Looking for a Tutor Near You?

Post Learning Requirement »
x

Choose Country Code

x

Direction

x

Ask a Question

x

x
x
For better view click here
x
Hire a Tutor
DISCOVER

Notes On Electricity

Loading...

Published in: Physics
1,014 Views

Physics

Aritra D / Kolkata

20 years of teaching experience

Qualification: M.A (Rabindra Bharati University - [RBU], Kolkata - 2005)

Teaches: English, Mathematics, Physics, BA Tuition, LIC, NDA, SSB Exam, SSC Exams, NET, SET, TET, UGC Net

Contact this Tutor
  1. Electricity We were introduced to the concept of negatively charged particles inside atoms known as electrons (e-) in the lesson on Matter. We also learnt that electric current is like a flowing river of electrons that move from negatively charged regions to positively charged regions. Electricity is also a form of energy like heat and light. Frictional Electricity The ancient Greeks observed electric and magnetic phenomena as early as 600 B.C. They found that a piece of amber, when rubbed, becomes electrified and attracts small pieces of feathers. The word electric comes from Greek word for amber meaning electron. You can perform simple activities to demonstrate the existence of charges and forces between them. If you run a comb through your dry hair, you will note that the comb begins to attract small pieces of paper. Do you know how does it happen? Let us perform two simple experiments to understand the reason. Take a hard rubber rod and rub it with fur or wool. Next, take a glass rod and rub it with silk. Suspend them (rubber rod and a glass rod) separately with the help of non-metallic threads, as shown in the figure below. n. rubber-rod Attraction rvbbet—vd (a) rubber-cod Force of attraction/repulsion between charges: a) a charged rubber rod repels another charged rubber rod: like charges repel each other; and b) a charged glass rod attracts a charged rubber rod: unlike charges attract each other. Now bring rubber rod rubbed with wool near these rods one by one. What do you observe? You will observe that • when a charged rubber rod is brought near the charged (suspended) rubber rod, they show repulsion (a); and • when the charged rubber rod is brought near the (suspended) charged glass rod, they show attraction (b).
  2. Similar results will be obtained by bringing a charged glass rod. Based on these observations, we can say that • A charged rubber rod attracts a charged glass rod but repels a charged rubber rod. • A charged glass rod repels a charged glass rod but attracts a charged rubber rod. From these activities, we can infer that the rubber rod has acquired one kind of electricity and the glass rod has acquired another kind of electricity. Moreover, like charges repel and unlike charges attract each other. Benjamin Franklin (1706—1790), suggested that the charge on glass rod is to be called positive and that on the rubber rod is to be called negative. We follow this convention since then. Once a body is charged by friction, it can be used to charge other conducting bodies by conduction, i.e., by touching the charged body with an uncharged body; and induction, i.e., by bringing the charged body close to an uncharged conductor and earthing it. Subsequently, the charged body and the earthing are removed simultaneously. Conservation of Charge In the activity described above, you have seen that when a glass rod is rubbed with silk, the rod acquires positive charge and silk acquires negative charge. Since both materials in the normal state are neutral (no charge), the positive charge on the glass rod should be equal in magnitude to the negative charge on silk. This means that the total charge of the system (glass + silk) is conserved. It is neither created nor destroyed. It is only transferred from one body of the system to the other. The transfer of charges takes place due to increase in the thermal energy of the system when the glass rod is rubbed; the less tightly bound electrons from the glass rod are transferred to silk. The glass rod (deficient in electrons) becomes positively charged and silk, which now has excess electrons, becomes negatively charged. When rubber is rubbed with fur, electrons from the fur are transferred to rubber. That is, rubber gains negative charge and fur gains an equal amount of positive charge. Any other kind of charge (other than positive and negative) has not been found till today. Quantisation of Charge In 1909, Millikan (Robert Millikan, 1886-1953) experimentally proved that charge always occurs as some integral multiple of a fundamental unit of charge, which is taken as the charge on an electron. This means that if Q is the charge on an object, it can be written as Q = Ne, where N is an integer and e is charge on an electron. Then we say that charge is quantised. It means that a charged body cannot have 2.5e or 6.4e amount of charge. An electron has charge — e and a proton has charge + e. Neutron has no charge. Every atom has equal number of electrons and protons and that is why it is neutral. From this discussion, we can draw the following conclusions: • There are only two kinds of charges in nature — positive and negative. • Charge is conserved. • Charge is quantised.
  3. Coulomb's Law You have learnt that two stationary charges either attract or repel each other. The force of attraction or repulsion between them depends on their nature. Coulomb studied the nature of this force and in 1785 established a fundamental law governing it. From experimental observations, he showed that the electrical force between two static point charges qi and q2 placed some distance apart is — directly proportional to their product; — inversely proportional to the square of the distance r between them; — directed along the line joining the two charged particles; and — repulsive for same kind of charges and attractive for opposite charges. The magnitude of force F can then be expressed as F = k (qi x q2)/r2 where k is the constant of proportionality whose value is 1/4TtEo for free space (i.e., vacuum) and k = 1/4TtE for a material medium. and E are known as permittivity. This means that if the same system of charges is kept in a material medium, the magnitude of Coulomb force will be different from that in free space. The constant k has a value, which depends on the units of the quantities involved. The unit of charge in Sl system is coulomb (C). The coulomb is defined in terms of the unit of current, called ampere. (You will learn about it later.) In Sl system of units, the value of k is k = 1/4TtEo = 9 x 109 (Nm2/c2) since Eo = 8.85 x 10-12 C2 N-l m-2 Thus in terms of force, one coulomb charge can be defined as: If two equal charges separated by one metre experience a force of 9 x 109 N, each charge has a magnitude of one coulomb. The value of electronic charge e is 1.60 x 10-19 C. You now know that the ratio of forces between two point charges qi and q2 separated by a distance r, when kept in free space (vacuum) and material medium, is equal to E/Eo: Fo (in Vacuum)/F (in Medium) = E/Eo = Er where Er is the relative permittivity or dielectric constant. Its value is always greater than one. It is to be noted that: Coulomb's law is also an inverse square law just like Newton's law of Gravitation. Coulomb's law holds good for point charges only. Coulomb's force acts at a distance, unlike mechanical force. Electric Field To explain the interaction between two charges placed at a distance, Faraday introduced the concept of electric field. The electric field E at a point is defined as the electric force F
  4. experienced by a positive test charge qo placed at that point divided by the magnitude of the test charge. Mathematically, we write E = F/qo. This is analogous to the definition of acceleration due to gravity, g = F/mo, experienced by mass mo in the gravitational field F. The electric field E is a vector quantity and has the same direction as the electric force F. Note that the electric field is due to an external charge and not due to the test charge. The test charge q0 should, therefore, be so small in magnitude that it does not disturb the field due to external charge. (In practice, however, even the smallest test charge will disturb the external field.) Electric Field due to a Dipole -g If two equal and opposite charges are separated by a small distance, the system is said to form a dipole. The most familiar example is H20. The figure given above shows charges +q and — q separated by a small distance 21. The product of the magnitude of charge and separation between the charges is called dipole moment, p: p = q x 21 Its Sl unit is coulomb-metre. The dipole moment is a vector quantity. This equation gives its magnitude and its direction is from negative charge to positive charge along the line joining the two charges (axis of the dipole). Having defined a dipole and dipole moment, we are now in a position to calculate the electric field due to a dipole. The calculations are particularly simple in the following cases. Electric Lines of Force (Field Lines) A very convenient method for depicting the electric field (or force) is to draw lines of force pointing in the direction of the field. The sketch of the electric field lines gives us an idea of the magnitude and direction of the electric field. The number of field lines passing through a unit area of a plane placed perpendicular the direction of the field is proportional to the strength of the field. A tangent at any point on the field lines gives the direction of the field at that point. Note that the electric field lines are only fictitious construction to depict the field. No such lines really exist. However, the behaviour of charges in the field and the interaction between charges can be effectively explained in terms of field lines. Some illustrative examples of electric field lines due to point charges are shown in the figure below. The field lines of a
  5. stationary positive charge point radially in outward direction. However, for stationary negative charge, the lines start from infinity and terminate at the point charge in radially inward direction (towards the point charge). You must understand that the electric field lines in both cases are in all directions in the space. Only those, which are in the plane containing the charge are shown here. -q Electrical field lines of single point charges: (a) The field lines of positive charge, and (b) the field lines of negative charge. Similarly, figure (a) below shows a sketch of electric field lines of two equal and similar positive charges placed close to each other. The lines are almost radial at points very close to the positive charges and repel each other, bending outwards. There is a point P midway between the charges where no lines are present. The fields of the two charges at this point cancel each other and the resultant field at this point is zero. Figure (b) depicts the field lines due to a dipole. The number of lines leaving the positive charge is equal to the number of lines terminating on the negative charge. Electric field lines due to a system of two point charges: (a) Two positive charges at rest, and (b) The field lines due to a dipole start from the positive charge and terminate on the negative charge. The following are the properties of electric field lines: • The field lines start from a positive charge radially outward in all directions and terminate at infinity. • The field lines start from infinity and terminate radially on a negative charge. • For a dipole, field lines start from the positive charge and terminate on the negative charge. • A tangent at any point on field line gives the direction of electric field at that point. • The number of field lines passing through unit area of a surface drawn perpendicular to the field lines is proportional to the field strength on this surface. • Two field lines never cross each other.
  6. Electric Flux and Gauss' Law Let us consider a sphere of radius r having charge +q located at its centre. The magnitude of electric field at every point on the surface of this sphere is given by E = k x q/r2 The direction of the electric field is normal to the surface and points outward. Let us consider a small element of area As on the spherical surface. As is a vector whose magnitude is equal to the element of area As and its direction is perpendicular to this element as given in the figure below. ås The electric flux A$ is defined as the scalar product of As and E: The total flux over the entire spherical surface is obtained by summing up all such contributions: Since the angle between E and As is zero, the total flux through the spherical surface is given by = Its x The sum of all elements of area over the spherical surface is 4Tt r2. Hence the net flux through the spherical surface is = k x (q/r2) x 4Ttr2 =4Ttkxq On substituting for k = 1/4TtEo, we get = (1/4TtEo) 4Ttq = q/Eo The spherical surface of the sphere is referred to as Gaussian surface. Equation cbE = q/Eo is known as Gauss' law. It states that the net electric flux through a closed Gaussian surface is equal to the total charge q inside the surface divided by €0. Gauss' law is a useful tool for determining the electric field. You must also note that Gaussian surface is an imaginary mathematical surface. It may not necessarily coincide with any real surface.
  7. Electrostatic Filter You must have seen black smoke and dirt particles coming out of a chimney of a thermal power station or brick kiln. The smoke consists of not only gases but also large quantities of small dust (coal) particles. The smoke along with the dirt is discharged into the atmosphere. The dust particles settle down on earth and pollute the soil. The gases contribute to global warming. These are extremely injurious to living systems (health). It is therefore essential that the dirt be removed from smoke before it is discharged into the atmosphere. A very important application of electrical discharge in gases by application of high electric field is the construction of a device called Electrostatic Filter or Precipitator. The basic diagram of the device is shown here. The central wire inside a metallic container is maintained at a very high negative potential (about 100 kV). The wall of the container is connected to the positive terminal of a high volt battery and is earthed. A weight W keeps the wire straight in the central part. The electric field thus created is from the wall towards the wire. The dirt and gases are passed through the container. An electrical discharge takes place because of the high field near the wire. Positive and negative ions and electrons are generated. These negatively charged particles are accelerated towards the wall. They collide with dust particles and charge them. Most of the dust particles become negatively charged because they capture electrons or negative ions. They are attracted towards the wall of the container. The container is periodically shaken so that the particles leave the surface and fall down at the bottom of the container. These are taken out through the exit pipe. The undesirable dust particles are thus removed from the gases and the clean air goes out in the atmosphere. Most efficient systems of this kind are able to remove about 98% of the ash and dust from the smoke. Electricity and its Importance
  8. In our daily life, we can see a large number of appliances operated by electricity. Let us know some appliances, which make use of electricity. 1. Electricity helps us to keep ourselves, warm in the winter, while it helps us to stay cold in the summer by being utilised in room heaters and air-conditioners, respectively. 2. It is used to produce light in bulbs and other lights. The fan, an important ventilation appliance, is operated by electricity. 3. It is used in refrigerator to keep fruits and vegetables fresh for a longer period. 4. Television, telephone, radio and computer utilise electricity for their working. 5. It is used to produce sound, as in a calling bell or blowing a horn, as in a car or operating the siren in a factory. 6. Even a small torch in a doctor's hand used during a surgery needs electricity. In industries, cranes using electromagnets can lift heavy metals. Electromagnets are made using electric current. Likewise, metals like iron and glasses can be melted at higher temperatures, which can be produced by high electric current. Heavy machineries, which use electric motors, also require electricity for their operations. Sources of Electricity Electricity can be generated from many different energy sources. For example, in a cell, the electricity is produced from chemical energy. The electricity that you use at house is generated from other energy sources such as coal, water, sunlight, wind and so on. Several devices can generate electricity. The most common sources are cells, batteries and generators. Cells You might have seen electric cells in devices like torch, TV remote, wall clock, toys and so on. An electric cell is a device, which produces electricity from stored chemical energy. The cells have a positive terminal and a negative terminal. The positive terminal is the knob on the top of the cell and the negative terminal is the flat end of the cell. On the other hand, button cells are used in wristwatches, calculators, hearing aids and so on. Dry Cell or Primary Cell Dry cell or Leclanche cell is a primary cell, invented by French engineer George Leclanche. It is now widely used in devices, such as, radio and TV remotes. You can identify the parts of dry cell by observing the vertical cross-section of the dry cell.
  9. rcd m•xim The dry cell consists of a carbon electrode packed in a porous pot. The space around the carbon rod is filled with manganese dioxide and activated charcoal. The porous pot is immersed in a jelly—like substance, which is a mixture of ammonium chloride, gelatine and little amount of water. The entire arrangement is kept in a cylindrical case made of thick zinc. The zinc container is again covered with a non-conducting case, which is made of cardboard in earlier days, and tin case in recent days. A brass cap is fixed with the carbon rod. Working: In the above arrangement, a chemical reaction occurs and liberates free electrons. In the cell, carbon rod acts as positive terminal and zinc base acts as negative terminal. When an electric component, such as a bulb or buzzer, is connected in between the brass cap and base, the free electrons move around the circuit and constitute an electric current. Hence, the bulb or buzzer works. A significant advantage of using dry cells is that they have good shelf life and hence well suited for clocks, remote controls and torches. Battery A battery can be constructed by combining more than two cells. A battery will have positive and negative terminals to connect the devices with. For example, in a TV remote, two cells are used to make a battery. The number of cells used to make a battery depends on the requirement of electricity. Secondary Cells In mobile phones, trucks and cars, we use storage cells or rechargeable cells. When an electric current is passed through these cells, reverse chemical reaction takes place and original chemical is restored. These cells can be used multiple times by recharging them. Such cells are called secondary cells. As these cells can accumulate electricity, so they are also known as storage cells. Electric Generators Electric cells and batteries cannot be used when the need of electricity is high. For example, the home appliances such as refrigerator, fan, television, washing machine, etc. cannot be operated using cells or batteries. The electric generator, also called dynamo, converts mechanical energy into electricity for domestic, commercial and industrial customers. The mechanical power may come from sources such as hydraulic turbines at dams or waterfalls, wind turbines, steam turbines using steam produced with heat from the combustion of coals, etc.
  10. Ctcuit Shaft arc.ut 1.7 1-IT Solar Cells Sun gives out its energy year around. Solar energy is everywhere and thus is most abundant and accessible source of energy. The most common applications of solar cells are in solar water heater, solar calculators, and solar cookers, etc. Solar cell is one of the sources of energy, which converts light energy into electrical energy. Solar cell is tiny in size and made of special materials called semi-conductors. When light energy falls on the surface of the cell, electrons will flow in the circuit and thus, produces electricity. Solar cells are interconnected serially on a large flat sheet making solar panels. The output from all solar cells is added and therefore, we can get high power. Solar panels are used to charge storage batteries, supply power to radio, TV and refrigerator. Advantages of solar energy: 1. No maintenance is required 2. No pollution and uses renewable energy Disadvantages: It is expensive. Electric Power Plants We might have observed two power outlets in the plug boards to draw electricity for appliances. Where are the outlets deriving the electricity from? We get electricity from power stations. In power stations, huge generators are deployed to produce high power electricity. Electricity from Water Electricity can be generated by flowing water. The power plant, which produces electricity from water, is called hydroelectric power station. If you live near a river that has dams, you might benefit from hydroelectricity. How does a dam convert flowing water into electricity? Hydroelectric dams are constructed on large rivers and are constructed with penstock, which is a special passage. This passage slopes downwards to create a flow of falling water. As the water falls down the penstock, it turns the blades of turbine, which in turn spins the metal shaft in the electric generator that
  11. produces electricity. Hydroelectric power is the most easily derivable renewable energy source. Electricity from Wind Energy In a wind power station, the energy of moving wind is used to spin the blades of windmills. This in turn runs an electric generator, which produces electricity. Thus, the wind energy is used to produce electricity. The amount of electricity produced increases if the speed of wind increases. The wind velocity must be at least 15 kmh-l in order to produce electricity.

Need a Tutor or Coaching Class?

Post an enquiry and get instant responses from qualified and experienced tutors.

Post Requirement

Related Study Notes

Upload Study Notes

If you have your own Study Notes which you think can benefit others, please upload on LearnPick. For each approved Study Note you will get 100 Credit Points and 100 Activity Score which will increase your profile visibility.

Upload Now
Home > Study Notes > Notes On Electricity