a compound into its constituents. For example, water can be decomposed using the process of electrolysis to obtain hydrogen and oxygen.
Figure 10 Decomposing water to separate hydrogen and oxygen
→ The vehicles, any object, moving over the surface of another object slows down due to the force of friction applied on it.
Friction
→ It is an opposing force that acts between surfaces in contact moving with respect to each other.
→ It always opposes relative motion between two surfaces.
Cause of friction
→ Friction is cause by the irregularities on the two surfaces in contact.
→ We are able to walk because of the force of friction.
Nature of surfaces for Friction
Smooth surfaces: less friction Rough surfaces: greater friction
Friction when object is pressed hard
Greater pressing force means Greater friction.
Friction according to mass of object
Greater mass: Greater friction
Sliding friction < Static friction Rolling friction < Sliding friction
Examples where friction is useful
(i) Walking (ii) Handling any object (iii) Rolling motion of ball or wheel
Examples where friction is harmful
(i) Energy dissipation of engine due to friction between surfaces in motion. (ii) More energy is lost in pulling or pushing an object in rough surfaces. (iii) Wear and tear of shoe soles and tyres.
(iv) Lubrication: Powder on carom board, oil in machine (v) Wheel: Wheels reduce friction (because rolling friction < sliding friction). (vi) Shoe soles and tires are threaded to increase friction for a better grip. (vii) Fluid friction is minimised by giving suitable shapes to vehicles moving through fluids.
Rolling Friction
→ When a body rolls over the surface of another body, the resistance to its motion is called rolling friction.
Sliding Friction
→ When a body slides over the surface of another body, the resistance to its motion is called sliding friction.
• Rolling friction < Sliding friction
→ Ball bearings change sliding friction into rolling friction. Fluid Friction
→ Fluid friction is minimised by giving suitable shapes to vehicles moving through fluids.
• Fluid friction is also called drag.
→ Friction force depends upon the speed of the object with respect to the fluid.
→ Friction force depends upon the shape of the and nature of the fluid.
→ Actions like picking, opening, shutting, kicking, hitting, lifting, flicking, pushing, pulling are often used to describe certain tasks.
→ Each of these actions usually results in some kind of change in the motion of an object.
Force
→ A push or pull on an object is called a force.
• Push: When an object is moving away from the applier of force.
• Pull: When an object is moving towards the applier of force.
→ Force is a push or a pull which changes or tends to change the state of rest or of uniform motion, or direction of motion or the shape or size of a body.
→ Force is any action that has the tendency to change the position, shape, or size of an object.
→ Interaction of one object with another object results in a force between the two objects.
→ The effect of force depends on the magnitude and direction of the force.
→ Force applied in the same direction added to one another.
→ Force applied in the opposite direction, the net force is given by the difference of two forces.
→ Force can move a body initially at rest.
→ Force can bring a moving body to rest.
→ Force can change the direction of a moving body.
→ Force can change the speed of a moving body.
→Force can change the shape of a body.
→ Force can change the size of a body.
Muscular force
→ It involves the action of muscles.
→ Animals make use of muscular force to carry out their physical activities and other tasks.
Friction
→ It is an opposing force that acts between surfaces in contact moving with respect to each other.
→ The direction of force of friction is always opposite to the direction of motion.
→ Frictional force always acts between two moving objects, which are in contact with one another.
→ Frictional force always acts opposite to the direction of motion.
→ Frictional force depends on the nature of the surface in contact.
→ Non-contact force come into play even when the bodies are not in contact.
Magnetic force
→ Force acting between two magnets or a magnet and a magnetic material. Example: iron, steel, nickel, cobalt etc.).
→ It can be attractive and repulsive.
Electrostatic force
→ Force due to electric charges.
→ It can be attractive and repulsive.
Gravitational force
→ It is a kind of attractive force that comes into play because of the mass of a body. Example: earth’s gravitational attraction. Pressure
→ The force acting per unit area of surface is called pressure.
• Pressure = Force/Area on which it acts
→ The unit of pressure is Newton per square meter (N/m2), which is also known as Pascal.
→ Smaller the area larger the pressure for the same force.
→ Liquids exerts pressure on the walls of the container.
→ Pressure exerted by liquids increases with depth.
→ Liquids exert equal pressure at the same depth.
→ The pressure at which water comes out of the holes is directly proportional to its depth.
Fluid
→ Substance which can flow and has no fixed shape
• Pressure due to a liquid column of height h (p) = hrg
where, h = Height of column r = Density of fluid g = Acceleration due to gravity
→ Pressure inside a fluid increases with increase in depth and density of the fluid.
→ Water and gas exert pressure on the walls of their container.
→ Atmosphere exerts pressure on the surface of the Earth.
• Atmospheric pressure = Weight of the atmosphere per unit area.
Pressure inside our body is equal to the atmospheric pressure and cancels the pressure from out side.
Air surrounding the Earth: atmosphere
Air exerts pressure on its surroundings: thrust on unit area is called atmospheric pressure
→ Testes are involved in the production of male gametes called sperms.
→ Millions of sperms are produced by the testes.
→ Each sperm consists of three parts: Head, middle piece and tail.
Female reproductive system
→ It consists of ovaries, oviduct and uterus.
→ Ovaries produce ova or eggs.
→ A single matured egg is released from ovary into oviduct every month.
→ Baby develops in the uterus.
→ Egg is also single celled like sperm.
Fertilization
→ The process of fusion of male and female gametes (egg and sperm) to form zygote is known as fertilization.
→ It is of two types: (i) Internal fertilization (ii) External fertilization
Internal fertilization
→ In this, the fusion of sperm and egg takes place inside the female’s body.
→ It occurs in cows, dogs and humans.
External fertilization
→ In this, the fusion of sperm and egg takes place outside the female’s body in a surrounding medium, generally water.
→ It occurs in frogs, fishes, starfish, etc.
Test tube baby
→ A baby conceived by fertilization that occurs outside the mother’s body is called test tube baby.
→ Development of the embryo
→ The zygote repeatedly divides to form a ball of cells.
→ The ball of cells then starts differentiating into tissues and organs. At this stage, it is called embryo.
→ Embryo gets attached to the wall of the uterus and develops various body parts such as hands and legs.
→ Foetus is a stage of embryo that shows main recognizable feature of mature organism.
→ Foetus develops for nine months inside the mother’s womb and is finally delivered.
Fertilization in Humans
→ Fusion of the nucleus of the sperm with the ovum to form a zygote. It occurs in the fallopian tube of females.
→ Zygote divides to form an embryo.
→ Embryo is implanted in the uterus.
→ Foetus develops inside the mother’s body for nine months (gestation period).
Asexual reproduction
→ The type of reproduction which involves only a single parent and the new individuals are formed without the fusion of gametes is known as asexual reproduction.
• Three common methods of asexual reproduction are: (i) Budding (ii) Fission (iii) Cloning
Budding
→ It involves the formation of new individual from the bulging of the parent body.
→ This phenomenon is very common in plants, fungi and animals such as Hydra and yeast.
Fission
→ Binary fission is the type of asexual reproduction that occurs in Amoeba.
→ It is a type of asexual reproduction in which a single cell divides into two halves.
Cloning
→ Cloning is the process used to create an exact copy of a cell, tissue or an organism.
→ Dolly, a sheep was the first mammal to be cloned. It was cloned by Ian Wilmut and his colleagues in 1996.
Oviparous animals
→ The animals that lay eggs are called oviparous animals.
→ The examples include all kinds of birds, lizards, snakes, and frogs.
Viviparous animals
→ The animals that give birth to young ones are called viviparous animals.
→ The examples include cows, dogs, and humans.
Metamorphosis
→ The biological process of transformation of larva into an adult is known as metamorphosis.
• The life cycle of frog consists of the following stages: Egg → Tadpole (larva) → Adult
→ Hormones controlling metamorphosis in frogs
→ Thyroxin (produced by the thyroid gland) initiates the process of a tadpole’s metamorphosis into an adult frog.
→ In the absence of thyroxin, the tadpole does not transform into an adult and remains in the tadpole stage.
Life cycle of silkworm
→ Silkworm grows on mulberry trees and feeds on its leaves.
→ During a stage in its life cycle, silkworm spins a cocoon around itself.
→ Silk is obtained from this cocoon.
Hormone responsible for metamorphosis in insects
→ In insects, metamorphosis is controlled by the insect hormones. Some of the insect hormones are: (i) Prothoracicotropic Hormone (PTTH) (ii) Ecdysone (iii) Juvenile Hormone (JH)
Discovery of the Cell → Cells are the basic structural units and the building blocks of all living organisms. → Cell was discovered by Robert Hooke in 1665 after observing a piece of cork under a magnifying device.
→ Robert Hooke coined the term “cell”.
Cell Theory
→ Schleiden and Schwann proposed the cell theory.
→ According to cell theory: Cells are the basic structural and functional units of life.
→ All living organisms are made up of one or more cells.
→ New cells arise from pre-existing cells.
Types of Organisms on the basis of cell
(i) Unicellular Organisms (ii) Multicellular Organisms
Unicellular organisms
• Number of Cells Organisms made of only a single cell are called unicellular organisms. For example: Amoeba and Paramecium
→ All the basic functions such as digestion, respiration, excretion, etc. in these organisms performed by a single cell.
Multicellular organisms
• Organisms made up of more than one cells are called multicellular organisms. For example: Humans, cow, rose, etc.
→ In these organisms, the cells show division of labour as particular set of cells are involved in performing a specific body function.
Shape of the Cells
→ Most of the cells have a definite shape.
→ Some cells such as that in Amoeba have no definite shape.
→ The human red blood cell (RBC) is spherical-shaped.
→ The muscle cells in humans are spindle-shaped.
→ The human nerve cells have elongated branched structure.
→ In plants and bacteria, the cell is enclosed in a protective covering called cell wall, which gives shape and rigidity to the cells.
Size of the Cells
→ The smallest cell is 0.1 to 0.5 micrometre in bacteria.
→ The largest cell is of size 170 mm x 130 mm, which is the egg of an ostrich.
→ Size of a cell has no relation with the size of an organism.
Cell Structure and Functions
→ In multicellular organisms, each organ system is made up of several organs.
→ Organs are further made up of tissues.
→ Tissues are groups of similar cells performing a specific function.
→ Number of cells Organisms made up of only a single cell are called unicellular organisms. For example: Amoeba and Paramecium
→ Single cell in unicellular organisms performs all the basic functions such as digestion, respiration, and excretion.
→ Organisms made up of more than one cells are called multicellular organisms. For example: Humans, cow, etc.
→ In multicellular organisms, the cells show division of labour as a particular set of cells are involved in performing a specific body function.
Types of cell
There are two types of cells.
(i) Prokaryotic cells (ii) Eukaryotic cells
• Prokaryotic cells
Cells which do not have a well defined nuclear membrane and the nuclear material lies freely in the cytoplasm of the cell. For example: bacteria, blue green algae.
• Eukaryotic cells
Cells having nucleus with well defined nuclear membrane. For example: plant and animal cells
Components of the cell
Cell membrane
→ It is the protective layer that surrounds the cell.
→ Cell membrane selectively allows the entry of only some substances and prevents the movement of other materials.
→ Hence, it checks the transport of substances in and out of the cell.
Cell wall
→ In plants, an extra protective covering of a polysaccharide, cellulose is present.
→ It is called cell wall that protects plant cells from environmental variations.
Cytoplasm
→ It is a jelly-like substance present between cell membrane and nucleus.
→ It contains various cell organelles such as mitochondria, Golgi bodies, lysosomes etc.
Nucleus
→ It is a dense spherical body located at the centre of the cell.
→ It is surrounded by porous nuclear membrane.
→ It contains spherical body called nucleolus.
→ It also contains thread-like structures called chromosomes.
Chromosomes
→ These are the structures that carry genes and play an important role in inheritance.
→ Genes are the structural and functional unit of inheritance.
→ The entire living substance in a cell is known as protoplast.
Vacuoles
→ Vacuoles are fluid-filled membrane-bound structures in the cell.
→ In plant cells, a single large vacuole is present.
→ In animal cells, numerous small vacuoles are present.
→ The membrane of the vacuole is called tonoplast.
→ This membrane encloses a fluid called cell sap.
Plastids
→ They are present only in plant cells.
→ Plastids that contain green colour pigment chlorophyll are known as chloroplasts.
→ It is the chlorophyll that gives green colour to the leaves.
→ Chloroplast traps solar energy and utilizes this energy to manufacture food for the plant.
• Plastids are of two types: (i) Leucoplasts (ii) Chromoplasts
→ Leucoplasts are colourless and are used to store food while chromoplasts are plastids containing pigments.
→ Chloroplasts are a type of chromoplasts.
• Chloroplasts consist of two regions: (i) grana (stacks of sac like membrane bound structures that contain pigment chlorophyll) (ii) stroma (ground substance containing enzymes and starch grains)
Endoplasmic Reticulum (ER)
(i) Rough Endoplasmic Reticulum (RER): It is important for the synthesis and packaging of proteins. (ii) Smooth Endoplasmic Reticulum (SER): It acts as storage organelle. It also helps in lipid (fat) synthesis.
Golgi Apparatus
→ It is made up of parallel arranged membrane-bound vesicles called cisternae.
→ It helps in storage, modification, and packaging of products in vesicles.
→ It helps in formation of glycoproteins and glycolipids.
Lysosomes
→ It is a membrane-bound structure that holds variety of enzymes.
→ Rich in all types of hydrolytic enzymes, which are active at acidic pH.
→ It is involved in the digestion of carbohydrates, proteins, lipids, and nucleic acids.
Mitochondria
→ It is a double membrane-bound structure.
→ The inner membrane of mitochondria is deeply folded to form cristae.
→ Cristae increase the surface area in the organelle.
→ It is the site of cellular respiration and hence known as ‘power house of cell’.
→ They have their own circular DNA.
→ They divide by fission.
Differences between plant and animal cells
Plant Cells
Animal Cells
Cell wall is present.
Cell wall is absent.
Nucleus is located in the periphery of the cell.
Nucleus is located in the centre of the cell.
Plastids are present.
Plastids are absent.
A large single vacuole is present in the centre of the cytoplasm.
Numerous small vacuoles are present in the cytoplasm.
→ Deforestation is the process of clearing of forests in order to use the land for industrial, agricultural, and other purposes.
Causes of Deforestation
→ Natural causes → Forest fire → Severe droughts → Man-made causes → Using land for agricultural purposes → Rapid urbanization → Procurement of wood for fuel and furniture
Consequences of Deforestation
→ Increase in the level of carbon dioxide in atmosphere, which leads to global warming
→ Lowering of ground water levels
→ Increase in pollution level and temperature
→ Decrease in fertility of soil and amount of rainfall
→ Increase in frequency of droughts and floods
→ Desertification- conversion of fertile lands into deserts.
Biodiversity
→ Biodiversity is the species richness of the biosphere. It is defined as the number and variety of life forms such as plants, animals and microorganisms in an area.
→ It supports all the essential living resources such as wild life, fisheries and forests.
→ Forests help in maintaining the delicate balance of nature.
→ Animals living in forests are called wild animals.
→ The plants found in a particular area are known as flora of that area.
→ The animals found in a particular area constitute fauna of that area.
→ Those species of plants and animals, which are found only in a particular area, are called endemic species.
→ Species is a group of organisms in population which are capable of interbreeding.
→ The animals, whose numbers are diminishing to a level that they might face extinction, are called endangered animals. For example: tiger, lion, and elephants
→ Project tiger was launched by the government of India to protect endangered tigers in their natural habitat.
→ The flora and fauna of a particular habitat can be protected through special protected areas.
Protected areas
A protected area is a clearly defined geographical space which is recognised and managed by government to achieve the long term conservation of nature, culture and preserve ecosystem.
Wildlife sanctuary
→ It is the place where wild animals are protected from hunting and are provided with suitable living conditions. For example: Madhumalai wildlife sanctuary in Tamil Nadu, Chilika bird sanctuary in Orissa, etc.
National parks
→ These are the areas reserved for wildlife. They are maintained and preserved by the government for the public to visit. For example: Ranthambore National Park in Rajasthan, Kanha National Park in Madhya Pradesh, etc.
→ Satpura National Park is the first reserve forest of India.
Biosphere Reserves
→ It is a large protected land for conservation of wild life, plant and animals resources, and the traditional life of the tribal groups living in the area. For example: Pachmarhi Biosphere Reserve and Nilgiri Biosphere Reserve in India.
Red Data Book
→ It is the source book maintained by IUCN (International Union for Conservation of Nature and Natural resources).
→ It keeps a track record of various endangered species of plants and animals.
Migration
→ It is the movement of birds and animals from their original habitat to other places at a particular time.
→ Migratory birds fly to distant areas every year during a particular time because of climatic changes their original habitat becomes very cold and inhospitable lack of food availability
→ Numerous migratory birds including ducks, geese, flamingos, and cranes fly to India every year.
Recycling of paper
→ One ton of paper is made from about seventeen fully-grown trees.
→ Papers should be recycled and reused to conserve forest.
→ Each paper can be recycled three to seven times.
→ Recycling of paper saves trees, energy and water.
→ It prevents the release of harmful chemicals emitted during paper manufacturing in nature.
Reforestation
→ Restoring of destroyed forests by planting new trees is called reforestation.
