physics notes

Physics notes for all exams

Heat

Our sense of touch is not always a reliable guide to the degree of hotness

of an object.

Temperature is a measure of the degree of hotness of an object.

Thermometer is a device used for measuring temperatures.

Clinical thermometer is used to measure our body temperature. The

range of this thermometer is from 35°C to 42°C. For other purposes, we

use the laboratory thermometers. The range of these thermometers is

usually from –10°C to 110°C.

The normal temperature of the human body is 37°C.

The heat flows from a body at a higher temperature to a body at a lower

temperature. There are three ways in which heat can flow from one

object to another. These are conduction, convection and radiation.

In solids, generally, the heat is transferred by conduction. In liquids

and gases the heat is transferred by convection. No medium is required

for transfer of heat by radiation.

The materials which allow heat to pass through them easily are

conductors of heat.

The materials which do not allow heat to pass through them easily are

called insulators.

Dark-coloured objects absorb radiation better than the light-coloured

objects. That is the reason we feel more comfortable in light-coloured

clothes in the summer.

Woollen clothes keep us warm during winter. It is so because wool is a

poor conductor of heat and it has air trapped in between the fibres.

Winds, Storms and Cyclones

Air around us exerts pressure.

Air expands on heating and contracts on cooling.

Warm air rises up, whereas comparatively cooler air tends to sink towards

the earth’s surface.

As warm air rises, air pressure at that place is reduced and the cooler

air moves to that place.

The moving air is called wind.

Uneven heating on the earth is the main cause of wind movements.

 Winds carrying water vapour bring rain.

High-speed winds and air pressure difference can cause cyclones.

It has become easier to monitor cyclones with the help of advance

technology like satellites and radars.

Self-help is the best help. Therefore it is better to plan in advance and

be ready with defence against any approaching cyclone.

 The following flow chart will help you to understand the phenomena

that lead to the formation of clouds and falling of rain and creation of

storms and cyclones:

Motion and Time

The distance moved by an object in a unit time is called its speed.

 Speed of objects help us to decide which one is moving faster than the

other.

The speed of an object is the distance travelled divided by the time taken

to cover that distance. Its basic unit is metre per second (m/s).

 Periodic events are used for the measurement of time. Periodic motion of

a pendulum has been used to make clocks and watches.

Motion of objects can be presented in pictorial form by their

distance-time graphs.

The distance-time graph for the motion of an object moving with a constant

speed is a straight line.

Electric Current and itsEffects

It is convenient to represent electric components by symbols. Using these,

an electric circuit can be represented by a circuit diagram.

When an electric current flows through a wire, the wire gets heated. It is

the heating effect of current. This effect has many applications.

Wires made from some special materials melt quickly and break when

large electric currents are passed through them. These materials are

used for making electric fuses which prevent fires and damage to electric

appliances.

When an electric current flows through a wire, it behaves like a magnet.

A current carrying coil of an insulated wire wrapped around a piece of

iron is called an electromagnet.

Electromagnets are used in many devices.

Light

Light travels along straight lines.

Any polished or a shining surface acts as a mirror.

An image which can be obtained on a screen is called a real image.

An image which cannot be obtained on a screen is called a virtual image.

 The image formed by a plane mirror is erect. It is virtual and is of the

same size as the object. The image is at the same distance behind the

mirror as the object is in front of it.

In an image formed by a mirror, the left side of the object is seen on the

right side in the image, and right side of the object appears to be on the

left side in the image.

A concave mirror can form a real and inverted image. When the object is

placed very close to the mirror, the image formed is virtual, erect and

magnified.

Image formed by a convex mirror is erect, virtual and smaller in size

than the object.

A convex lens can form real and inverted image. When the object is

placed very close to the lens, the image formed is virtual, erect and

magnified. When used to see objects magnified, the convex lens is called

a magnifying glass.

A concave lens always forms erect, virtual and smaller image than the

object.

White light is composed of seven colours.

FORCE AND PRESSURE

Force could be a push or a pull.

 A force arises due to the interaction between

two objects.

Force has magnitude as well as direction.

 A change in the speed of an object or the

direction of its motion or both implies a change

in its state of motion.

 Force acting on an object may cause a change

in its state of motion or a change in its shape.

A force can act on an object with or without

being in contact with it.

 Force per unit area is called pressure.

Liquids and gases exert pressure on the walls

of their containers.

The pressure exerted by air around us is

known as atmospheric pressure.

FRICTION

Friction opposes the relative motion between

two surfaces in contact. It acts on both the

surfaces.

Friction depends on the nature of surfaces in

contact.

 For a given pair of surfaces friction depends

upon the state of smoothness of those

surfaces.

 Friction depends on how hard the two surfaces

press together.

Static friction comes into play when we try to

move an object at rest.

 Sliding friction comes with play when an

object is sliding over another.

 Sliding friction is smaller than static friction.

 Friction is important for many of our activities.

Friction can be increased by making a surface rough.

The sole of the shoes and the tyres of the vehicle are treaded to increase friction.

The friction is sometimes undesirable.

 Friction can be reduced by using lubricants.

When one body rolls over another body, rolling friction comes into play. Rolling friction is

smaller than the sliding friction.

 In many machines, friction is reduced by using ball bearings.

 Fluid friction can be minimised by giving suitable shapes to bodies moving in fluids.

SOUND

Sound is produced by vibrating objects.

 In human beings, the vibration of the vocal

cords produces sound.

Sound travels through a medium (gas, liquid

or solid). It cannot travel in vacuum.

The eardrum senses the vibrations of sound, It sends the signals to the brain. This process is called hearing.

The number of oscillations or vibrations per second is called the frequency of oscillation.

The frequency is expressed in hertz (Hz)  Larger the amplitude of vibration, louder is the sound.

 Higher the frequency of vibration, the higher is the pitch, and shriller is the sound.

 Unpleasant sounds are called noise.

 Excessive or unwanted sounds lead to noise pollution. Noise pollution may pose health problems for human beings.

Attempts should be made to minimise noise pollution.

 Plantation on the roadside and elsewhere can reduce noise pollution.       

CHEMICAL EFFECTS OF ELECTRIC
CURRENT

Some liquids are good conductors of electricity and some are poor conductors.

Most liquids that conduct electricity are

solutions of acids, bases and salts.

The passage of an electric current through a conducting liquid causes chemical reactions.

The resulting effects are called chemical effects of currents.

The process of depositing a layer of any desired metal on another material, by means of electricity, is called electroplating.

SOME NATURAL PHENOMENA

Some objects can be charged by rubbing with other objects.

There are two kinds of charges — positive charge and negative charge

 Like charges repel and unlike charges attract each other.

The electrical charges produced by rubbing are called static charges.

When charges move, they constitute an electric current.

An electroscope may be used to detect whether a body is charged or not.

 The process of transfer of charge from a charged object to the earth is called earthing.

 The process of electric discharge between clouds and the earth or between different clouds causes lightning.

 Lightning strike could destroy life and property.

 Lightning conductors can protect buildings from the effects of lightning.

An earthquake is a sudden shaking or trembling of the earth.

Earthquake is caused by a disturbance deep inside the earth’s crust.

 It is not possible to predict the occurrence of an earthquake.

Earthquakes tend to occur at the boundaries of earth’s plates. These boundaries are known as fault zones.

 Destructive energy of an earthquake is measured on the Richter scale. The earthquake measuring 7 or more on Richter

scale can cause severe damage to life and property.

We should take necessary precautions to protect ourselves from earthquakes.

LIGHT

Light is reflected from all surfaces.

Regular reflection takes place when light is incident on smooth, polished and regular surfaces.

Diffused/irregular reflection takes place from rough surfaces.

Two laws of reflection are (i) The angle of incidence is equal to the angle of reflection.

(ii) Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane.

Image formed in a plane mirror undergoes lateral inversion.

Two mirrors inclined to each other give multiple images.

Beautiful patterns are formed in a kaleidoscope because of multiple reflections.

Sunlight, called white light, consists of seven colours.

Splitting of light into its constituent colours is known as dispersion.

 Important parts of the eye are cornea, iris, pupil, lens, retina and optic nerve.

A normal eye can see nearby and distant objects clearly. Visually challenged persons can read and write using Braille system.

Visually challenged persons develop their other senses more sharply to improve their interaction with their environment.

STARS AND THE SOLAR SYSTEM

The phases of the moon occur because we can see only that part of the moon which reflects the light of the Sun towards us.

Stars are celestial bodies that emit light of their own. Our sun is also a star.

It is convenient to express distances of stars in light years.

Stars appear to move from east to west.

The pole star appears to be stationary from the Earth, because it is situated close to the direction of the axis of rotation of the Earth.

Constellations are groups of stars that appear to form recognisable shapes.

The solar system consists of eight planets and host of asteroids, comets and meteors.

A body revolving around another body is called a satellite.

Moon is the natural satellite of the Earth.

Some planets also have natural satellites.

Venus is the brightest planet in the night sky.

Jupiter is the largest planet of the solar

system.

The artificial satellites revolve around the

Earth. They are much closer than the moon.

Artificial satellites are used for weather

forecasting, long distance communication and

remote sensing.

POLLUTION OF AIR AND WATER

Air pollution is the contamination of air by impurities which may have a harmful impact on the living organisms and the non-living components.

Pollutants are the substances which contaminate air and water.

Carbon monoxide, nitrogen oxides, carbon dioxide, methane and sulphur dioxide are the major pollutants of air.

Increasing levels of greenhouse gases like CO2 are leading to global warming.

Water pollution is the contamination of water by substances harmful to life.

Sewage, agricultural chemicals and industrial waste are some of the major contaminants of water.

Water which is purified and fit for drinking is known as potable water.

 Water is a precious natural resource. We must learn to conserve it.

MATTER IN OUR SURROUNDINGS

Matter is made up of small particles.

The matter around us exists in three states— solid, liquid and gas.

The forces of attraction between the particles are maximum in solids, intermediate in liquids and minimum in gases.

The spaces in between the constituent particles and kinetic energy of the particles are minimum in the case of solids, intermediate in liquids and maximum in gases.

The arrangement of particles is most ordered in the case of solids, in the case of liquids layers of particles can slip and slide over each other while for gases, there is no order, particles just move about randomly.

The states of matter are inter-convertible. The state of matter can be changed by changing temperature or pressure.

Sublimation is the change of gaseous state directly to solid state without going through liquid state, and vice versa.

Boiling is a bulk phenomenon. Particles from the bulk (whole) of the liquid change into vapour state.

Evaporation is a surface phenomenon. Particles from the surface gain enough energy to overcome the forces of attraction present in the liquid and change into the vapour state.

The rate of evaporation depends upon the surface area exposed to the atmosphere, the temperature, the humidity and the wind speed.

Evaporation causes cooling.

Latent heat of vaporisation is the heat energy required to change 1 kg of a liquid to gas at atmospheric pressure at its boiling point.

Latent heat of fusion is the amount of heat energy required to

change 1 kg of solid into liquid at its melting point.

Quantity Unit Symbol

Temperature kelvin  K

Length metre  m

Mass kilogram  kg

Weight newton  N

Volume cubic metre  m3

Density kilogram per cubic metre kg m–3

Pressure pascal    Pa

A mixture contains more than one substance (element and/or compound) mixed in any proportion.

Mixtures can be separated into pure substances using appropriate separation techniques.

A solution is a homogeneous mixture of two or more substances. The major component of a solution is called the solvent, and the minor, the solute.

The concentration of a solution is the amount of solute present per unit volume or per unit mass of the solution/solvent.

Materials that are insoluble in a solvent and have particles that are visible to naked eyes, form a suspension. A suspension is a heterogeneous mixture.

Colloids are heterogeneous mixtures in which the particle size is too small to be seen with the naked eye, but is big enough to scatter light. Colloids are useful in industry and daily life. The particles are called the dispersed phase and the medium in which they are distributed is called the dispersion medium.

Pure substances can be elements or compounds. An element is a form of matter that cannot be broken down by chemical reactions into simpler substances. A compound is a substance

composed of two or more different types of elements, chemically combined in a fixed proportion.

Properties of a compound are different from its constituent elements, whereas a mixture shows the properties of its constituting elements or compounds.

MOTION

Motion is a change of position; it can be described in terms of the distance moved or the displacement.

•
The motion of an object could be uniform or non-uniform depending on whether its velocity is constant or changing.

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The speed of an object is the distance covered per unit time, and velocity is the displacement per unit time.

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The acceleration of an object is the change in velocity per unit time.

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Uniform and non-uniform motions of objects can be shown through graphs.

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The motion of an object moving at uniform acceleration can be described with the help of three equations, namely v = u + at s = ut + ½ at2 2as = v2 – u2

where u is initial velocity of the object, which moves with uniform acceleration a for time t, v is its final velocity and s is the distance it travelled in time t.

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If an object moves in a circular path with uniform speed, its motion is called uniform circular motion.

FORCE AND LAWS OF MOTION

First law of motion: An object continues to be in a state of rest or of uniform motion along a straight line unless acted upon by an unbalanced force.

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The natural tendency of objects to resist a change in their state of rest or of uniform motion is called inertia.

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The mass of an object is a measure of its inertia. Its SI unit is kilogram (kg).

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Force of friction always opposes motion of objects.

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Second law of motion: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.

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The SI unit of force is kg m s–2. This is also known as newton and represented by the symbol N. A force of one newton produces an acceleration of 1 m s–2 on an object of mass 1 kg.

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The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity.Its SI unit is kg m s–1 .

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Third law of motion: To every action, there is an equal and opposite reaction and they act on two different bodies.

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In an isolated system (where there is no external force), the total momentum remains conserved.

GRAVITATION

The law of gravitation states that the force of attraction between any two objects is proportional to the product of their masses and inversely proportional to the square of the distance between

them. The law applies to objects anywhere in the universe. Such a law is said to be universal.

• 
Gravitation is a weak force unless large masses are involved.

• 
Force of gravitation due to the earth is called gravity.

• 
The force of gravity decreases with altitude. It also varies on the surface of the earth, decreasing from poles to the equator.

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The weight of a body is the force with which the earth attracts it.

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The weight is equal to the product of mass and acceleration due to gravity.

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The weight may vary from place to place but the mass stays constant.

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All objects experience a force of buoyancy when they are immersed in a fluid.

• 
Objects having density less than that of the liquid in which they are immersed, float on the surface of the liquid. If the   density of the object is more than the density of the liquid in

which it is immersed then it sinks in the liquid

WORK AND ENERGY

Work done on an object is defined as the magnitude of the

force multiplied by the distance moved by the object in the

direction of the applied force. The unit of work is joule:

1 joule = 1 newton × 1 metre.

•
Work done on an object by a force would be zero if the

displacement of the object is zero.

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An object having capability to do work is said to possess energy.

Energy has the same unit as that of work.

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An object in motion possesses what is known as the kinetic

energy of the object. An object of mass, m moving with velocity

v has a kinetic energy of

mv2

1

2 .

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The energy possessed by a body due to its change in position or shape is called the potential energy. The gravitational potential energy of an object of mass, m raised through a height, h from the earth’s surface is given by m g h.

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According to the law of conservation of energy, energy can only be transformed from one form to another; it can neither be created nor destroyed. The total energy before and after the transformation always remains constant.

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Energy exists in nature in several forms such as kinetic energy, potential energy, heat energy, chemical energy etc.

The sum of the kinetic and potential energies of an object is called its mechanical energy.

• 
Power is defined as the rate of doing work. The SI unit of power is watt. 1 W = 1 J/s.

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The energy used in one hour at the rate of 1kW is called 1 kW .

SOUND

Sound is produced due to vibration of different objects.

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Sound travels as a longitudinal wave through a material medium.

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Sound travels as successive compressions and rarefactions in the medium.

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In sound propagation, it is the energy of the sound that travels and not the particles of the medium.

• 
Sound cannot travel in vacuum.

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The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation.

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The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength, ë.

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The time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period, T.

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The number of complete oscillations per unit time is called the frequency (í), 1 T

.

•The speed v, frequency í, and wavelength ë, of sound are related by the equation, v = ëí.

he speed of sound depends primarily on the nature and the temperature of the transmitting medium.

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The law of reflection of sound states that the directions in which the sound is incident and reflected make equal angles with the normal to the reflecting surface at the point of incidence and the three lie in the same plane.

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For hearing a distinct sound, the time interval between the original sound and the reflected one must be at least 0.1 s. • The persistence of sound in an auditorium is the result of  repeated reflections of sound and is called reverberation.

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Sound properties such as pitch, loudness and quality are determined by the corresponding wave properties.

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Loudness is a physiological response of the ear to the intensity of sound.

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The amount of sound energy passing each second through unit area is called the intensity of sound.

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The audible range of hearing for average human beings is in the frequency range of 20 Hz – 20 kHz. Sound waves with frequencies below the audible range are termed “infrasonic” and those above the audible range are     termed “ultrasonic”.

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Ultrasound has many medical and industrial applications.

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The SONAR technique is used to determine the depth of the sea and to locate under water hills, valleys, submarines,icebergs, sunken ships etc.

Light – Reflection andRefraction

Light seems to travel in straight lines.

Mirrors and lenses form images of objects. Images can be either real or virtual, depending on the position of the object.

The reflecting surfaces, of all types, obey the laws of reflection. The refracting surfaces obey the laws of refraction.

New Cartesian Sign Conventions are followed for spherical mirrors and lenses.

More to Know!

Light – Reflection and Refraction 185

Mirror formula,1 1 1+ = v u f , gives the relationship between the object-distance (u), image-distance (v), and focal length (f) of a spherical mirror.

The focal length of a spherical mirror is equal to half its radius of curvature.

 The magnification produced by a spherical mirror is the ratio of the height of the image to the height of the object.

A light ray travelling obliquely from a denser medium to a rarer medium bends away from the normal. A light ray bends towards the normal when it travels obliquely from a rarer to a denser medium.

Light travels in vacuum with an enormous speed of 3×108 m s-1. The speed of light is different in different media.

The refractive index of a transparent medium is the ratio of the speed of light in vacuum to that in the medium.

In case of a rectangular glass slab, the refraction takes place at both air-glass interface and glass-air interface. The emergent ray is parallel to the direction of incident ray.

Lens formula, 1 1 1 – = v u f , gives the relationship between the object-distance (u), image-distance (v), and the focal length (f) of a spherical lens.

Power of a lens is the reciprocal of its focal length. The SI unit of power of a lens is dioptre.

The Human Eye andthe Colourful World

The ability of the eye to focus both near and distant objects, by adjusting its focal

length, is called the accommodation of the eye.

The smallest distance, at which the eye can see objects clearly without strain, is called the near point of the eye or the least distance of distinct vision. For a young adult with normal vision, it is about 25 cm.

The common refractive defects of vision include myopia, hypermetropia and presbyopia. Myopia (short-sightedness – the image of distant objects is focussed before the retina) is corrected by using a concave lens of suitable power.

Hypermetropia (far-sightedness – the image of nearby objects is focussed beyond the retina) is corrected by using a convex lens of suitable power. The eye loses its power of accommodation at old age.

The splitting of white light into its component colours is called dispersion.

Scattering of light causes the blue colour of sky and the reddening of the Sun at

sunrise and sunset.

Electricity

A stream of electrons moving through a conductor constitutes an electric current.Conventionally, the direction of current is taken opposite to the direction of flow of electrons.

The SI unit of electric current is ampere.

To set the electrons in motion in an electric circuit, we use a cell or a battery. A cell generates a potential difference across its terminals. It is measured in volts (V).

Resistance is a property that resists the flow of electrons in a conductor. It controls the magnitude of the current. The SI unit of resistance is ohm (Ù).

Ohm’s law: The potential difference across the ends of a resistor is directly proportional to the current through it, provided its temperature remains the same.

The resistance of a conductor depends directly on its length, inversely on its area of cross-section, and also on the material of the conductor.

The equivalent resistance of several resistors in series is equal to the sum of their individual resistances. 

A set of resistors connected in parallel has an equivalent resistance Rp given by 1 2 3 1 1 1 1 ... Rp R R R = + + + 􀂄 The electrical energy dissipated in a resistor is given by W = V × I × t

The unit of power is watt (W). One watt of power is consumed when 1 A of current flows at a potential difference of 1 V.

The commercial unit of electrical energy is kilowatt hour (kWh). 1 kW h = 3,600,000 J = 3.6 × 106 J.

Magnetic Effects of Electric Current

Take a straight thick copper wire and place it between the points X and Y in an electric circuit, as shown in Fig. 13.1. The wire XY is kept perpendicular to the plane of paper.

Horizontally place a small compass near to this copper wire. See the position of its needle.

Pass the current through the circuit by inserting the key into the plug.

Observe the change in the position of the compass needle.

Magnetic Effects ofElectric Current

A compass needle is a small magnet. Its one end, which points towards north, is called a north pole, and the other end, which points towards south, is called a south pole.

A magnetic field exists in the region surrounding a magnet, in which the force of the magnet can be detected.

Field lines are used to represent a magnetic field. A field line is the path along which a hypothetical free north pole would tend to move. The direction of the magnetic field at a point is given by the direction that a north pole placed at that point would take. Field lines are shown closer together where the magnetic field is greater.

A metallic wire carrying an electric current has associated with it a magnetic field.

The field lines about the wire consist of a series of concentric circles whose direction is given by the right-hand rule.

The pattern of the magnetic field around a conductor due to an electric current flowing through it depends on the shape of the conductor. The magnetic field of a solenoid carrying a current is similar to that of a bar magnet.

An electromagnet consists of a core of soft iron wrapped around with a coil of insulated copper wire.

A current-carrying conductor when placed in a magnetic field experiences a force. If the direction of the field and that of the current are mutually perpendicular to each other, then the force acting on the conductor will be perpendicular to both and will be given by Fleming’s left-hand rule. This is the basis of an electric motor.

An electric motor is a device that converts electric energy into mechanical energy.

The phenomenon of electromagnetic induction is the production of induced current in a coil placed in a region where the magnetic field changes with time. The magnetic field may change due to a relative motion between the coil and a magnet placed near to the coil. If the coil is placed near to a current-carrying conductor, the magnetic field may change either due to a change in the current through the

conductor or due to the relative motion between the coil and conductor. The direction of the induced current is given by the Fleming’s right-hand rule.

A generator converts mechanical energy into electrical energy. It works on the basis of electromagnetic induction.

In our houses we receive AC electric power of 220 V with a frequency of 50 Hz. One of the wires in this supply is with red insulation, called live wire. The other one is of black insulation, which is a neutral wire. The potential difference between the two is 220 V. The third is the earth wire that has green insulation and this is connected to a metallic body deep inside earth. It is used as a safety measure to ensure that any leakage of current to a metallic body does not give any severe shock to a user.

Fuse is the most important safety device, used for protecting the circuits due to short-circuiting or overloading of the circuits.

Sources of Energy

Our energy requirements increase with our standard of living.

In order to fulfil our energy requirements, we try to improve the efficiency of energy usage and also try and exploit new sources of energy.

We also need to look for new sources of energy because the conventional sources of energy like fossil fuels are in danger of getting exhausted soon.

The energy source we select would depend on factors like the ease and cost of extracting energy from the source, the efficiency of the technology available for using that source of energy and the environmental impact of using that source.

Many of the sources ultimately derive their energy from the Sun.

Our Environment

The various components of an ecosystem are interdependent.

The producers make the energy from sunlight available to the rest of the ecosystem.

There is a loss of energy as we go from one trophic level to the next, this limits the number of trophic levels in a food-chain.

Human activities have an impact on the environment.

The use of chemicals like CFCs has endangered the ozone layer. Since the ozone layer protects against the ultraviolet radiation from the Sun, this could damage  the environment.

The waste we generate may be biodegradable or non-biodegradable.

The disposal of the waste we generate is causing serious environmental problems.