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- Electricity is a very important form of energy which can be easily converted into other forms of energy. Electricity can be produced at one place and can be transmitted to long distances.
- Electricity is a branch of Physics which deals with charges, stationary and moving. For convenience, electricity is divided into two types electrostatics and current electricity.
- The pioneers of this branch Physics are Gilbert, Thale, Faraday, Benjamin Franklin, Ampere, Volta, Coulomb, Thevenin, and Maxwell.
Structure of Atom:
- An atom consists of positively charged protons, negatively charged electrons, and neutral neutrons. The total number of protons in an atom is equal to the total number of electrons. Thus net positive charge balances the net negative charge. Hence atom is electrically neutral.
- Protons and neutrons are present in central core called nucleus. Hence nucleus carries a positive charge. The negatively charged electrons revolve around the positively charged nucleus in circular orbits. There is force attraction between the negatively charged electrons and the positively charged nucleus which provides the necessary centripetal force for the circular motion of electrons around the nucleus.
- The attractive force between the electron and the nucleus decreases with the increase in the distance of the electron from the centre of the nucleus. Thus the electrons present in the last orbit and last subshells (orbitals) are loosely attached to the nucleus. These orbitals are called valence orbitals and such electrons are called valence electrons,
- By applying suitable method, these valence electrons are removed from valence orbitals and made to move in a particular direction or can be made to transfer to another substance.
- Around 600 BC (Before Christ), a Greek mathematician Thales discovered that amber (a resinous material) rubbed with animal fur attracted light objects like pieces paper, feathers, and treads. Both the amber and the fur acquired this property of attracting lighter objects. Amber in the Greek language is called ‘electron’. From this word, Thale coined a word ‘electricity’. Even though other people may have noticed this before, Thales was the first to record his findings. We don’t have his writings, but from other people’s reports of his work, we can guess at his experiments. At this time, magnetism was also confused with static electricity.
- In this experiment, it is said that the amber and fur have acquired electrical property and process of acquiring electrical property is called electrification.
- When a body is rubbed over another, the transfer of valence electrons from one substance to another takes place. The body which loses valence electrons become electron deficient and acquires a positive charge, while the body which gets the electrons becomes electron rich and acquires a negative charge. As the electricity is produced by rubbing (friction), this electricity is called as frictional electricity.
- A charge may be defined as the amount of electricity present in a body. S.I. unit of charge is coulomb (C) named after Charles Coulomb. It is a scalar quantity and its dimensions are [LoMoA1T1].
- In the early sixteenth century, Gilbert performed the similar example as Thales had performed but he used other materials like a glass rod, ebonite rod etc. He also gave characteristics of charges developed on the bodies.
- When a glass rod is rubbed with a silk cloth, the loosely attached valence electrons of glass rod get transferred to the silk cloth. Thus in case of glass rod becomes electron deficient and acquires a positive charge, while the silk cloth has the excess of negative charge and acquires a negative charge. The total charge of the system i.e. the glass rod and the silk cloth remains zero.
- When an ebonite rod is rubbed with a fur, the loosely attached valence electrons of fur get transferred to the ebonite rod. Thus ebonite rod has e the cess of electrons and acquires a negative charge, while the fur is electron deficient and acquires a positive charge. The total charge of the system i.e. the ebonite rod and the fur remains zero.
- Gilbert charged two glass rods and found that the two glass rods repel each other. Then he charged two ebonite rods and found that the two ebonite rods repel each other. From this, he concluded that the like charges repel each other. He charged glass rod and bought a charged ebonite rod near to glass rod, he found that the two rods attract each other. From this, he concluded that the unlike charges attract each other.
- Thus there are two types of charges positive and negative. Charge produced on glass rod by rubbing it with a silk cloth is considered as a positive charge, while charge produced on ebonite is considered as a negative charge.
- For electrification, two material bodies are involved. The following list gives some objects which are arranged in the order such that if the two objects from the list are rubbed together, they get electrified. The object appearing first in the list acquires a positive charge and the object which appears later in the list acquires a negative charge.
|1. Cat’s skin||2. Fur||3. glass||4. Cotton|
|5. Silk||6. Wood||7. Indian rubber||8. Resin|
|9. Amber||10. sulphur||11. ebonite|
Characteristics of Charges:
- There are two types of charges, namely positive and negative.
- Like charges ( positive and positive or negative and negative) repel each other. Unlike charges (positive and negative) attract each other.
- When neutral body accepts excess of electrons then it acquires a negative charge, while a neutral body loses electrons, then it acquires a positive charge.
- The total charge on a body (either positive or negative) is an integral multiple of the magnitude of the charge on an electron.
- When a glass rod is rubbed with a silk cloth, the glass rod acquires a positive charge and the silk cloth acquires negative charge, but the total charge of the system i.e. combined charge on a glass rod and silk cloth is zero.
- The charge per unit surface area is called surface charge density. It is more at sharp curves and pointed tips.The charge always remains on the outer surface of a conductor. On the uniform surface, it gets distributed uniformly.
- If the charge is produced on insulator, then it remains stationary at the supplied position on the insulator, while if the charge is produced on a conductor, then it gets distributed on the surface of the conductor.
- The charge supplied to a conductor always reside on the outer surface.
- Electrical charge is a scalar quantity.
Important Experiments to Study Characteristics of Electrical Charges:
Coulomb’s or Biot’s Experiment:
- Coulomb demonstrated that charge always resides on the outside surface of a conductor with the aid of two hemispherical cups called Coulomb’s hemispheres which fitted exactly round an insulated metal sphere. These Coulomb’s hemispheres have insulating holders attached to them.
- The sphere mounted on the insulated stand is first charged, and afterwards, the hemispheres are fitted over it by holding at insulating handles. On removing the hemispheres they are found to be charged, but the sphere becomes chargeless. This shows that all the charge on the sphere must have passed to the outside of the hemispheres.
Faraday’s Butterfly Net Experiment:
- Michael Faraday used an insulated cotton net to act as a hollow conductor. at cone apex of the net, a silk thread is attached which extends on both the sides, pulling which the net surfaces can be turned inside out.
- The charge is given to inside surface of the net and it is observed that the charge gets distributed on the outer surface of the net.
- Now the string is pulled from the ringside so that the surfaces flip. i.e. the inside surface becomes the outside surface and the outside surface becomes the inside surface. Again charge is found residing on the outer surface.
Methods of Charging a Body:
Charging by Friction:
- When a body is rubbed to another, there is a transfer of electrons from one body to another due to friction. The body losing electrons is positively charged and the body gaining electrons is negatively charged. Amount of gained and lost electron is equal to each other. Hence the total charge of the system is conserved.
- When a glass rod is rubbed with a silk cloth, glass loses electrons and gets positively charged while the silk cloth gains electrons and gets negatively charged.
- Charge Producers:
- The charge producers consist of two wands, one with dark cploured material and one with white coloured material attached to a conductive disk.
- After rubbing the dark and white surfaces of the two charge producers together. The disk with the white surface will acquire a positive charge; the disk with the dark surface will acquire a negative charge.
Charging by Conduction:
- In the electrically neutral body, there are equal numbers of electrons and protons.The body can be charged by changing this balance by some external agency.
- When a negatively charged rod touches to the neutral body mounted on an insulating stand, then some of the electrons from the rod pass to the neutral body. As a result, the neutral body is negatively charged by contact due to conduction of electrons from the negatively charged rod to the neutral body.
- If the rod is positively charged, then some of the electrons from neutral body pass to the rod and the neutral body becomes electron deficient and acquires a positive charge by contact due to conduction of electrons from the neutral body to the positively charged rod.
- When charged object touches to a neutral object, they both have the same charge. after contact, they start repelling each other due to same nature of the charge.
- When two charged bodies touch each other, the total charge of the system is conserved and they share the total charge according to their capacities.
Charging by Induction:
- Conductors can also be charged without contact.
- Let us consider a negatively charged rod is brought near (without contact) a neutral body mounted on an insulating stand, which is a good conductor of electricity.The rod repels the electrons in the conductors. Hence electrons move towards the far end and protons stay at near end Thus, the near end acquires positive charge while the far end acquires a negative charge. Total charge is zero. Now the far end is grounded. The negative charge on the far end is a free charge it moves towards the earth, while the positive charge is a bound charge remains on the body. Now earthing is removed at the negatively charged rod moved away, the body retains the positive charge.
- Let us consider a positively charged rod is brought near (without contact) a neutral body mounted on an insulating stand, which is a good conductor of electricity.The rod attracts the electrons in the conductors. Hence electrons move towards the near end and protons stay at the far end Thus, the near end acquires negative charge while the far end acquires a positive charge. Total charge is zero. Now the far end is grounded. The positive charge on the far end is a free charge it is neutralized, while the negative charge is a bound charge remains on the body. Now earthing is removed at the negatively charged rod moved away, the body retains the negative charge.
Detection of Charge on a Body:
- If the size of a body to be tested is very large, then an instrument called proof plane is used. The proof plane is a brass or an aluminium-covered conductive disk attached to an insulated handle. It is used to carry the sample of the charge on charged conductive surfaces to transfer to the electroscope.
- To collect a sample, the proof plane is rested on the surface of a charged body. When the proof plane is detached it carries the same nature of charge as that carried by the charged body.
Gold Leaf Electroscope:
- This instrument is used for detecting and measuring static electricity. It works on the principle that the like charges repel each other.
- It consists of evacuated glass jar placed on a nonconducting surface like wood. The mouth of the jar is sealed. A brass rod passes through the seal. inside the jar, at the lower end of the brass rod, two flattened gold foils are fixed parallel to each other. Sometimes only one gold foil is fixed and thin brass plate at the lower end of the brass rod acts as a parallel plate. At the bottom and lower lateral sides of the jar, tin foils are fixed (optional), which help the gold foils to retain their charge for a longer time. A brass disc is provided at the top of the brass rod.
- When a charge is put on the disc at the top it spreads down to the plate and the gold leaves. Now both the leaves and plate will have the same charge. Similar charges repel each other and hence the leaves diverge from each other. Bigger the charge the more is the divergence of the leaves.
- After the use of electroscope, the gold leaves can be made to come together by touching the disc or earthing the disc of the electroscope.
- The electroscope can be charged in two ways: (a) by contact – a charged rod is touched on the surface of the disc and some of the charges are transferred to the electroscope. This is not a very effective method of charging the electroscope. or (b) by induction – a charged rod is brought up to the disc and then the electroscope is earthed, the rod is then removed. The two methods give the gold leaf opposite charges.
Pith Ball Electroscope:
- A pith ball electroscope is a pith ball hanging from a copper hook by help pf a silk thread. It is used to test whether an object is charged or not.
- When a non-charged object is brought near a non-charged pith ball electroscope, the pith ball will not move. If the object is charged then the pith ball will move towards the charged object because it is attracted to it. Now both the pith ball and charged object has same nature of charge hence the pith ball moves away from the charged object. Now if the oppositely charged body is brought near the pith ball it gets attracted.
- The amount of repulsion or attraction depends on the strength of a charge on the charged body.
- Benjamin Franklin discovered atmospheric electricity. When he was flying a kite made up of silk cloth and iron wire as the cross of the kite on a rainy day, he got electricity shocks through the silk thread connected to the kite. From this, he concluded that the clouds contain electric charges.
Reason for the Charge on Clouds:
- The heat from the sun and hot air causes evaporation of water bodies like the sea, river, etc. to vapourize. These vapours rise to height and get accumulated in the sky. This accumulation of vapour particles is the cause of formation of clouds.
- When the molecules of cloud rub with each other, and thus they get charged due to friction. A large amount of electrostatic charge (either positive or negative) gets accumulated on the clouds, resulting in the increase in its electrical potential.
- When two clouds of high electrical potential but carrying opposite charges come to each other, the air between them gets ionized and the electrons jump from negatively charged cloud to positively charged cloud. It produces a dazzling white streak of light called the lightning.
- During lightning, a large amount of heat is produced, which creates pressure waves, which are transferred in all the directions, and produce a very loud sound called the thunder.
- Sometimes during lightning, the accumulated charge on clouds tends to conduct to the surface of the earth. It is to be noted that the charge ties to strikes a conductor that is very near to it like tall buildings and trees. A negatively charged cloud induces a positive charge at the top of the building and a negative charge at its base. Thus a potential difference is created between the top and base of the building. During a rain, the moist air creates a path between the cloud and the top of the building creating a path for the flow of the charge. When the electrons from the cloud reach the top of the building a very high current is set between the top of the building towards the base, which may cause a fire to the building. Thus lightning can have a very devastating effect. To protect high rise buildings from this destruction a lightning rod is put at the top of the building which is connected to the earth. Thus now the electrical current has the easy path through the lightning rod. When a charge is received it is passed to the ground through the lightning rod and not through building itself. Thus building is safeguarded from the devastating effect of the lightning.
Quantization of Electric Charge:
- The fact that all observable charges are always some integral multiple of elementary charge e = 1.6 × 10-19 C is known as quantization of the charge.
Thus q = ± ne, where n = 1, 2, 3, …..
- e = 1.6 × 10-19 C is the magnitude of lowest possible charge which is carried by an electron and proton.
- The cause of quantization of the charge is due to the fact that when one body is rubbed with the other, only integral number of electrons are transferred. There is no scientific explanation for quantization in electrodynamics theory and modern physics but it can be verified experimentally.
- At the microscopic level, Gell-Mann and Zweig postulated that all elementary particles are built out of more elementary constituents called quarks. Protons and neutrons are made up two types of quarks i) up quarks denoted by ‘u’ carrying charge +2e/3 and ii) down quarks denoted by ‘d’ carrying charge – e/3. According to quark model the composition of proton is (uud) carrying charge (2e/3 +2e/3 – e/3 = e) and that of neutron is (udd) carrying charge (2e/3 – e/3 – e/3 = 0). Till now the existence of quarks is not detected experimentally but their existence is proved indirectly. In future when they are detected experimentally only we have to change the definition of quantization from e to e/3. The idea of quantization will remain the same.
Coulomb is not a Practical Unit or it is Very Large Unit:
- Let us consider a body giving 1 billion (109) electrons per second. Let us calculate the time to create a charge of 1 C
We have q = ne
∴ number of electrons required = n = q/e = 1/1.6 × 10-19 = 6.25 × 1018
Time for obtaining these electrons = t = 6.25 × 1018/ 109
= 6.25 × 109 seconds = 6.25 × 109 / (365 × 24 × 60 × 60) = 198.2 years
This indicates that the coulomb is very large unit, hence practical units
like millicoulomb (mC), microcoulomb(μC), nanocoulomb (nC) are used.
Example – 1:
- How much electronic charge is required to make 1 coulomb.
- Given: Total charge = q = 1 C, Electronic charge = e = 1.6 × 10-19 C
- To find: Number of electronic charge = n = ?
We have q = ne
∴ n = q/e = 1/1.6 × 10-19 = 6.25 × 1018
Number of electronic charge is 6.25 × 1018
Example – 2:
- How many electrons should be removed from a conductor so that it acquires a positive charge of 3.5 μC.
- Given: Total charge = q = 3.5 μC = 3.5 × 10-6 C, Magnitude of the charge on electron = e = 1.6 × 10-19 C
- To find: Number of electrons removed = n = ?
We have q = ne
∴ n = q/e = 3.5 × 10-6/1.6 × 10-19 = 2.1875 × 1013
Number of electrons removed is 2.1875 × 1013
Example – 3:
- calculate the positive charge and negative charge on the water in a cup holding 250 g of water.
- Given: Mass of water
- To find: Number of electrons = n = ?
The molecular formula for water is H2O. Its molecular mass is 18 g mol-1
Number of Moles of water = Given mass/molecular mass = 250 /18 = 13.89
1 mol of water contains 6.022 × 10²³ molecules of water
Number of molecules in 13.89 moles of water = 13.89 × 6.022 × 10²³
= 83.66 × 10²³
Each molecule of water contains 2 hydrogens ( 1 electron each) and 1 oxygen (8 electrons)
Number of electrons in each water molecule = 1 × 2 + 8 × 1 = 10
Total number of electrons in a cup = 83.66 × 10²³ × 10 = 83.66 × 1024
Total negative charge on water = 83.66 × 1024 × 1.6 × 10-19 =1.34 × 107 C
As water is electrically neutral, total positive charge =1.34 × 107 C
Ans: The total negative charge is – 1.34 × 107 C and the total positive charge is + 1.34 × 107 C
Example – 4:
- Find the number of electrons moving through an electric bulb per second rated with power 100 W at 230 V.
- Given: Power of bulb = P = 100 W, Voltage = V = 230 V
- To find: Number of electrons passed = n = ?
P = VI =V q/t
∴ q = P t /V = (100 × 1)/230 = 0.4348 C
∴ n = q/e = 0.4348/1.6 × 10-19 = 2.72 × 1018
Number of electrons passed is 2.72 × 1018
Principle of Conservation of Charges:
- Electric charge can neither be created nor be destroyed but it is transferred from one part of a system to another part of the system so that the total charge of an isolated system remains constant.
- Example – 1: When a glass rod (electrically neutral) is rubbed with a silk cloth(electrically neutral), the loosely attached valence electrons of glass rod get transferred to the silk cloth. Thus in case of glass rod becomes electron deficient and acquires a positive charge, while the silk cloth has the excess of negative charge and acquires a negative charge. The total charge of the system i.e. the glass rod and the silk cloth remains zero.
- Example – 2: When a γ ray photon having energy equal or greater than 1.01 MeV passes near very close to the nucleus, the electric field created by the nucleus would annihilate γ rays photon and create a pair of an electron and positron. This phenomenon is known as pair production. It is represented as
We can see that the total charge on either side is equal (zero)
- Example – 3: When electron and positron come very close to each other, they disappear forming two γ ray photons each of energy o.51 MeV. This phenomenon is known as an annihilation of matter. It is represented as
e– + e + → γ + γ
We can see that the total charge on either side is equal (zero)
- Example – 4: Consider following reaction showing α decay of uranium.
92U238 → 90U234 + 2He4
We can see that the total charge on either side is equal (+ 92e)
- Example – 5: Consider nuclear fission reaction
92U235 + 0n1 → 156Ba141 + 36Kr92 + 3 0n1 + Energy
We can see that the total charge on either side is equal (+ 92e)
- Electric charges have additive nature. The total electric charge on a body is equal to the algebraic sum of all electric charges located anywhere on the body. When doing the algebraic sum a due importance should be given to the sign (positive or negative) should be given.
Example – 5:
- Two identical spheres carrying charges -2 μC and 14 μC are made to contact each other and then separated. Find charge on each sphere after separation
- Given: Charge on first sphere q1 = -2 μC, Charge on the second sphere = q2 = 14 μC
- To find: Charge on each sphere =?
Total charge on the system = -2 + 14 = 12 μC
As the two spheres are identical the charge will get equally distributed among them
Hence charge on each sphere = 12/2 =6 μC
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