Physics Notes for NEET Chapter 20 PDF

60 Heating and Chemical Effect of Current 1129 E3 Chapter 20 ID Heating and Chemical Effect of Current (3) Resistance of electrical appliance : If variation of Joules Heating resistance with temperature is neglected then resistance of any resistance R then the work done by the electric field on char...

60 Heating and Chemical Effect of Current 1129 E3 Chapter 20 ID Heating and Chemical Effect of Current (3) Resistance of electrical appliance : If variation of Joules Heating resistance with temperature is neglected then resistance of any resistance R then the work done by the electric field on charge q electrical appliance can be calculated by rated power and rated U When some potential difference V is applied across a to flow through the circuit in time t will be W = qV = Vit = i Rt 2 resistor. V R2. PR (4) Power consumed (illumination) : An electrical appliance (Bulb, heater, …. etc.) consume rated power (PR) only if applied W Vit i 2 Rt V 2t    Cal. This relation is called joules J 4  2 4  2 4  2R voltage (VA) is equal to rated voltage (VR) i.e. If VA = VR so produced heating. Electric Power by the resistance R forms U The rate at which electrical energy is dissipated into other of energy is called electrical power i.e. 2 W V  Vi  i 2 R  t R ST P voltage i.e. by using R  is Heat H D YG V 2t  Joule. This work appears as thermal energy in the R (1) Units : It’s S.I. unit is Joule/sec or Watt Bigger S.I. units are KW, MW and HP, remember 1 HP = 746 Watt (2) Rated values : On electrical appliances (Bulbs, Heater, Geyser …. etc.). Wattage, voltage, ……. etc. are printed called rated values e.g. If suppose we have a bulb of 40 W, 220 V then rated power (PR) = 40 W while rated voltage (VR) = 220 V. Pconsumed = PR. If VA < VR then Pconsumed  R  V2 VR2 so PConsumed (Brightness )   A2 PR  VR VA2 R also we have  . PR   (5) Long distance power transmission : When power is transmitted through a power line of resistance R, power-loss will be i2 R Now if the power P is transmitted at voltage V then P  Vi i.e. i  (P / V ) So, Power loss  P2 R V2 Now as for a given power and line, P and R are constant so Power loss (1 / V 2 ) 1130 Heating and Chemical Effect of Current So if power is transmitted at high voltage, power loss will P1, V be small and vice-versa. This is why long distance power P2 , V transmission is carried out at high voltage. Supply Electricity Consumption V (1) The price of electricity consumed is calculated on the Fig. 20.2 power. (ii) If ‘n’ identical bulbs are in parallel. Ptotal  nP (2) The unit Joule for energy is very small hence a big practical unit is considered known as kilowatt hour (KWH) or (iii) which dissipates in one hour in an electrical circuit when the electrical power in the circuit is 1 KW thus 1 KWH = 1000 W  parallel combination, bulb of greater wattage will give more bright light and more current will pass through it. Chemical Effect of Current ability of current is called chemical effect (shown by dc not by ac). (1) Electrolytes : The liquids which allows the current to pass through them and also dissociates into ions on passing D YG (1) Series combination P1, V P2, V Supply V U Fig. 20.1 ST in U Total W att Total Hours 1000 Combination of Bulbs 1 Ptotal  (ii) If ‘n’ bulbs are identical, Ptotal  (iii) i.e. ID (4) Important formulae to calculate the no. of consumed (i) Total power consumed 1 R Current can produce or speed up chemical change, this 3600 sec = 3.6  106 J. units is n  Pconsumed (Brightness)  PR  i  E3 board of trade unit (B.T.U.) or simple unit. (3) 1 KWH or 1 unit is the quantity of electrical energy 60 basis of electrical energy and not on the basis of electrical 1 1  ...... P1 P2 salts, acids and bases in water, etc. Those liquids which do not allow current to pass through them are called insulators (e.g. vegetable oils, distilled water etc.) Solutions of cane sugar, glycerin, alcohol etc. are examples of non-electrolytes. (2) Electrolysis : The process of decomposition of electrolyte solution into ions on passing the current through it is called electrolysis. P N Pconsumed (Brightness)  V  R  current through them are called electrolytes e.g. solutions of Practical applications of electrolysis are Electrotyping, 1 Prated i.e. in series combination bulb of lesser wattage will give more bright light and p.d. appeared across it will be more. (2) Parallel combination (i) Total power consumed Ptotal  P1  P2  P3......  Pn extraction of metals from the ores, Purification of metals, Manufacture of chemicals, Production of O2 and H2, Medical applications and electroplating. (3) Electroplating : It is a process of depositing a thin layer of one metal over another metal by the method of electrolysis. The articles of cheap metals are coated with precious metals like silver and gold to make their look more attractive. The Heating and Chemical Effect of Current 1131 article to be electroplated is made the cathode and the metal to are be deposited is made the anode. A soluble salt of the precious of platinum over metal is taken as the electrolyte. (If gold is to be coated then (Pt) cathode made collects auric chloride is used as electrolyte). the and anode respectively in the ratio 60 of 2 : 1 Faraday's Law of Electrolysis E3 (1) First law : It states that the mass (m) of substance deposited at the cathode during electrolysis is directly proportional to the quantity of electricity (total charge q) passed (4) Voltameter : The vessel in which the electrolysis is electrolyte. It is also known as electrolytic cell. Electrolyte Cu Cathode CuSO4 may be of CuCl2 or any C At cathode Cu Cu S.I. unit of electrochemical equivalent of a substance is kilogram coulomb–1 (kg-C–1). deposited Table 20.2 : E.C.E. for certain substances material but anode Cu lost Hence, the electrochemical equivalent of substance may be when one coulomb of charge passes through the electrolyte. D YG Rh A plates Deposition defined as the mass of its substance deposited at the cathode, cathode Cu voltameter m = z  1 or z = m U Anode/ (E.C.E.) of the substance. Therefore we have m  zit. If q = 1 coulomb, then we have Table 20.1 : Types of voltameters Volatameter constant of proportionality z is called electrochemical equivalent ID carried out is called a voltameter. It contains two electrodes and through the electrolyte i.e. m  q or m = zq = zit, where the Element Atomic weight number Hydrogen 1.0008 1 1 10.4  10–9 Oxygen 15.999 8 2 82.9  10–9 Aluminium 26.982 13 3 93.6  10–9 Chromium 51.996 24 3 179.6  10–9 must be of deposited U Cu AgNO3 Valency E.C.E. (Z) in Atomic kg / C Cathode Rh may be of Ag Nickel 58.710 28 2 304.0  10–9 any deposited Copper 63.546 29 2 329.4  10–9 material Zinc 65.380 30 2 338.7  10–9 but anode Silver 107.868 47 1 1118  10–9 must be of Gold 196.966 79 3 681.2  10–9 ST Ag voltameter – + Anode AgNO3 Agsolution Cathode At cathode Ag Water voltameter O2 + H2 – A – + Rh Both Acidulated H2 and O2 electrode water gases are (2) Second law : If same quantity of electricity is passed through different electrolytes, masses of the substance 1132 Heating and Chemical Effect of Current deposited at the respective cathodes are directly proportional to their chemical equivalents i.e. m  E  m1 E  1 m2 E2 It is an arrangement in which the chemical energy is converted into electrical energy due to chemical action taking Let m be the mass of the ions of a substance liberated, place in it. whose chemical equivalent is E. Then, according to Faraday’s (1) Primary cell : Is that cell in which electrical energy is second law of electrolysis, m  E or m = constant  E or produced due to chemical energy. In the primary cell, chemical m  constant E reaction is irreversible. This cell can not be recharged. Chemical equivalent E also known as equivalent weight in Atomic mass ( A) gm i.e. E  Valancy (V ) cell and Dry cell etc. between chemical equivalent and electrochemical equivalent : Suppose that on passing same amount of electricity q through two different electrolytes, masses of the two substances liberated are m1 and m2. If E1 and E2 are So z1 E  1 z2 E2 (2) Secondary cell : A secondary cell is that cell in which the electrical energy is first stored up as a chemical energy and when the current is taken from the cell, the chemical energy is reconverted into electrical energy. In the secondary cell chemical reactions are reversible. The secondary cells are also called storage cell or accumulator. The commonly used ID their chemical equivalents, then from Faraday’s second law, we m E m1 z  1 have 1  1. Also from Faraday’s first law m2 E2 m2 z2 secondary cells is lead accumulator. (3) Defects In a primary cell : In voltaic cell there are two  zE main defects arises. Cu U (4) Faraday constant : As we discussed above E  z E A  E  Fz  z . ‘F’ is proportionality constant  F VF D YG called Faraday’s constant. As z  60 Relation E3 (3) Examples of primary cells are Voltaic cell, Daniel cell, Leclanche m E and z  Q F so E m hence if Q = 1 Faraday  F Q Polarisation + – Zn Electrolyte dilute H2SO4 Cu Local action Zn Fig. 20.5 then E  m i.e. If electricity supplied to a voltameter is 1 Faraday then amount of substance liberated or deposited is (in gm) equal to the chemical equivalent. Electro Chemical Cell e– U e– Copper (cathode) Zinc (anode) ST Salt bridge SO42– 2Na+ iron, carbon etc. on the surface of commercial Zn rod used as an electrode. The particles of these impurities and Zn in contact with sulphuric acid form minute voltaic cell in which small local electric currents are set up resulting in the wastage of Zn even when the cell is not sending the external current. Removal : By amalgamating Zn rod with mercury (i.e. the 2e– Zn2+ Cu2+ SO4 2– Cu2+(aq)+2e– Cu(s) Local action : It arises due to the presence of impurities of Zn SO42– Zn(s) Zn2+(aq)+2e– Fig. 20.4 surface of Zn is coated with Hg). Polarisation : It arises, when the positive H2 ions, which are formed by the action of Zn on sulphuric acid, travel towards the Cu rod and after transferring, the positive charge converted into H2 gas atoms and get deposited in the form of neutral layer of a gas on the surface of Cu rod. This weakens the action of cell. Heating and Chemical Effect of Current 1133 Removal : Either by brushing the anode the remove the (i) Seebeck arranged different metals in the decreasing order layer or by using a depolariser (i.e. some oxidising agent MnO2, of their electron density. Few metals forming the series are as CuSO4 etc which may oxidise H2 into water). below. Sb, Fe, Cd, Zn, Ag, Au, Cr, Sn, Pb, Hg, Mn, Cu, Pt, Co, Ni, Bi Thermo electric effect of current (ii) Thermo electric emf is directly proportional to the 60 distance between the two metals in series. Farther the metals in the series forming the thermo couple greater is the thermo emf. Thus maximum thermo emf is obtained for Sb-Bi thermo couple. E3 (iii) The current flow at the hot junction of the thermocouple is from the metal occurring later in the series towards that occurring earlier, Thus, in the copper-iron thermocouple the If two wires of different metals are joined at their ends so as This may be remembered easily by the hot coffee. ID to form two junctions, then the resulting arrangement is called a current flows from copper (Cu) to iron (Fe) at the hot junction. (3) Variation of thermo emf with temperature : In a “Thermo couple”. thermocouple as the temperature of the hot junction increases Seeback Effect keeping the cold junction at constant temperature (say 0oC). U (1) Definition : When the two junctions of a thermo couple are maintained at different temperatures, then a current starts D YG flowing through the loop known as thermo electric current. The The thermo emf increases till it becomes maximum at a certain temperature. potential difference between the junctions is called thermo E G of a few micro-volts per degree electric emf which is of the order temperature difference (V/oC). Fe ST U Hot O Cu tn ti t Fig. 20.7 Ice Fig. 20.6 (2) Seebeck series : The magnitude and direction of thermo emf in a thermocouple depends not only on the temperature difference between the hot and cold junctions but also on the nature of metals constituting the thermo couple. (i) Thermo electric emf is given by the equation E t  1  t 2 where  and  are thermo electric constant 2 having units are volt/oC and volt/oC2 respectively (t = temperature of hot junction). For E to be maximum (at t = tn)  dE  0 i.e.  +  tn = 0  t    dt (ii) The temperature of hot junction at which thermo emf becomes maximum is called neutral temperature ( t n). Neutral temperature is constant for a thermo couple ( e.g. for Cu- Fe, t n = 270 o C) 1134 Heating and Chemical Effect of Current (iii) Neutral temperature is independent of the temperature of cold junction. formed. One is between iron and tin and the other is between tin and copper, as shown in figure (iv) If temperature of hot junction increases beyond neutral temperature, thermo emf start decreasing and at a particular Cu temperature it becomes zero, on heating slightly further, the Cu E Sn Sn direction of emf is reversed. This temperature of hot junction is called temperature of inversion (ti). Sn E Fe Fe 60 t t (v) Relation between tn , ti and tc is t n  i c 2 Fig. 20.9 (4) Thermo electric power : The rate of change of thermo called thermoelectric power. It is also given by the slope of parabolic curve representing the variation of thermo emf with temperature of the hot junction, as discussed in previous section. If the soldering metal does not lie between two metals (in Seebeck series) of thermocouple then the resultant emf will be subtractive. Peltier Effect ID  dE  The thermo electric power   is also called Seebeck  dt  E3 emf with the change in the temperature of the hot junction is When current is passed through a junction of two different metals, the heat is either evolved or absorbed at the junction. emf with respect to t, we have thermoelectric power dE d 1 P  ( t   t 2 ) P dt dt 2 This effect is known as Peltier effect. It is the reverse of U coefficient. Differentiating both sides of the equation of thermo Slope  D YG  P    t The equation of the thermo  electric power is of the type y  mx  c, so the graph of Seebeck effect. (When a positive charge flows from high potential to low potential, it releases energy and when positive charge flows from low potential to high potential it absorbs energy.) t thermo electric power is as shown. + – Fig. 20.8 Fe – + Fe Cu Cu (5) Laws of thermoelectricity Heated U (i) Law of successive temperature : If initially temperature limits of the cold and the hot junction are t1 and t2, say the Cooled thermo emf is E tt12. When the temperature limits are t2 and t3, (Heat absorbed) Heated Cooled (Heat evolved) (Heat evolved) (Heat absorbed) Fig. 20.10 ST then say the thermo emf is E tt23 then E tt12  E tt23  E tt13 where E tt13 is the thermo emf when the temperature limits are E tt13 (ii) Law of intermediate metals : Let A, B and C be the three metals of Seebeck series, where B lies between A and C. According to this law, E AB  E BC  E CA When tin is used as a soldering metal in Fe-Cu thermocouple then at the junction, two different thermo couples are being Peltier co-efficient () : Heat absorbed or liberated at the junction is directly proportional to the charge passing through the junction i.e. H  Q  H = Q ; where  is called Peltier coefficient. It’s unit is J/C or volt. Peltier co-efficient of a junction is the amount of heat absorbed or liberated per sec. When 1 amp of current is passed to the thermo couple. Heating and Chemical Effect of Current 1135 It is found that   T dE  T  S ; where T is in Kelvin and dT dE  P  Seebeck coefficient S dT Thomson’s co-efficient : In Thomson’s effect it is found that heat released or absorbed is proportional to Q i.e. H  Q Thomson's Effect  H  Q where  = Thomson’s coefficient. It’s unit is In Thomson’s effect we deal with only metallic rod and not Joule/coulomboC or volt/oC and  = temperature difference. If Q = 1 and  = 1 then   H so the amount of heat (That’s why sometimes it is known as homogeneous thermo energy absorbed or evolved per second between two points of a electric effect. When a current flows thorough an unequally conductor having a unit temperature difference, when a unit heated metal, there is an absorption or evolution of heat in the current is passed is known as Thomson’s co-efficient for the (i) E3 material of a conductor. body of the metal. This is Thomson’s effect. It can be proved that Thomson co-efficient of the material of Positive Thomson’s effect : In positive Thomson’s effect it is found that hot end is at high potential and cold end is at low potential. Heat is evolved when current is passed from conductor   T d2E electric constant  dS dt is passed from Heat colder end to hotter Heat end.evolved The metals which absorbed i , Zn... etc. i effect are Cu, Sn, Ag, Cd shows positive Thomson's Hot Cold dT 2  dS   T    T   ; where  = Thermo  dT  ID hotter end to the colder end and heat is absorbed when current Cold 60 with thermocouple as in Peltiers effect and Seebeck’s effect. Application of Thermo Electric Effect (1) To measure temperature : A thermocouple is used to measure very high (2000oC) as well as very low (– 200oC) U temperature in industries and laboratories. The thermocouple Fig. 28.11 (2) To detect heat radiation : A thermopile is a sensitive instrument Sb-Bi, all connected in series. evolved when current is passed from colder end to the hotter end and heat is absorbed when current flows from hotter end to U colder end. The metals which shows negative. Thomson's effect are Fe, Co, Bi, Pt, Hg... etc. ST i Hot Cold S.No. 1. detection of heat radiation and A thermoppile consists of a number of thermocouples of low potential and the cold end is at higher potential. Heat is Heat evolved for measurement of their intensity. It is based upon Seebeck effect. Negative Thomson’s effect : In the elements which show negative Thomson’s effect, it is found that the hot end is at used Sb T1 Heat radiations D YG (ii) used to measure very high temperature is called pyrometer. G Bi Heat absorbed Fig. 20.13 T2 i Cold Fig. 20.12 Table 20.3: Heating effect and Thermo-electric effects Joule's effect Peltier's effect Seebeck effect Thomson's effect directly Heat produced or absorbed at Here temperature difference of Thomson's heat is proportional proportional to the square of the a junction is proportional to the junction is used to produce to the current passing through current current through the junction. thermo e.m.f. and vice versa. the conductor. Heat produced passing is through a 1136 Heating and Chemical Effect of Current conductor. 2. This effect is produced due to This effect is produced when This when This effect is produced when collision of free electrons with current junctions of a themocouple are parts of same conductor are positive ions of the current junction of suitable materials. kept at different temperatures. kept at different temperature. It is a reversible effect is passed through effect produced carrying conductor. 3. It is not a reversible effect. It is a reversible effect It is a reversible effect 4. Heat produced depends upon Heat exchange depends upon This resistance nature nature of materials used to nature form junctions and temperature temperature of junctions. different parts of the conductor. also) of the of conductors and temperature of the junctions. conductor. 5. It is basically a heating effect It can be heating as well as Different cooling effect. different temperature. junctions are at radiations from a match stick lighted at a distance of 50 metres from the thermopile. electric refrigerator is based on Peltier effect.  Different bulbs U (3) Thermoelectric refrigerator : The working of thermo- This effect depends upon of conductor and difference of It is heating as well as cooling effect. If VApplied < VRated then % drop in output power of (P  Pconsumed )  100 electrical device  R PR ID This instrument is so sensitive that it can detect heat upon 60 temperature thus depends E3 (and effect 25W 100W 1000W 220V 220V 220V D YG (4) Thermoelectric generator : Thermocouple can be used to generate electric power using Seebeck effect in remote areas. (5) Thero-couple meter : The current to be measured passes through a resistance where heat is generated in the amount of i2R joule/sec. The hot junction of the thermocouple is in contact with this resistance, and resulting thermoelectic U current gives deflectionGin the galvanometer G. Hot ST i Fig. 20.14 Cold  Resistance R25 > R100 > R1000  Thickness of filament t1000 > t100 > t40  Brightness B1000 > B100 > B25  Time taken by heater to raise the temperature by  of m kg (or m litre) water is given by t  4180 ( or 4200) m  p  Necessary series resistance to glow a bulb, if VApplied > VRated  VRated  V   VR R   Applied  PR   (PR = Rated power of bulb)  When some potential difference applied across the conductor then collision of free electrons with ions of the lattice result’s in conversion of electrical energy into heat energy  If a heating coil of resistance R, (length l) consumed power P, when voltage V is applied to it then by keeping V constant if it is cut in n equal parts then resistance of each part will be R/n and from Pconsumed  1 , power consumed by each R Heating and Chemical Effect of Current 1137 part P'  nP.  In series a device of higher power rating consumes less power.  Consider that n bulbs are connected in series across V volt supply. If one bulb gets fused and (n – 1) bulbs are again 60 connected in series across same supply, the illumination will be more with (n – 1) bulbs then n bulbs but risk of fusing of bulbs will increases. or geyser is switched on, it will draw a heavy current from the source so that terminal voltage of source decreases. Hence power consumed by the bulb decreases, so the light of bulb r ~ U K ID Heater becomes less.  If  is the density of the material deposited and A is the area of deposition then the thickness (d) of the layer of the material m Zi t  ; where m = A A D YG deposited in electroplating process is d  deposited mass, Z = electro chemical equivalent, i = electric current.  Charging current for a secondary cell  e.m.f. of charger  e.m.f. of cell Total resistance of the circuit U  Efficiency of a cell is given by   E3  When a heavy current appliance such us motor, heater R where R is rR external resistance and r is internal resistance. ST  The efficiency of cell is 50% when the power dissipated in the external circuit is maximum.  Thermo couple can be compared to a heat engine. It absorbs heat at the junction (source) converts heat into electric energy (which appears as the circulating electric current) and rejects the remaining heat to cold junction (Sink).