Heating effect of electric current:
A long piece of nichrome wire is ties between two nails as shown in the figure. Touch the wire. Now switch on the circuit and touch the wire. Switch off the circuit and touch the wire. The wire gets hot when an electric current pass through it. This is called as the heating effect of the electric current. The electric room heater or an electric heater used for cooking contain a coil of wire. This coil of wire is called an element. When the appliances are switched on after connecting to the electric supply, their elements become red hot and give out heat. The amount of heat produced in a wire depends on its material, length and thickness. Thus, for different requirements, the wires of different materials and different lengths and thicknesses are used. The wires used for making electric circuits do not normally become hot. On the other hand, the elements of some electric appliances become so hot that they are easily visible. The filament of an electric bulb gets heated to such a high temperature that it starts glowing.
If a large current pass through a wire the wire may become so hot that it may even melt and break. Wires made from some special materials melt quickly and break when large electric currents are passed through them. These wires are used for making electric fuses. In buildings, fuses are installed in all electrical circuits. There is a maximum limit on the current which can safely flow through a circuit. If by accident the current exceeds this safe limit, the wires may become overheated and may cause fire. If a proper fuse is there in the circuit, it will blow off and break the circuit. A fuse is thus a safety device which prevents damages to electrical circuits and possible fires.
Magnetic effect of electric current:
A small compass needle is placed in the cardboard as shown in the figure. A bar magnet is brought near the compass needle, the needle deflects. Switch on the circuit and then observe the compass needle. It deflects. Needle of a compass is a tiny magnet. When electric current passes through a wire, it behaves like a magnet. This is the magnetic effect of the electric current. The first person to observe this was a scientist named Hans Christian Oersted.
Electromagnet:
A long piece of insulated wire is wound tightly around a nail in the form of a coil. Some pins are placed near the end of the nail. When the current is switched on, the pins cling to the end of the nail. When the current is switched off, the pins fall. Therefore, the coil behaves like a magnet when electric current flows through it. When the current is switched off, it loses its magnetism. These coils are called electromagnets. The electromagnets can be made very strong and can lift very heavy loads. The electromagnets are also used to separate magnetic material from the junk. Doctors use tiny electromagnets to take out small pieces of magnetic material that have accidentally fallen in the eye. Many toys also have electromagnets inside them.
Electric Bell:
An electric bell consists of a coil of wire wound on an iron piece. The coil acts as an electromagnet. An iron strip with a hammer at one end is kept close to the electromagnet. There is a contact screw near the iron strip. When the iron strip is in contact with the screw, the current flows through the coil which becomes an electromagnet. It pulls the iron strip. In the process, the hammer at the end of the strip strikes the gong of the bell to produce a sound. When the electromagnet pulls the iron strip, it breaks the circuit. The current through the coil stops flowing and it is no longer an electromagnet. It no longer attracts the iron strip. The iron strip comes back to its original position and touches the contact screw again. This completes the circuit. The current flows in the coil and the hammer strikes the gong again. The hammer strikes the gong every time the circuit is completed. This is how the bell rings.
Symbols of electric current:
Electric cell:
The longer line represents the positive terminal of the cell and the shorter, thicker line represents the negative terminal of the cell.
Symbol:
Electric bulb: 
Symbol: 
Switch:
In ON
position: Symbol:
In OFF position 
Symbol: 
Battery: Combination of two or more cells such that the positive terminal of one cell is connected to the negative terminal of another cell is called battery. Many devices such as torches, transistors, toys, TV remote controls, use batteries.
Symbol: 
Wire:
Symbol:
Electric circuit:
Circuit diagram when the switch is ON:
When the switch is ON, the circuit is complete. The circuit is said to be closed and the current flows throughout the circuit. In the bulb, there is a thin wire called the filament which glows when an electric current passes through it.
Circuit diagram when the switch is OFF:
When the switch is OFF, the circuit is incomplete. The circuit is said to be open and no current flows through any part of the circuit.
Hot and cold:
In our daily lives, we come across objects that are hot and objects that are cold. Tea is hot while ice is cold. Fire is hot while snow is cold. Some objects are hot while some objects are cold. Some objects are hotter than other objects and some objects are colder than other objects. We often decide if the object is hot or cold by touching it. But sometimes our sense of touch can deceive us. Suppose there are three mugs of water A, B and C. A contains hot water, B contains cold water and C contains mix of hot and cold water. We dip our left hand in mug A and right hand in mug B for few minutes. Now, when we dip both our hands in mug C, our left hand feels hot and right hand feels cold. We are unable to decide whether the water is hot or cold. Therefore, a reliable measure of the hotness of an object is its temperature. Temperature is measured by the device known as thermometer.
Measuring Temperature:
A clinical thermometer is used to measure our body temperature. A clinical thermometer consists of a long, narrow, uniform glass tube. It has a bulb at one end. This bulb contains mercury. Outside the bulb, a small shining thread of mercury can be seen. There is a scale on this thermometer which is in Celsius, indicated by °C. A clinical thermometer reads temperature from 35°C to 42°C.
To read a thermometer, first we need to note the temperature difference indicated between the two bigger marks. Also note down the number of divisions between these marks. Suppose the bigger marks read one degree and there are five divisions between them. Then, one small division can read = 0.2 C.
To measure our body temperature, we need to wash the thermometer and ensure that the mercury level is below 35°C. If it’s not, give it a few jerks. Place the thermometer under the tongue and after one minute take out the thermometer and note down the reading. The normal temperature of human body is 37°C. The temperature of human body normally does not go below 35°C or above 42°C. That is the reason that this thermometer has the range 35°C to 42°C.
Laboratory Thermometer:
Laboratory thermometers are used to measure the temperature of other objects. The range of a laboratory thermometer is generally from –10°C to 110°C. Different types of thermometers are used for different purposes. The maximum and minimum temperatures of the previous day, reported in weather reports, are measured by a thermometer called the maximum-minimum thermometer.
Laboratory thermometers has to be held vertically while measuring. Suppose you want to measure the temperature of the water in a bucket. It has to be held vertically and the temperature has to be measured while it is in the water. As soon as the thermometer is taken out from the water, the mercury level drops.
Transfer of heat:
The heat flows from a hotter object to a colder object. When we keep a frying pan on a flame, the pan becomes hot. The heat is transferred from hot object to a cold object. When the pan is removed from the flame, it cools down after some time. Again, the heat is transferred from hot object to cold object (to the surroundings).
The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction. In solids, the heat is generally transferred by the process of conduction.
Conductors: The materials which allow heat to pass through them easily are conductors of heat. For ex, aluminum, iron and copper.
Insulators: The materials which do not allow heat to pass through them easily are poor conductors of heat are known as insulators. For ex, plastic and wood.
The water and air are poor conductors of heat. Take a round flask and two-third of it is filled with water. A crystal of potassium permanganate is placed at the bottom of the flask gently using a straw. It is kept on a tripod and a candle is placed beneath the flask. When water is heated, the water near the flame gets hot. Hot water rises up. The cold water from the sides moves down towards the source of heat. This water also gets hot and rises and water from the sides moves down. This process continues till the whole water gets heated. This mode of heat transfer is known as convection. It is the same case with air. The air near the heat source gets hot and rises. The air from the sides comes in to take its place. In this way the air gets heated.
The people living in the coastal areas experience an interesting phenomenon. During the day, the land gets heated faster than the water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze. To receive the cooler sea breeze, the windows of the houses in coastal areas are made to face the sea. At night it is exactly the reverse. The water cools down more slowly than the land. So, the cool air from the land moves towards the sea. This is called the land breeze.
The sunlight reaches us through radiation. The transfer of heat through radiation can take place whether a medium is present or not. When we sit in front of a room heater, we get heat by this process. A hot utensil kept away from the flame cools down as it transfers heat to the surroundings by radiation. Our body too, gives heat to the surroundings and receives heat from it by radiation. All hot bodies radiate heat. When this heat falls on some object, a part of it is reflected, a part is absorbed and a part may be transmitted. The temperature of the object increases due to the absorbed part of the heat.
Kinds of clothes we wear in summer and winter:
In summer, we wear light colored clothes and in winter we wear dark colored clothes. Dark surfaces absorb more heat and, therefore, we feel comfortable with dark colored clothes in the winter. Light colored clothes reflect most of the heat that falls on them and, therefore, we feel more comfortable wearing them in the summer.
Woollen clothes keep us warm in winter:
In the winter, we use woollen clothes. Wool is a poor conductor of heat. Moreover, there is air trapped in between the wool fibres. This air prevents the flow of heat from our body to the cold surroundings. So, we feel warm.
Images formed by lenses:
Magnifying lens is a lens used to read very small print. It is also used to observe the body parts of a cockroach or an earthworm. Lenses are widely used in spectacles, telescopes and microscopes. Lenses which are thicker in the middle than at the edges are called convex lenses. Lenses which are thinner in the middle than at the edges are called concave lenses. Lenses are transparent and light can pass through them. A convex lens converges the light. It is also known as converging lens. A concave lens diverges the light. It is known as diverging lens.
Convex lens:
Concave lens:
A convex lens is placed in the path of sunrays. A paper is placed and the convex lens is adjusted such that we see a bright spot on the paper. When we change the convex lens to concave lens, we do not see a bright spot. Image formed by a convex lens has different nature and size for different position. Image formed by a concave lens is always virtual, erect and smaller in size than the object.
Sunlight- white or colored?
A rainbow is a natural phenomenon which appears in the sky after rain when the sun is low in the sky. There are seven colors in the rainbow- red, orange, yellow, green, blue, indigo and violet. We can also see these colors when we blow a soap bubble, or when light is reflected from the surface of a compact disk (CD).
When the sunlight is passed through a prism, it reflects seven colors. Sunlight is said to be a white light but it contains seven colors. A newton’s disc contains a circular cardboard sheet, which is divided into seven segments. Each segment is given one color of the rainbow. When the disc is rotated in daylight, it appears to be white.
Light travels along a straight line:
A beam of sunlight enters a room through a narrow opening or a hole. Beam of light is emitted from the headlamps of scooters, cars and engines of trains, and torch. When we look at a candle through a straight pipe, we can see the candle. But when we look at the candle through a bent pipe, we can’t see it. This shows that the light travels in straight line.
Reflection of light:
The change in the direction of light by a mirror is called reflection of light. A shining stainless-steel plate or a shining steel spoon can change the direction of light. The surface of water can also act like a mirror and change the path of light.
To show the reflection of light, we can do an activity. Take a torch and cover the glass with a chart paper with three slits on it. A beam of light is directed on the mirror from the torch. We can see that the light reflects by the mirror. As the position of the torch is changed, the direction of the reflected light also changes.
When we place a candle in front of a mirror, the mirror shows the same candle on its surface, that candle is known as the image of the candle formed by the candle. The candle is known as object. An image formed by a plane mirror is erect and of the same size as the object. The image formed by the plane mirror is at the same distance behind the mirror as the object is in front of it. The image formed by a plane mirror can not be obtained on a screen.
Right or Left:
In a mirror, the ‘right’ appears ‘left’ and the ‘left’ appears ‘right’. Lift your left hand in front of a mirror, you can see that the image formed raises right hand. When we look at an ambulance, we can see that ‘AMBULANCE’ is written from right to left. It is because when the driver of a vehicle ahead of an ambulance looks in their rear-view mirror, they can read ‘AMBULANCE’ written on it and give way to it. It is the duty of every one of us to allow an ambulance to pass without blocking its way.
Spherical Mirrors:
When we look at the outer side of the stainless-steel spoon, we can see an erect image of ourselves. When we look at the inner side of the spoon, we see and erect image which is larger in size. When we increase the distance of the spoon from our face, we may see our image inverted. The curved shining surface of a spoon acts as a mirror. The most common example of a curved mirror is a spherical mirror. If the reflecting surface of a spherical mirror is curved inside, it is called a concave mirror. If the reflecting surface is curved outward, then it is a convex mirror. The inner surface of a spoon acts like a concave mirror, while its outer surface acts like a convex mirror.
Take a concave mirror and hold it facing the sun. Try to get the light reflected by the mirror on a sheet of paper. Adjust the distance of the paper until you get a sharp bright spot on it. Hold the mirror and the sheet of paper steady for a few minutes. The bright spot formed is the image of the sun. This image is formed on the screen. An image formed on a screen is called a real image. The image formed by a plane mirror could not be obtained on a screen. Such an image is called a virtual image. The image formed by a concave mirror can be smaller or larger in size than the object. The image may also be real or virtual. Concave mirrors can be used for many purposes. Doctors use concave mirrors for examining eyes, ears, nose and throat. Concave mirrors are also used by dentists to see an enlarged image of the teeth. The reflectors of torches, headlights of cars and scooters are concave in shape.
Convex mirrors can form images of objects spread over a large area. So, these help the drivers to see the traffic behind them.
Measuring speed:
We can easily measure speed when we know the time and distance. We can determine which object is moving faster than the others depending upon their speed. Speed can be calculated in km/h or m/s. Rockets, launching satellites into earth’s orbit, often attain speeds up to 8 km/s. On the other hand, a tortoise can move only with a speed of about 8 cm/s.
If we know the speed and time of an object, we can find the distance moved by an object by
The speed can be measured by meters. We see meters on our motorcycles, on dashboard of our cars, buses etc. These are called speedometers and they measure speed in km/h. The meter that measures the distance moved by a vehicle is known as odometer.
Distance-time graph:
The pictorial representation of distance and time is easy to evaluate. Therefore, we can use different methods to make a graph.
Bar graph: 
Pie chart:
Line graph:
OX is the x-axis and time is measured on x-axis. OY is the y-axis and distance is measured on y-axis. O is the origin.
Scale has to be chosen for distance and time. In this graph, for time: 1 min= 1 cm and for distance 1 km= 1 cm.
For different values of distance and time, the graph is plotted. These points are joined and a straight line is formed. This is the distance-time graph for the motion of the car.
If the distance-time graph is a straight line, it indicates that the object is moving with a constant speed. However, if the speed of the object keeps changing, the graph can be of any shape.
Slow or fast:
Speed is the total distance covered divided by the total time taken.
Speed:
There are objects that move slow and there are objects that move fast. We can determine which object is moving fast or slow based on the distance moved by the objects in a given interval of time. In a race, we say that the fastest person is the one who takes less time cover the distance.
To determine which object is moving faster, we compare the distances moved by them in a unit time. The distance covered by an object in a unit time is known as the speed of the object. 
Uniform motion: An object moving along a straight line with a constant speed is said to be in uniform motion.
Non-uniform motion: If the speed of an object moving along a straight line keeps changing, its motion is said to be non-uniform.
Measurement of time:
In ancient days, they didn’t have clocks to now the time. They noticed that many events in nature repeat themselves after definite intervals of time. They found that sun rises everyday in the morning. The time between one sunrise and the next was called a day. A month was measured from one new moon to the next. A year was fixed as the time taken by the earth to complete one revolution of the sun. We need to measure time intervals which are much shorter than a day. Therefore, we use time measuring devices such as clocks and watches. They make use of a periodic motion. A simple pendulum is the most well-known periodic motion. 
A simple pendulum consists of a small metallic ball or a piece of stone suspended from a rigid stand by a thread. The metallic ball is called the bob of the pendulum. The pendulum is at rest in its mean position. When the bob of the pendulum is released after taking it slightly to one side, it begins to move to and fro. The to and fro motion of a simple pendulum is an example of a periodic or an oscillatory motion. The pendulum is said to have completed one oscillation when oscillation its bob, starting from its mean position O, moves to A, to B and back to O. The pendulum also completes one oscillation when its bob moves from one extreme position A to the other extreme position B and comes back to A. The time taken by the pendulum to complete one oscillation is called its time period.
Nowadays most clocks or watches have an electric circuit with one or more cells. These clocks are called quartz clocks and time measured by them are far more accurate.
Units of time and speed:
The basic unit of a time is second. Its symbol is s. Larger units of time are minutes (min) and hours (h). The basic unit of speed is m/s. Different units of time are used depending on the need. We express our age in years while distance in km. The pulse of a normal healthy adult at rest beats about 72 times in a minute that is about 12 times in 10 seconds. Many time measuring devices were used in different parts of the world before the pendulum clocks became popular. Sundials, water clocks and sand clocks are some examples of such devices.
Cyclones are very destructive; they destroy so many lives and households. Orissa was hit by a cyclone with wind speed of 200 km/h on 18 October 1999. The cyclone smashed 45,000 houses making 7,00,000 people homeless. On 29 October the same year, a second cyclone with wind speed of 260 km/h hit Orissa again. It was accompanied by water waves about 9 m high. Thousands of people lost their lives. Property worth crores of rupees was destroyed. The cyclone affected agriculture, transport, communication, and electricity supply.
Air exerts pressure:
If we fill a soft plastic bottle with hot water, empty it and then immediately close the cap. Place the bottle under running water and we notice that the shape of the bottle changes. If the wind is in our direction, it is easier for us to do things. While flying a kite or rowing a boat, if the wind is coming from behind us then it easier to fly the kite and row the boat. It is difficult for us to ride a bicycle when the wind is in opposite direction to us. This shows that air exerts pressure. The bicycle tube changes its shape when air is filled and it bursts when overfilled.
The plastic bottle changes its shape when placed under running water because the steam inside the bottle condenses into water, reducing the amount of air inside. The pressure of air inside the bottle decreases than the pressure exerted by the air from outside the bottle. As a result, the bottle gets compressed.
High speed winds are accompanied by reduced air pressure:
A small piece of paper is crumpled and is placed into the mouth of an empty bottle. When we blow on the ball to force it into the bottle, it is difficult for us to push it inside. When we blow into the mouth of the bottle, the air near the mouth has higher speed. This decreases the pressure there. The air pressure inside the bottle is higher than near the mouth. The air inside the bottle pushes the ball out.
Two balloons of same size with little water into it is hung 8-10cm apart on a stick. When we blow in between the balloons, we observe that the balloons move towards each other. When we blow air, the pressure between them is reduced and the pressure outside the balloon push them towards each other.
Increased wind speed is accompanied by a reduced air pressure. Moving air is called wind. Air moves from the region where the air pressure is high to the region where the pressure is low. The greater the difference in pressure, the faster the air moves.
Air expands on heating:
A boiling tube is taken and a balloon is stretched over the neck of the tube. Hot water is poured in a beaker and the boiling tube is inserted with the balloon in hot water. The balloon inflated. Same thing is done with cold water. It is observed that in this case, the balloon deflates.
Two paper bags of same size are placed on the two ends of a metal or wooden stick. A piece of thread is tied in the middle of the stick. Hold the stick by the thread as in a balance. A burning candle is placed below one of the bags. The bag under which a candle was placed, rises up. As the warm air rises up, it pushes the bag above the candle. On heating the air expands and occupies more space. When the same thing occupies more space, it becomes lighter. The warm air is, therefore, lighter than the cold air. That is the reason that the cold air smoke goes up. In nature there are several situations, where warm air rises at a place. The air pressure at that place is lowered. The cold air from the surrounding areas rushes in to fill its place. This sets up convection in air.
Wind currents are generated due to uneven heating on the earth:
Uneven heating between the equator and the poles:
The regions close to equator get maximum heat from the Sun and the air gets warm in the regions. The warm air rises and the cooler air from the regions in the 0–30 degrees latitude belt on either side of the equator moves in. These winds blow from north and south towards the equator. At the poles, the air is colder than that at latitudes about 60 degrees. The warm air at the latitudes rise up and cold winds from polar regions moves in. In this way, wind circulation is set up from the poles to the warmer latitudes. The winds would have flown in the north-south direction from north to south, or from south to north. A change in direction is however, caused by the rotation of the earth.
Uneven heating of land and water:
In summer, near the equator the land warms up faster and the temperature of land is higher than that of oceans. The warm air near the land rises and the winds from ocean flows towards the land. These winds are called monsoon winds. In winter, the wind flows from land to ocean. The monsoon winds from the ocean contain water which brings rain. Rain brings happiness. Farmers in our country depend mainly on rains for their harvests. There are many folk songs associated with clouds and rain. Rains can also be disastrous in the case of cyclones and thunderstorms.
Thunderstorms and Cyclones:
Thunderstorms develop in hot, humid tropical areas like India very frequently. The rising temperature produce strong uprising winds. These winds carry water droplets upwards, where they freeze and fall down. The swift movement of the falling water droplets along with the rising air create lightning and sound. This is called thunderstorm.
Water takes up heat from the atmosphere to change into vapor. When water vapor changes back to liquid as raindrops, this heat is released in the atmosphere which warms the air around. The air tends to rise and causes a drop in pressure. More air rushes to the centre of the storm. This cycle is repeated. It forms a very low-pressure system with very high-speed winds revolving around it. This is called a cyclone. Factors like wind speed, wind direction, temperature and humidity contribute to the development of cyclones. The centre of a cyclone is a calm area. It is called the eye of the storm.
Destruction caused by cyclones:
Cyclones can be very destructive. Strong winds push water towards the shore. The water waves produced by the wind are so powerful that a person cannot overcome them. The low pressure in the eye lifts water surface in the centre. The rising water may be as high as 3–12 meters. It appears like a water-wall moving towards the shore. As a result, the seawater enters the low-lying coastal areas, causing severe loss of life and property. It also reduces the fertility of the soil. Continuous heavy rainfall may further worsen the flood situation. High-speed winds accompanying a cyclone can damage houses, telephones and other communication systems, trees, etc., causing tremendous loss of life and property. It is also known as hurricane or typhoon.
Tornadoes: A tornado is a dark funnel shaped cloud that reaches from the sky to the ground. Most of the tornadoes are weak. A violent tornado can travel at speeds of about 300 km/h. Tornadoes may form within cyclones. The whole coastline of India is vulnerable to cyclones, particularly the east coast. The west coast of India is less vulnerable to cyclonic storms both in terms of intensity and frequency of the cyclones.
Effective safety measures:
A cyclone forecast and warning service. Rapid communication of warnings to the Government agencies, the ports, fishermen, ships and to the general public. Construction of cyclone shelters in the cyclone prone areas, and Administrative arrangements for moving people fast to safer places.
We should not ignore the warnings issued by the meteorological department through TV, radio, or newspapers. We should — make necessary arrangements to shift the essential household goods, domestic animals and vehicles, etc. to safer places; avoid driving on roads through standing water, as floods may have damaged the roads; and keep ready the phone numbers of all emergency services like police, fire brigade, and medical centres.
If we live in cyclone prone area: Do not drink water that could be contaminated. Always store drinking water for emergencies. Do not touch wet switches and fallen power lines. Do not go out just for the sake of fun. Do not pressurise the rescue force by making undue demands. Cooperate and help your neighbours and friends.
Advanced Technology:
The satellites and radars issue cyclone warning 24 hours in advance and expected storm 48 hours in advance by cyclone alert or cyclone watch. The message is broadcast every hour or half hour when a cyclone is nearer the coast. Several national and international organisations cooperate to monitor the cyclone-related disasters.
