CHAPTER -12 ELECTRICITY
GIST OF THE LESSON
- Positive and negative charges: The charge acquired by a glass rod when rubbed with silk is called positive charge and the charge acquired by an ebonite rod when rubbed with wool is called negative charge.
- Coulomb: It is the S.I. unit of charge. One coulomb is defined as that amount of charge which repels an equal and similar charge with a force of 9 x 109 N when placed in vacuum at a distance of 1 meter from it. Charge on an electron = -1.6 x 10-19 coulomb.
- Static and current electricities: Static electricity deals with the electric charges at rest while the current electricity deals with the electric charges in motion.
- Conductor: A substance which allows passage of electric charges through it easily is called a ‘conductor’. A conductor offers very low resistance to the flow of current. For example copper, silver, aluminium etc.
- Insulator: A substance that has infinitely high resistance does not allow electric current to flow through it. It is called an ‘insulator’. For example rubber, glass, plastic, ebonite etc.
- Electric current: The flow of electric charges across a cross-section of a conductor constitutes an electric current. It is defined as the rate of flow of the electric charge through any section of a conductor.
Electric current = Charge/Time or I = Q/t
Electric current is a scalar quantity.
- Ampere: It is the S.I. unit of current. If one coulomb of charge flows through any section of a conductor in one second, then current through it is said to be one ampere. 1 ampere = 1 coulomb/1 second or 1 A = 1C/1s = 1Cs-1 1 milliampere = 1 mA = 10-3 A 1 microampere = 1µA = 10-6 A
- Electric circuit: The closed path along which electric current flows is called an ‘electric circuit’.
- Conventional current: Conventionally, the direction of motion of positive charges is taken as the direction of current. The direction of conventional current is opposite to that of the negatively charged electrons.
- Electric field: It is the region around a charged body within which its influence can be experienced.
- Electrostatic potential: Electrostatic potential at any point in an electric field is defined as the amount of work done in bringing a unit positive charge from infinity to that point. Its unit is volt. Positive charges move from higher to lower potential regions. Electrons, being negatively charged, move from lower to higher potential regions.
- Potential difference between two points: The Potential difference between two points in an electric field is the amount of work done in bringing a unit positive charge from one to another. Potential difference = Work done/Charge or V = W/Q
- One volt potential difference: The Potential difference between two points in an electric field is said to one volt if one joule of work has to be done in bringing a positive charge of one coulomb from one point to another. 1 volt = 1 joule/1 coulomb or 1 V = 1J/1C
- Galvanometer: It is device to detect current in an electric circuit.
- Ammeter: It is device to measure current in a circuit. It is always connected in series in a circuit.
- Voltmeter: It is a device to measure potential difference. It is always connected in parallel to the component across which the potential difference is to be measured.
- Ohm’s law: This law states that the current passing through a conductor is directly proportional to the potential difference cross its ends, provided the physical conditions like temperature, density etc. remains unchanged.
V α I or V = RI
The proportionality constant R is called resistance of conductor.
- Resistance: It is a property of a conductor by virtue of which it opposes the flow of current through it. It is equal to the ratio of the potential difference applied across its ends and the current flowing through it.
Resistance = Potential difference/Current or R = V/I
- Ohm: It is the S.I. unit of resistance. A conductor has a resistance of one ohm if a current of one ampere flows through it on applying a potential difference of one volt across its ends. 1 ohm = 1 volt/1 ampere or 1Ω = 1V/1A
- Factors on which resistance of a conductor depends: The resistance R of a conductor depends
i) Directly on its length L i.e. R α L. ii) inversely on its area of cross-section A i.e. R α 1/A iii) on the nature of material of the conductor on. On combining the above factors, we get R α L/A R = ρ * L/A The proportionality constant ρ is called
resistivity of conductor.
- Resistivity: It is defined as the resistance offered by a cube of a material of side 1 m when current flows perpendicular to its opposite faces. Its S.I. unit is ohm-meter (Ωm). Resistivity, ρ = RA/L
- Equivalent resistance: If a single resistance can replace the combination of resistances in such a manner that the current in the circuit remains unchanged, then that single resistance is called the equivalent resistance.
- Laws of resistances in series: i) Current through each resistance is same. ii) Total voltage across the combination = Sum of the voltage drops. V= V1 + V2 + V3 iii) Voltage drops across any resistor is proportional to its resistance. V1 = IR1, V2 = IR2, V3 = IR3 iv) Equivalent resistance = Sum of the individual resistances. Rs = R1 + R2 + R3 v) Equivalent resistance is larger than the largest individual resistance.
- Laws of resistances in parallel: i) Voltage across each resistance is same and is equal to the applied voltage. ii) Total current = Sum of the currents through the individual resistances. I = I1 + I2 + I3 iii) Currents through various resistances are inversely proportional to the individual resistances. I1 = V/R1, I2 = V/R2, I3 = V/R3 iv) Reciprocal of equivalent resistance = Sum of reciprocals of individual resistances. 1/Rp = 1/R1 + 1/R2 + 1/R3 v) Equivalent resistance is less than the smallest individual resistance.
- Joule’s law of heating: It states that the heat produced in a conductor is directly proportional to (i) the square of the current I through it (ii) proportional to its resistances R and (iii) the time t for which current is passed. Mathematically, it can be expressed as H = I2Rt joule = I2Rt/4.18 cal or H = VIt joule = VIt/4.18cal
- Electric energy: It is the total work done in maintaining an electric current in an electric circuit for given time.
Electric energy, W = VIt = I2Rt joule
- Electrical power: Electrical power is the rate at which electric energy is consumed by an appliance.
P = W/t = VI = I2R = V2/R
- Watt: It is the S.I. unit of power. The power of an appliance is 1 watt if one ampere of current flows through it on applying a potential differences of 1 volt across its ends. 1 watt = 1 joule/1 second =1 volt x 1 ampere or 1 W = 1 Js-1 = 1 VA 1 kilowatt = 1000 W 1Mega watt= 106 watt 1Gigawatt=109 watt
- Kilowatt hour: It is the commercial unit of electrical energy. One kilowatt hour is the electric energy consumed by an appliance of 1000 watts when used for one hour. 1 kilowatt hour (kWh) = 3.6 x 106 J
HOTS QUESTIONS (SOLVED / UNSOLVED)
|Q.1.||Why is the tungsten metal more coiled in the bulb and not installed in straight parallel wire form?|
|Ans.||The coiled wire of tungsten increases the surface area of the wire in very less space so as to emit more light and helps in glowing with more intensity.|
|Q.2.||Why are fairy decorative lights always connected in parallel?|
|Ans.||When the fairy lights are connected in series the resistance offered will be greater and brightness of the bulbs will be affected. But in parallel connection all the bulbs will glow with same intensity and if any more bulbs gets fused the other bulbs will continue to glow.|
|Q.3.||What will happen when -a) Voltmeter is connected in series?b) Ammeter is connected in parallel?|
|Ans.||a) Negligible current will pass through the circuit because the voltmeter has a very high resistance.b) Ammeter will get damaged due to flow of large amount of current through it, because it has low resistance.|
MAGNETIC EFFECTS OF ELECTRIC CURRENT
KEY CONCEPTS & GIST OF THE LESSON
v Magnet: (i) is an object that attracts objects made of iron, cobalt & nickel. (ii) Comes to rest in North-South direction, when suspended freely.
v Magnets are used: (i) In radio & stereo speakers, (ii) In refrigerator doors, (iii) in audio & video cassettes players, (iv) On hard discs & floppies of computers & (v) in children’s toys.
v Magnetic field: The area around a magnet where a magnetic force is experienced is called a magnetic field. It is a quantity that has both direction & magnitude.
v Magnetic field lines: Magnetic field is represented by field lines. They are lines drawn in a Magnetic field along which a North magnetic pole moves. Magnetic field lines are called as Magnetic lines of force.
Refer to figure 13.3 & 13.4 page no. 225 of N.C.E.R.T Text book)
v Properties of Magnetic field lines:
(i) They do not intersect each other.
(ii) It is taken by convention that magnetic field lines emerge from
North pole and merge at the South pole. Inside the magnet, their
direction is from South pole to North pole. Therefore magnetic field
lines are closed curves.
v Magnetic field lines due to a current through a straight conductor (wire)- consist of series of concentric circles whose direction is given by the Right hand thumb rule.
v Right hand thumb rule: If a current carrying straight conductor is held in your right hand such that the thumb points towards the direction of current, then the wrapped fingers show the direction of magnetic field lines.
(Refer to figure 13.7, page no. 228 of N.C.E.R.T Text book)
v Magnetic field lines due to a current through a circular loop
(Refer to figure 13.8, page no. 228 of N.C.E.R.T Text book)
v The strength of the magnetic field at he centre of the loop(coil)depends on:
(i) The radius of the coil- The strength of the magnetic field is inversely proportional to the radius of the coil. If the radius increases, the magnetic strength at the centre decreases.
(ii) The number of turns in the coil: As the number of turns in the coil increase, the magnetic strength at the centre increases, because the current in each circular turn is having the same direction, thus the field due to each turn adds up.
(iii) The strength of the current flowing in the coil: as the strength of the current increases, the strength of thee magnetic fields also increases.
v Solenoid: (Refer to figure 13.10, page no. 229 of N.C.E.R.T Text book)
v (i) A coil of many turns of insulated copper wire wrapped in the shape of a cylinder is called a Solenoid. (ii) Magnetic field produced by a Solenoid is similar to a bar magnet.
(iii) The strength of magnetic field is proportional to the number of turns & magnitude of current.
v Electromagnet: An electromagnet consists of a long coil of insulated copper wire wrapped on a soft iron core.
(Refer to figure 13.11, page no. 229 of N.C.E.R.T Text book)
v Fleming’s Left hand rule: Stretch the thumb, forefinger and middle finger of left hand such that they are mutually perpendicular. Forefinger points in the direction of magnetic field and centre finger in the direction of current, then the thumb gives the direction of force acting on the conductor.
(Refer to figure13.13, page no. 231 13.13 of N.C.E.R.T Text book)
v Electric motor: A device that converts electric energy to mechanical energy.
(Refer to figure 13.15, page no. 232 of N.C.E.R.T Text book)
v Principle of Electric motor: When a rectangular coil is placed in a magnetic field and a current is passed through it, force acts on the coil, which rotates it continuously. With the rotation of the coil, the shaft attached to it also rotates.
v Electromagnetic induction: Electricity production as a result of magnetism (induced current) is called Electromagnetic induction.
v Fleming’s Right hand rule: gives the direction of induced current.
Stretch the thumb, forefinger and middle finger of right hand such that they are mutually
perpendicular. Forefinger points in the direction of magnetic field and centre finger in the
direction of induced current, then the thumb gives the direction of motion of the conductor.
v Electric generator: A devise that converts mechanical energy to electric energy.
(Refer to figure 13.19, page no. 236 of N.C.E.R.T Text book)
Electric generator is of two types- (i) A.C generator (ii) D. C generator
v Principle of Electric generator: Electromagnetic induction
v Domestic electric circuits: (Refer to figure 13.20, page 238 of N.C.E.R.T Text book)
v We receive electric supply through mains supported through the poles or cables. In our houses we receive AC electric power of 220V with a frequency of 50Hz.
The 3 wires are as follows- (i) Live wire- (Red insulated, Positive)
(ii) Neutral wire- (Black insulated, Negative)
(iii) Earth wire- (Green insulated) for safety measure to ensure
that any leakage of current to a metallic body does not give
any serious shock to a user.
v Short circuit: is caused by touching of live wires and neutral wire
v Fuse: is a protective device used for protecting the circuits from short circuiting and over loading
v Important diagrams-
- Magnetic field lines around a bar magnet.
- Right hand thumb rule
- Magnetic field lines through and around a current carrying solenoid.
- An electromagnet.
- A simple electric motor
- Electric generator
v Important activities-
- Magnetic field lines around a bar magnet
- Direction of electric current in a simple electric circuit.
- Direction of Magnetic field lines depends on the direction of electric current.
HOTS QUESTIONS (SOLVED)
- On what effect of an electric current does an electromagnet work?
A. Magnetic effect of electric current
- What is the frequency of AC (Alternating Current) inIndia?
- 50 Hz
- On what effect of an electric current does a fuse work?
A. Heating effect of electric current
- An Alternating Current has a frequency of 50 Hz. How many times it changes direction in one second?
A. Since Alternating Current changes its direction twicw in a cycle, it will change its direction 100 times (50×2) in one second as its frequency is 50Hz.
- What is short circuiting in an electric circuit?
A. Short circuiting in an electric circuit is a situation in which the live wire touches the neutral wire.
- What kind of quantity is magnetic field?
A. It is a vector quantity as it has both direction & magnitude.
- How is Solenoid different from a circular coil?
A. Solenoid is different from a circular coil in the sense that the length of the solenoid is much greater than its diameter
- On what factor the strength of magnetic field of a solenoid depends?
A The strength of magnetic field of a solenoid is proportional to the number of turns of the coil and the magnitude of the current.
10. What kind of motor is used in: (i) A fan (ii) A battery operated toy
A. (i) A fan has AC motor (ii) A battery operated toy has DC motor.
HOTS QUESTIONS (UNSOLVED)
- Name the sources of direct current.
- Why don’t two magnetic lines intersect each other?
- What is the role of split ring in an electric motor?
- What is an earth wire?
- Why do we use power of two different current ratings at our homes?
- What would be the frequency of an Alternating Current, it its direction changes after every 0.01 second?
- Which of the two requires a thin fuse wire to draw: 5A or 15A of current?
- It is advised to keep magnets away form computers, T.V. Give reason
- In a domestic circuit, MCB (Miniature Circuit Breaker) of one room trips again and again. What could be the reason for this?
- What is the application of a solenoid?
- What is the application of a solenoid?
SOURCES OF ENERGY
KEY CONCEPTS & GIST OF THE LESSON
v Characteristics of a good fuel:
(iv) High calorific value
(v) Less smoke
(vi) Less residue after burning
(vii) Easy availability
(ix) Easy to store and transport
v Fossil fuels: were formed millions of years ago, when plants and animal remains got buried under the earth and were subjected to high temperature and pressure conditions. E.g.: Coal, Petroleum, etc. These fossil fuels are non renewable sources of energy and cause environmental problems due to pollution.
v Thermal power plants:
(i) Use coal, petroleum and natural gas to produce thermal electricity.
(ii) Electricity transmission is very efficient.
(iii) The steam produced by burning the fossil fuels runs the turbine to produce electricity
v Hydro power plant:
(Refer to figure 14.3, page no. 246 of N.C.E.R.T Text book)
(i) It is the most conventional renewable energy source obtained from water falling from a great height.
(ii) It is clean & non polluting source of energy.
(iii) Dams are constructed to collect water flowing in high altitude rivers. The stored water has a lot of potential energy.
(iv) When water is allowed to fall from a height, potential energy changes to kinetic energy, which rotates the turbines to produce electricity.
v Disadvantages of Hydro power plant:
(i) Highly expensive to construct.
(ii) Dams cannot be constructed on all river sites.
(iii) Large areas o human habitation and agricultural fields get submerged.
(iv) People face social and environmental problems.
v Non conventional sources:
(1) Bio mass:
- It is the source of the conventionally used fuels that are used in our
country. E.g.: Cow dung cakes, fire-wood, coal, charcoal
- Bio gas: It is a mixture of gases produced during decomposition of bio mass in the absence of Oxygen. (Anaerobic Respiration). Methane is the major component of bio gas.
- Bio gas plants: Animal dung, sewage, crop residues, vegetable wastes, poultry droppings, etc. are used to produce Bio gas in Bio gas plants.
- (Refer to figure 14.4, page no. 247 of N.C.E.R.T Text book)
(2) Wind energy:
- It can be converted into mechanical and electrical energy.
- Kinetic energy of the wind is used in running of wind mills, which are used to lift water, grind grains, etc.
- Wind mill-(Refer to figure 14.5, page no. 247 of N.C.E.R.T Text book)
- Advantages: (i) Eco friendly (ii) Renewable
- Disadvantages: (i) Wind speed not uniform always.
(ii) Needs a large area to erect series of wind mills.
(iii) Big amount of investment is needed.
(iv) Out put is less as compared to investment
(3) Solar energy:
- Solar radiations can be converted electricity through solar cells (photovoltaic cells).
- Photovoltaic cells convert solar radiations directly into electricity through silicon solar cells.
- Solar cells arrange on a large flat sheets form a solar panel.
- Solar cookers are painted black from outside and a large glass plate to trap solar radiations by green house effect.
- (Refer to figure 14.6, page no. 249 of N.C.E.R.T Text book)
- Advantages of Solar cookers:
(i) Eco friendly
(iii) Used in rural areas.
(iv) Retains all the nutrients in food due to slow cooking.
- Disadvantages of solar cooker:
(i) Silicon cells are expensive.
(ii) Solar radiations are not uniform over earth’s surface.
(iii) Cannot be used at night or on cloudy days.
(iv) Cannot be used to make chapattis for frying as these
require a temperature of 1400C or more.
(Maximum temperature of 1000C only can be
achieved in a solar cooker)
- Other solar devices- Solar water heater, Solar furnace
(4) Geo thermal energy:
(i) Energy harnessed from the heat of the sun is called Geo thermal energy.
(ii) Magma is formed when this heat melts the rocks. The molten rocks and hot gases are called magma
(iii) The magma gets collected at some depths below the earth’s surfaces. These places are called ‘Hot spots”
(iv) When underground water comes in contact these hot spots, it changes into steam, which can be used to generate electricity.
- Advantages of Geo thermal energy:
- Disadvantages of Geo thermal energy:
(i) Only few sites available for harnessing energy.
(5) Nuclear energy:
(i) Energy released when some changes take place in the nucleus of the atom of a
substance, is called Nuclear energy.
(ii) It is used for heat generation, fuel for marine vessels.
- Advantages of Nuclear energy:
(i) Alternative source of energy due to depletion of fossil fuels.
(ii) From a small amount of fuel, a large amount of energy is released.
- Disadvantages of Nuclear energy:
(i) Risk of nuclear waste leakage
(ii) High cost of setting up of nuclear plant
(iii) Pollution of environment.
(6) Energy from the sea-
(A) Tidal energy: Locations inIndia–Gulf of Kutch, Gujrat & W. Bengal
(i) Depends upon harnessing the rise and fall of sea level due to tidal action.
(ii) Dams are constructed across a narrow part of sea and turbine converts tidal
energy into electrical energy.
Disadvantages: Uniform tidal action is not seen
(B) Wave energy:
(i) Kinetic energy of the waves of sea are used to rotate turbines..
(ii) These turbines generate electrical energy
v Important diagrams-
- Hydro power plant
- Bio gas plant
- A wind mill
- A solar cooker
HOTS QUESTIONS (SOLVED)
- Name the materials used for making solar cells.
A. Silicon, Germanium and Selenium
- What fraction of solar energy reaches the earth’s surface?
- Name the process that produces a large amount of energy in the sun.
A. Nuclear fusion
- Why is biogas called a clean fuel?
A. Because it- (i) leaves no ash (ii) does not cause pollution (iii) does not produce any
- Why Hydrogen cannot be used as a domestic fuel?
AHydrogen cannot be used as a domestic fuel, in spite of its high calorific value, because it is explosive in nature and difficult to store & transport.
- What are OTEC power plants? How do they operate?
- (i) It is Ocean Thermal Energy Conversion.
(ii) These plants can operate if the temperature difference between water at the surface and
water at depths up to 2 km is 293K (200C) or more.
(iii) The warm surface water is used to boil a volatile liquid like Ammonia. The vapour of the
liquid is used to turn the turbine.
(iv) The cold water from the depth is pumped up and condensed water again changes into
- Though wood is renewable source of energy, but the use of wood as a fuel is not wise decision. Explain.
A. Wood is obtained by cutting trees. A newly plant sapling usually takes about 10-15 years to grow and mature into a tree. Cutting of trees on a large scale cases depletion of forests leading to disturbance of balance in nature. The balance between Oxygen & Carbon dioxide is also disturbed. Therefore the use of wood as a fuel is not wise decision
- Explain, why a sheet of glass is used in solar heating devices?
A. Glass has a property that allows the infra red rays of short wavelength from the sun to get in the device but does not allow infra red rays of long wavelength to leave the solar heating device. Therefore, heat is trapped inside the heating device. A kind of green house effect is produced, which helps to increase the temperature inside the heating device.
- Explain, why is a reflector used in a solar cooker?
A. The reflector is used to increase the area over which the solar energy is collected so that more and more heat rays of the sun enter the solar cooker. It increases the efficiency of solar cooker.
HOTS QUESTIONS (UNSOLVED)
- What is the use of black painted surface in solar heating devises.
- Why are bio gas plants considered to be boon to the farmers? Give reason.
- Hydroelectricity generated at a dam may be considered another form of solar energy. Why?
- How is the slurry left over after the generation of biogas in biogas plant used?
- Why is charcoal considered to be a better fuel than wood?
- Why a solar cooker cannot be used for frying or making chapattis?
- In parabolic reflector type coolers, even temperature up to 1800C- 2000C can be attained. How?
- Modern chulahs are more efficient than traditional chulahs. Why?
- How is hydro energy converted into electrical energy?
- Explain, why only a part of the solar energy that strikes the upper regions of atmosphere reaches the surface of the earth?