1. Wave Motion
i. Theory of Wave Motion click here
ii. Wave Motion, Short Quesion & Answer click here
iii. Wave Motion, Numericals click here 2.Mechanical Wave
i. Theory of Mechanical Wave click here
ii. Mechanical Wave, Short Question & Answer click here
iii. Mechanical Wave, Numericals click here 3. Wave In Pipes and Strings
i. Theory of Wave in Pipes and strings click here
ii. Wave in Pipes, Short Quesion & Answer click here
iii. Wave in Pipes, Numericals click here 4. Direct Current Circuit
i. Theory of Direct current circuit click here
ii. Conductor/Super conductor click here
iii. D.C. circuit, short Question & Answer click here 5. Thermoelectric effect of Current
i. Theory of Thermoelectric effect, Seebeck effect click here
ii. Thermoelectric effect, Peltier/Thomsons effect click here
iii. Thermoelectric effect, short Question and Answer click here 6. Chemical Effect of Current
i. Theory of Chemical Effect of current click here
ii. Theory of electrolysis/Faraday's Law of electrolysis click here
iii. Chemical effect of current, Short Quesion & Answer click here
Chemical Effect of Currrent/Short Question Answers
1. Distinguish between conduction of electricity through a metallic wire and an electrolyte?
Ans Metallic conduction differs from electrolytic conduction due to following reasons:
(a) The conduction of electricity in metals is due to the drifting of free electrons, while the conduction of electricity is electrolyte is due to
the movement of positive and negative ions.
(b) No chemical changes take place when current is passed through metal but chemical decomposition of the electrolyte occurs when current is passed through it.
(c) No matter is transformed during electric conduction metals but matter is trnasported in the form of positive and negative ions in case of electrolytes. 2. Explain why the anode of copper voltameter is made of copper?
Ans When current is passed through CuSO4 solution in a copper voltameter, copper ions move towards cathode (being positively charged) while sulphate
ions towards anode (being-very charged). Copper ions transfer their charge to cathode and become copper atoms. The sulphate ions on the anode interact with copper atoms and form CuSO4.
The cathode gain as much weight as the anode loses. This makes necessary for the anode in a copper voltameter to be made for copper. 3. Expalin how you can use a voltameter for measuring current.
Ans Suppose that on passing current through a voltameter for known time t, the increase in mass of the cathode is m. Then according to Faraday's first law
m = Z I t
where z is ECE of the substance. It follows that
i.e. knowing m, t and z, the current passed through the voltmeter can be determined.< 4. Pure water and dry salt are both non-conductorof electricity but the solution of salt in water is a conductor of electricity. Expalin why?
Ans There are no free ions to carry the electricity in pure water or dry salt but when dry salt is dissolved in water, ionization takes place. Salt ionizes inot Na+ and
Cl- ions which carry electricity and solution becomes conducting. 5. What is the difference between electrochemical equivalent and chemical equivalent?
Ans Electrochemical equivalent of a substance may be defined as the mass of ions liberated, when one coulomb of charge passes through the electrolyte.
On the other hand, chemical equivalent = atomic weight/valency
6. What do you understand by Faraday? What is its value?
Ans The quantity of charge required to deposit one gram equivalent (i.e. one chemical equivalent expressed in grams) of any substance, is known as Faraday. Its value is
96500C mol− 7. A simple voltaic cell has an emf equal to 1.0V. When the circuit is open, is there a net field, which would give rise to a force on a test charge
(i) inside the electrolyte of cell (ii) outside the cell?
Ans (i) There are two electric fields inside the cell, one is the electrostatic field due to emf of the cell while another field is of non-electrostatic origin (due to
chemical reactions taking place inside the cell). (ii) Outside the cell, the net field is the electrostatic field between the two plates of the cell (due to charges on the plates). 8. When a secondary cell of emf. 2.0 V is being charged by an external supply, is the terminal voltage of the secondary cell greater or less than 2.0 V?
Ans When a secondary cell is being charged, current inside the cell flows in the direction of electrostatic field (i.e., from positive plate ot the negative
plate). Now the electrostatic field is greater than the non electrostatic field, and therefore, the terminal voltage (which is determined by electrostatic field) is
always greater than the emf. (which is determined by non-electrostatic field). 9. The capacity of a storage cell is marked as 3.5 A at 1h discharge rate. What does this signify? Can the cell provide 16 A for 15 minutes?
Ans The storage cell can supply 3.5 amp current continuously for 1 hour or 0.35 amp current for 10 hours. Now maximum current that can be drawn continuously for
15 minutes = 3.5 × 4 = 14.0 amp. As 16 amp >14 amp, hence cell can not supply 16 amp current for 15 minutes. 10. You are given a primary and a secondary cell of the same emf. From which cell you will be able to draw larger current and why?
Ans Since the internal resistance of a resistance of a secondary cell is very small, hence a larger current can be drawn from this cell as compared to that
from a primary cell. 11. Do electrons carry current inside an electrolytic cell?
Ans No, the current carries inside an electrolytic cell are the positive and negative ions. 12. Name the current carries in the external circuit of an electrolytic cell?
AnsThe current carriers are the electrons. 13. Is the current in the external circuit more or less than the total current inside the cell?
Ans From continuity of current, the current in the external circuit is equal to the toal current of the positive and negative ions inside the cell. 14. Why does voltameter measure current more accurately than an ammeter?
Ans A voltameter measures current in terms of mass of ions (m) deposited and electrochemical equivalent (Z) of the substance (i.e. I = m/Zt). Since values of m and
z are measured to 3rd decimal place and 5th decimal place respectively. The time (t) of passage of current is also measured accurately. The relative error in the measured
of current by voltameter will be very small as compared to that when measured by ammeter directly. Hence voltameter measures current more accurately than an ammeter. 15. What is the function of charcoal and manganese dioxide used in the porous pot of Leclanche cell?
Ans Charcol and manganese dioxide are used in porous pot of leclanche cell as oxidizer. Manganese dioxide removes polarization effect. It oxidises layer of hydrogen
ions and removes the back emf. 16. What is meant by Faraday's Constant?
Ans Faraday constant (F) is the amount of charge which will liberate or deposit one gram equivalent (or chemical equivalent in grams) of a substance during electrolysis. It may
also be defined as the amount of chrge required to liberate or deposit one mole of monovalent substance is electrolysis. Its value is approximately 96500 coul mol-1.
Chemical Effect of Current : Theory of Electrolysis : Faraday's Law of Electrolysis:
Chemical Effect of Current (Short Question and Answer)click here
Introduction
When an electric current is passed through solid metallic conductors, it gets heated up as well as it produces a magnetic field in the surrounding medium. Thus,
solid conductors show both the heating and magnetic effect of current. On the other hand, when current is passed through liquid, different liquids behave differently.
Some liquids like distilled water, vegetable oil do not allow current to pass through them and are called insulators. Liquids like molten metal, mercury allow the passage
of current without dissociating into ions. There are some liquids like NaCl solutions, AgNO3 solution, CuSO4 solution etc. that allow the current
to pass through them by dissociating into ions. Such liquids are called electrolystes. This phenomenon of it, is called chemical effect of current. This effect
was studied by M Faraday in 1833.
Some Important Terms
Electrolysis : It is the process of decomposition of an electrolyte into its constituent ions by the passage of electric current through it. Voltameter: The vessel in which electrolysis is carried out is called voltameter. It is also called electrolytic cell. Electrodes: The metal rods or plates which are partially dipped in the electrolyte solution for passing the current through it are called electrodes. Anode : The electrode connected to the positive terminal of an external battery is called anode. Cathode : The electrode connected to the negative terminal of an external battery is called cathode. Ionisation: The process of decomposition of a compound into its constituent ions is called ionisation Anions: The negatively charged ions which move towards the anode during electrolysis are called anions. Cations: The ions which carry positive charge and move towards cathode during electrolysis are called cations. Chemical equivalent: It is the ratio of atomic mass and valency of a substance,
chemical equivalent =
Theory of electrolysis
Arrhenius in 1887 put forward an ionic dissociation theory to explain the process of electrolysis. According to this theory, the molecules of an electrolyte exist
anelectolyte exist in the form of ions even if the electrolyte is in the solid state. For example a common salt molecule (NaCl) exists in the form of ions even in
crystalline state as NaCl → Na+ + Cl-. These ions are held together by the electronstatic force of attraction.
When NaCl is dissolved in water, the force between them is lesser (1/80 times) and hence they become nearly free from each other's attraction and therefore they get
separated. when a potential difference is applied across the electrolyte, the cations, (Na+) move towards cathode and anion (Cl-) move
towards anode. One reaching to the respective electrodes, the ions get discharged (becomes neutral) and them appear as deposits on the electodes or get liberated as
free gas. This expalins the conduction of electric current through an electrolyte.
The electrolytes conduct electricity, but their conductivity is very low (about 10-6 times that of good conductor) because of the following reasons:
(a) Ions have very large mass compared to electons. So they drift slowly under an electric field.
(b) Number of ions per unit volume in an electrolyte is much less than the number of free electrons in a metallic conductor.
(c) Electrolyte solution is dense and disordered and therefore ions have difficulty to drift through it.
Faraday's Laws of Electrolysis
Michael Faraday performed a number of experiments of electolysis and summarized his conclusions in the following two laws, known as the Faraday's laws of electrolysis.
First Law
The mass of a substance liberated or deposited on an electrode during electrolysis is directly proportional to the quantity of electric charge passed through the electolyte.
If m is the mass of the substance deposited on the cathode when an electric charge Q is passed through the electolyte, then
m α Q
m = Z Q
m = Z I t
where Z is a constant of proportionality called electrochemical equivalent of the substance.
Second Law
If the same amount of electric charge is passed through different electrolytes, the masses of the substances liberated or deposited are proportional to their chemical equivalents.
Let m1, m2 and m3 be the masses of the substances liberated in three different voltameters when the same electric current is passed
through them and E1, E2 and E3 be their respective chemical equivalents. Then
or = constant.
Electrochemical Equivalent and its Experimental Determination
From Faraday's first law of electrolysis,
m = Z Q
If Q = 1 coulomb, then Z = m
Hence electrochemical equivalent (z) of a substance is defined as the mass of the substance liberated or deposited in electrolysis by the passage of 1 coulomb of
charge. Its SI unit is kg/C. Its value for copper is 3.29 × 10 − 7 kg/C, for silver 1.18 × 10 − 6 kg/C, for hydrogen
1.05 × 10− 11 kg/C.
Relation between Electrochemical Equivalent Z and Chemical Equivalent (E) of a substance
Let m1 and m2 be the mass of the substance liberated when the same quantity of charge Q is passed. If Z1 and Z2 be their
respective electrochemical equivalents, then from Faraday's first law of electrolysis,
.............(i)
If E1 and E2 be their respective chemical equivalents, then from Faraday's second law,
or,
.............(ii)
From (i) and (ii), we get
.............(iii)
Therefore, E α Z
Hence the electrochemical equivalent of an element is directly proportional to its chemical equivalent
or, E = F Z ..................(iv)
where F is constant and is called Faraday's constant.
This is the relationship between Z and E.
Faraday's Constant
Faraday's constant is defined as the ratio between the chemical equivalent of a substance to its electrochemical equivalent.
Important Practical Application of Electrolysis
Following are some important application of electrolysis I. Electroplating : This is the process of coating of one metal (Gold, Copper, silver etc.) over another metal by electrolysis. The particles os cheap
metals can be coated with precious metals to make them more attractive or to prevent corrosion. 2. Purification of metals : This method is used in the refining of metals like copper, zinc, tin etc. The anode is made of impure metal and the cathode is made
of pure metal. The electrolyte used is soluble salt of pute metal. On passing the current, the pure metal is deposited on the cathode. 3. Manufacture of chemicals: By electrolysis chemical, like sodium hydroxide, sodium chloride, pure sodium etc. can be manufactured. 4. Chemical analysis: The chemical composition of salts can be found out by electrolysis. 5. Medical application: Electrolysis is used for stimulating nerves especially fr treating polio and for removal of unwanted hair from any part of body. 6. Printing industry: The metal copies of types used in printing books, gramophones, records, block etc. can be made by using the process of electrolysis. 7. Production of gases for commercial use: Oxygen and hydrogen are obtained by the electrolysis of acidulated water, on commercial scales. 8. Electrolysis capacitors: are prepared by depositing a thin film of aluminium oxide and aluminium anode during electrolysis which acts as dielectric between the two
electrodes. The electrolysis is a mixture of boric acid, glycerin and ammonium hydroxide due to oxide deposit, capacitors has very large capacitance.
Thermoelectric Effect (Peltier and Thomson's Effect) :Click here Thermoelectric Effect(Seebeck Effect) :Click here
Thermoelectric Effect Short Question Answers
1. What is thermoelectric emf ?
Ans :- The production of electricity by keeping the junctions of two dissimilar metals at different temperatures is called thermoelectric effect. The e.m.f thus produced across the junctions is called thermoelectric e.m.f. The magnitude and direction of thermoelectric e.m.f. depends on the nature of the materials forming the thermocouple and the temperature difference between the two junctions. 2. What is the cause of electric current in thermo electricity ?
Ans :- The origin of potential difference due to diffence of temperature between two parts of the same conductor or between two junctions of a thermocouple is the cause of electric curent in thermo electricity. 3. Does the thermoelectric effect obey the law of conservation of energy ? Explain how ?
Ans :- The production of thermo emf in a thermocouple is the result of conversion of the net heat absobed in the thermocouple into electric eneergy. Hence, thermoelectric effect obeys the law of conservation of energy. 4. How does the thermoelectric series helps us to know the direction of flow of current in a thermocouple ?
Ans :- If the themocouple formed of the two metals form the themoelectric series, the current flows from the metal occuring earlier in the series to the metal occuring later in the series of the cold junction. 5. If the temperature of cold junction of a thermo couple is lowered, what will be the effect on (i) neutral temperature and (ii) temperature of inversion ?
Ans :- The value of neutral temperature of the given thermocouple will emain unchanged being independent of temperature of cold junction but the value of temperature of inversion of this thermocouple will increase, as θ i = 2 θ n − θ c. 6. What are the two tactors on which thermo-electric emf produced in a thermo couple depends. ?
Ans :- The factos are (i) nature of two metals forming the thermocouple and (ii) temperature difference between two junctions of the thermocouple. 7. What is thermo electric power ?
Ans :- Rate of change of thermo emf with temperature difference between the two junctions, is called as thermo-electric power. 8. Why do we generally prefe Sb-Bi thermocouple in all experimental work ?
Ans :- The thermo emf generated is large if the metal forming the thermocouple are far apart, in the thermo electric series. That is the case with Sb and Bi because Sb is the first member of the series and Bi is the last member of the series.
Peltier Effect : Thomson's Effect : Difference between Peltier's and Joule's Effect :
Thermoelectric Effect(Seebeck Effect) :Click here Thermoelectric Effect (Short Question and Answer) :Click here
Peltier Effect
When an electric currrent is passed through a thermocouple, heat is either absorbed or released at the junctions, depending on the direction of current flow. This effect is called Peltier effect. This is a reversible effect. That is, when direction of current is reversed, the heat evolved or absorbed is interchanged at the junction. Peltier effect is the inverse process of Seebeck effect.
The rule which tells whether heat is evolved or absorbed at any junction is Peltier effect is that, if the direction of Seebeck emf is from Cu to Fe at the hot junction, then an external emf applied in the same direction will produce cooling at this junction and heating at the other junction.
Peltier Coefficient
Peltier coefficient at any temperature for the junction of two metals is the product of absolute temperature and thermoelectric power at that temperature. Perltier's coefficient, denoted by π is given by
π = T P
or, π = T
Thomson's Effect
In 1851, William Thomson found that when an electric current is passed through a non-uniformly heated conductor, heat is evolved or absorbed. This phenomenon of evolution or absorption of heat (other than Joule's heat) along the length of a conductor on passing current through it when its two ends are kept at different temperature is known as Thomson's effect.
If an electric current is passed through a copper wire from its hotter end to the colder end, the heat is evolved and the wire becomes hot.
If the current is reversed, heat is absorved along the conductor.
Similarly, if an electric current is passed through an iron wire from its hotter end to the colder end, the heat is absorbed and the wire gets cooled. If current is revrsed, the heat is evolved along the conductor. so Thomson's effect is reversible.
The substances, which behave like copper, are said to have a positive Thomson's effect. some of the substances that show this effect are silver, zinc, antimony, cadmium etc. The substances which behave like iron are said to have a negative Thomson's effect. Other substances showing this effect are bismuth platinum, cobalt etc. Thomson's effect of lead is nil. So it is used as the standard metal in thermoelectricity.
Cause of Seebeck Effect
Different metals have different free electron density. When two different metals are brought into contact, the free electrons tend to diffuse from the metal with greater electron density to the other with lower electron density. Due to diffusion, a potential difference is set up at the junction of two metals, called contact potential. When both junctions are at the same temperature, the contact potential at the junctions will be equal and opposite. Hence no current flows through the thermocouple. But if one junction is kept at a higher temperature, the rate of diffusion of free electrons at the junction will increase. As a result of it the contact potential at the two junctions will become different and hence there will be an effective emf in the circuit called the thermo emf.
Cause of Peltier Effect
When two dissimilar metals are joined, contact potential is established at the junctions i.e. the potential of one must become above that of the other. For example, in Cu-Fe thermocouple, potential of Fe is greater than potential of Cu. At one junction current flows from lower potential to higher potential (from Cu to Fe) and the energy is required for this purpose, which is absorbed from the junction and hence it is cooled. At another junction current flows from higher potential to lower potential. The energy is given out at this junction due to which the junction becomes hot.
Cause of Thomson's Effect
When two ends of a conductor are kept at different temperature, the number of free electrons in the higher temperature region will have higher than those in the lower temperature region. So there is diffusion of electrons from one region into another and this gives rise to a potential difference or an emf. This emf is called Thomson's emf. When an electric current is passed through the wire either work is done on the charge carries or by the charge carriers. Therfore, the thermal energy is either evolved or absorbed depending upon the work done.
Differences between Peltier's Effect and Joule's Effect
Peltier's Effect
Joule's Effect
1. It takes place only at the junction.
1. It takes place throughout the conductor
2. Heat is evolved or absorbed
2. Heat is always produced
3. It is reversible process
3. It is irreversible process
4. Heat is produced or absorbed is proportional to the current
4. Heat produced is proportional to the square of the current
5. It depends upon the direction of current
5. It is indpendent of the direction of current
6. A mount of heat evolved or absorbed depends on the nature of the metals and temperature of conductor.
6. Amount of heat produced depends on resistance of the conductor
Distinction between Thomson's Effect and Joule's Effect
Thomsons Effect
Joule's Effect
1. Heat is evolved or absorbed along the wire
1. Heat is always produced along the length of the conductor
2. It is reversible effect
2. It is an irreversible effect
3. Heat produced in proportional to the current.
3. Heat produced is proportional to the square of the current.
4. For the evolution of heat, temperature difference is required along the length.
4. Temperature difference is not required.
5. It depends on direction of current.
5. It is independent of the direction of current.
6. Amount of heat produced depends upon temperature difference between the ends of the conductor.
6. Amount of heat produced depends upon resistance but not on temperature difference.
Application of Thermoelectric Effect
There are many applications of thermoelectric effect. Some of them are discussed below. Thermopile
It is a device used for detection and measurement of heat radiation. It is based on Seebeck effect.
It is constructed on the principle that if a number of thermocouples are connected in series, then the thermo emf gets multiplied.
It consists of a number of Bi-Sb thermocouples connected in series so that the thermo emfs produced in all thermocouples are added. One set of junctions is blackened and exposed to heat radiation while the other set of the junctions are protected from heat radiation by an ensulating cover. A sensitive galvanometer connected to the circuit detects the thermo emf produced by the rediation. The thermocouple is also used for the measurement of high temperature of a furnace.
Thermoelectric Generator
It is based on the thermoelectric effect. By heating one junction, and keeping other junction at room temperature of a thermocouple, electric current flows through the circuit. The electric power generated in this method can be used to operate electronic devices in remote areas.
Thermoelectric Effect/Seebeck Effect : Variation of Thermo emf with Temperature : Relation Connecting Thermoelectric Constants α, β , θ n, θ i :
Thermoelectric Effect(Peltier and Thomson's Effect):Click here Thermoelectric Effect(Short Question and Answer):Click here
Introduction
In the same way as the heat energy can be obtained from electrical energy, the electrical energy can also be generated from the heat energy. The electricity thus generated is called thermoelectricity. The phenomenon in which electrical energy produced by means of thermal energy is called thermoelectric effect. This effect involves following three related effects:
(a) Seebeck effect (b) Peltier effect (c) Thomson's effect.
Seebeck's Effect
If two different metal wires are joined to form a closed circuit and two junctions are kept at different temperatures, a small emf is set up in the circuit and small current flows in the circuit in a definite direction. This effect is called thermoelectric effect or Seebeck effect which is discovered by a German physicist Thomas J Seebeck. The emf developed in the circuit is called thermo emf and the current called thermoelectric current.
Thermocouples
A couple of wires of dissimilar metals forming a loop and producing thermoelectricity is called thermocouple. As an iron-copper thermocouple. The magnitude of emf produced and the direction of current depends on the pair of metals selected from the thermoelectric series and temperature of the junctions. In iron-copper thermocouple, current flows from iron to copper at the cold junction. The direction of current flow changes if heating and cooling of the junction are reversed.
Thermoelectric Series
An arrangement of metals in series in which any two metals can be used to form thermocouple is called thermoelectric series. When a thermocouple is formed from a pair of metals in the sires, the direction of the current flow through the cold junction is from the metal which occurs earlier in the series to the one which occurs later. Greater the difference in the order in the series, higher is the value of emf produced. The thermoelectric series is given below.
Antimony, Iron, Zinc, Silver, Lead, Copper, Platinum, Cobalt, Bismuth.
Thus for copper-iron thermocouple, the current will flow from iron to copper through the cold junction. Similarly in antomony-bismuth thermocouple, the direction of thermoelectric current is from antimony to bismuth throught the cold junctions. Also, for the same temperature difference between the two juncitons, the thermo emf developed in antimony-bismuth temperature is more than that in copper-iron thermocouple.
Thermoelectric effect is used in measurement of temperature and radiant energy. This effect produce trouble in circuit used for precise measurement of current or detecting small current.
Variation of Thermo emf with temperature
To study the variation of thermo emf with temperature, an iron-copper thermocouple is taken as shown in fig. One junction is immersed in an oil bath and the other junction is kept melting ice whose temperature is kept constant. The temperature of oil bath is increased gradually by heating it. When the temperatures of both junctions are at the same (00C), the galvanometer shows on deflection and so, no emf is produced.
...................................
As the temperature of the hot junction is increased, and the cold junction is kept at (00C), the deflection of galvanometer also increases i.e. emf also increase till it becomes maximum at θ n called neutral temperature. The temperature of the hot junction at which the thermo emf becomes maximum is known as neutral temperature (θ n).
As the temperature of hot junction is increased beyond neutral temperature, thermo emf starts to decrease and ultimately becomes zero at temperature θ i called temperature of inversion. The temperature of the hot junction at which thermo emf is zero and changes its polarity is called the temperature of inversion θ i.
If the temperature is increase beyond θ i, the direction of thermo emf is reversed. The inversion temperature depends upon the temperature of cold junction and nature of meals used in the thermocouple. For copper-iron thermocouple, neutral temperature is about 250 0 and temperature of inversion is about 500 0C. The variation of thermo emf with temperature is shown in fig. The variation of thermo emf with temperature θ is given by
where α and β are constants. The values of these constants depends on the materials of conductor and the temperature difference of two junctions.
If θ c is the temperature of the cold junction, then we have
θ i − θ n = θ n − θ c
or, 2 θ n = θ c + θ i
or, θ n =
Relation Connecting Thermoelectric Constants α, β , θ n, θ i
The cold junction of thermocouple is kept at 00C and θ is the temperature of the hot junction. The thermo emf is . Differentiating this equation with temperature, we get
At α = α n, E is maximum and so . It can be seen that the slope of E-θ graph, is zero at point P. So,
0 = α + β θ
or, θ n =
When θ = θ i, then E = 0 and
θ i(α + β θ i) = 0
Since θ i can't be zero, then (α + β θ i) = 0
(θ i =
Thermoelectric power
The rate of change of thermo emf with temperature is called thermoelectric power. It is denoted by P and given by
1. A large number of free electrons are present in metals. Why is there no current in the absence of electric field across it?
Ans In the absence of electric field, the free electrons in the metal have random motions. During motion, they collide with positive ions of the metal again and again and
after each collision, their direction changes. So there is no net flow of charge carriers in a particular direction and hence no current flows. 2. In a resistance box, there is a resistance marked infinity. What is the length of the wire used for making this infinite resistance?
Ans No wire is used to produce an infinte resistance. The infinite resistance is produced by an air gap as air is an insulator. 3. The electron drift speed in metals is small in the order of few mm s − and the charge of the electron is also very small, 10 − 19 C, but we
can still obtain a large amount of current in a metal. Why?
Ans The current in metal depends not only on charge (e) and drift speed (Vb), but also on the number density (n) of free electrons. Although e and vd are small,
we can still obtain a large current because n is very large, -1029 m− 3 4. A steady current is flowing in a metallic conductor of non-uniform cross-section. Which of these quantities is constant along the conductor: current, current density,
electric field, drift speed?
Ans Since the current is steady, it remains constant in the conductor. All other quantities vary inversely with area of cross-section. 5. Is Ohm's law universally applicable in all conducting materials? Explain with examples of material.
AnsNo. Examples of materials that are not applicable to Ohms law (i) semiconductor diodes, vaccum diodes, transistors, etc. The I-V graph is a curve but not a straight line. 6. There is animprression among many people that a person touching a high power line gets shock. Is it true? Expalin.
Ans This expression is not true. The current disorganizes our nervous system. It makes the person to lose temporarily his ability to exercise his nervous control to get himself
free from high power line. It is dangerous only if we touch to ground with power line at the same time. At this condition, the current will pass through our body and produce harmful effect. 7. The electron drift in a conductor arises due to the force experienced by electrons in the electric field. This force should cause acceleration of electrons. But the electrons acquire a steady drift velocity. Why ?
Ans The reason is that electrons suffer a large number of collisions with the positive ions of the conductor. Although the electric field makes the electron to accelerate, the collision reduces the acceleration.
So the gain in speed between collisions is lost in the next collision. Thus the net acceleration averages out to zero and the electron acquires a constant average drift speed. 8. Why are standard resistors made of alloys such as constantan and manganin ?
Ans The temperature coefficient of resistance for constantan and manganin is very small. This means that there is a negligible change in the resistance due to moderate changes in temperature.
Having high value of α, the wire occupies less space in devices for high value of resistance. 9. Two wires, one of copper and other of manganin, have equal lengths and equal resistance. Which wire is thicker?
Ans 10. Why is an ammeter connected in series in a circuit ?
Ans An ammeter is a low resistance galvanometer. When it is connected in series in a circuit, the resistance of the circuit does not increase appreciably and consequently and consequently
the current in the circuit remains unaffected. 11. Why voltmeter connected in parallel across a circuit element ?
Ans A voltmeter is a high resistance device. When it is connected in parallele across any component of a circuit, it draws a very small current from the main circuit. Most of the current passes through
the element. Hence the p.d. across the component is not affected. 12. What happens when an ammeter is placed in parallel with circuit ?
Ans When an ammeter is connected in parallel in a circuit, the resistance is considerably reduced. A large current flows through the circuit, which can damage the ammeter. Moreover, the ammeter
wil read the current passing through it only and not the current in the circuit. 13. What happen when voltmeter is connected in series in circuit ?
Ans When voltmeter is connected in series, the resistance of the circuit becomes high. The current decreases considerably. Voltmeter will not read actual potential difference. 14. Though the direction of electric current is given by the direction of flow of positive charges but even then it is regarded as a scalar quantitiy. Explain why ?
Ans : Current is regarded as scalar quantity due to the following facts:
(i) Currents can be added or subtracted according to ordinary rules of algebra.
(ii) It is dot product of two vectors i.e. I = J.A.
(iii) Strength and direction of current always remains unchanged even if (a) wire carrying current has different cross-sections at different points
along its length and (b) wire may be bent at different angles at its different points. 15. Though electrons are constantly in motion within metals, but there is not current until a potential difference is established across it. Expalin why?
Ans : Electrons in metals move randomly in all directions and there is no net motion in any direction in the absence of a potential difference. When a potential difference is applied across the metal an
electric field is established within the metal, and electrons drift towards higher potential which generates an electric current. 16. Although the drift velocity of electron is very small, yet an electric bulb lights up almost instantly when switched on, whatever be the distance of the bulb from the switch. Why?
Ans : As soon as the switch is made on, an electric field is established in the circuit instantly with the speed of light, causing at every point a local electron drift. Hence, current in the circuit is established
almost instanlty when the switch is made on the bulb also lights up instantly. This implies that information for the flow of current is transmitted through the propagation of electromagnetic waves
(electric impulse) and not with drift velocity of the electrons. 17. A steady current is flowing in a cylindrical conductor. Is there any electric field within the conductor?
Ans : Yes, current flows in a conductor only when electric field established within the conductor exerts force on the free electrons. 18. Why don't we consider the drift velocity of positive ions ?
Ans : The positive ions in a conductor also experience a force in the presence of an electric field. Since the positive ions are heavy and tightly bound in the metal, they are hardly able to
move making the drift velocity negligibly small. 19. A wire is stretched to double its length. What will be its new resistivity ?
Ans : The resistivity of the wire will remain the same as it only depends upon the nature of the material of the wire and is independent of the lenght or area of cross-section of the wire.20. Is the value of temperature coefficient always positve?
Ans : No, the value of temperature coefficient of resistance is positive only for metals and alloys and is negative for semiconductors and insulators. 21. Why do we use connecting wires made of copper?
Ans : Copper has low resistivity and therefore it is a good conductor. Moreover, it is diamagnetic and so does not get magnetised due to current flowing through it. Thus it does not
disturb the current flowing in the circuit. 22. What are the factors on which resistance of a conductor depend?
Ans : The resistance of the conductor depends upon length (l) of the conductor such that R α l, Area of cross section (A) of the conductor such that R α ,
Temperature and nature of the materials of the conductor.
Therefore, at a given temperature,
R α
i.e. R =
where ρ is the proportionality constant which is called the resistivity or specific resistance of the material of the conductor. 23. You are given n wires each of resistance r. what is the ratio of maximum to minimum resistance that can be obtained from these wires ?
Ans :Since in series combination effective resistance increases, therefore, when n wires each of resistance R connected in series the maximum resistance is obtained is given by
Rs = R + R + R + ...... = n R
or Rs = n R ............(i)
In parallel combination effective resistance decreases, so minimum resistance is given by
or, Rp = ..................(ii)
where Rs and Rp be the effective resistance in series and parallel combination.
Dividing Eq. (i) by Eq. (ii), we get
The required ratio = n2. 24. You are given 2 wires each of resistance R. What is the ratio of maximum to minimum resistance that can be obtained from these wires?
Ans : In series combination effective resistance increases and in parallel combination effective resistance decreases. Therefore, when 2 wires each of resistance R are connected
in series, the maximum resistance is obtained and when they are connected in parallel minimum resistance is obtained.
Rs = R + R = 2R
i.e. Rs = 2R ................(i)
Rp =
i.e. Rp = ................(ii)
where, Rs and Rp be the effective resistance in series and parallel combination.
Dividing Eq. (i) by (ii), we get
= 4
the required ratio of Rs and Rp is 4:1. 25. A wire is stretched to double its length. What happens to its resistance and resistivity?
Ans : If R be the resistance of the wire of rsistivity ρ whose length is l and area of cross section is A. Then resistance
R = .
Therefore, R =
or, R = , where A l = v is the volume of the conductor
If the wire is stretched to double its length, then the new length is l' = 2 l and the new resistance R' can be written as
R' = , where, v =v' because volume remains constant.
Also ρ' = ρ, because the material of the conductor is same
Therefore R' = [since R =]
Thus, when a wire is stretched to double its length, its resistance becomes 4 times of the original resistance. Since the resistivity depends upon the material of the conductor, therefore the resistivity
remains same for the same conductor.
