Effects of Electric Current PDF

All the devices here work using electric current. How do they give...

All the devices here work using electric current. How do they give different forms of energy! Have you also felt the same doubt? Some electrical devices are shown in the house of the child. What are they? Let's try to write them down. Electric bulb Electric fan It is electrical energy that is given to the devices. But don't they give us different forms of energy? Write down the energy changes in them with respect to their use.  Device Use Energy change e Electric bulb | i | eee, — nee e Induction cooker | To get heat Electrical energy — Heat e Storage battery (while charging) | vl| ceseeteesseeseeseesen DD NR CS I NE IR CO EP NE RY DD TT COFFUUTDD—_—___—_ —— > coomMoonocoonconoIOs Table 1.1 It is clear from the table that electrical energy can be transformed into different forms of energy. The useful form of energy into which a device converts electrical energy, 1s considered as the effect of electric current on that device. e Weare familiar with so many electrical devices in our daily life. Write down in your science diary the effect of electric current on each of them. You might have studied in your chemistry class the chemical effect of current. Now we shall learn more about the heating and the lighting effects of electricity. Heating effect of electric current Among those we use in daily life, which are the devices that give heating effect of electric current? e Electric iron How does electrical energy change into heat energy in such devices? Let’s do an experiment. Materials required: - A nichrome wire of approximate length 5 cm - 6 V storage battery - Connection wires Construct a circuit as shown in Fig. 1.1  R ———MW- Nichrome wire 6V S A Fig. 1.1 How does the nichrome wire become red hot while passing electricity through the circuit? Analyse this based on the concept that energy can neither be created nor destroyed. It can only be converted from one form to another (Law of Conservation of Energy) In this case which form of energy was converted into heat energy? How does this energy change occur? Let's analyse by examining the current and voltage in the circuit. >— % www _—> Wy A + HE{IN @) Io Y S A JF Fig. 1.2 We can measure the voltage between the ends of the resistor R (nichrome wire) using a voltmeter V and the current through it, using an ammeter A. If the ammeter shows a current I ampere on applying a potential difference V across the resistor of resistance R Q, Current I = Then, the charge that flows through the conductor in t second, The potential difference between two points will be one volt if one joule of work is done in moving one coulomb of charge from one point to the other. © ether t th tte teh th hl  One joule of work is required to move one coulomb of charge under one volt potential difference. Hence the work W to be done to move one coulomb of charge under a The English scientist James potential difference V will be, W = V joule. Prescott Joule, born in 1818, If so, the work to be done to move a charge Q under a conducted studies about the salient potential difference V is W = QV. The work required for features of heat energy and about moving the electric charge through the conductor is done the mechanical movements caused by the battery connected to the circut. The power P supplied by heat. by the battery to the circuit in a time t second is P = m_ On substituting the equation of work in this we get p - YL V He became famous for his two ie, P= VI. laws related to the magnitude of Therefore the energy supplied by the battery to the circuit heat due to current and how the int second = Pt = VIt. energy of gas influences pressure, The electrical energy expended by the battery in the circuit volume and temperature. containing the nichrome wire is converted into heat. Therefore H = VIt The heat is developed since a current is available in the circuit in accordance with the voltage applied. If so why did the nichrome wire in the circuit alone become red hot? Let's examine the influence of resistance in changing the electrical energy into heat energy. According to Ohm's Law, V = IR. On substituting this in the equation H=VIt H = IR(t) = FRt From this it can be understood why the nichrome wire alone becomes red hot. This process by which heat is developed in a circuit on passing current through it is known as the Joule Heating or Ohmic Heating. Haven’t you understood the factors influencing the heat developed when a current passes through a conductor? i a a a el a ae et tee]  Effects of Electric Current Joule's Law The heat generated (H) ina current carrying conductor is directly proportional to the product of the square of the current (I) in the conductor, the resistance of the conductor (R) and the time (t) of flow of current. H oc PRt -. H= PRt joule lis the current in ampere, R is the resistance in ohm and t is the time in second. Complete the following table on the basis of Joule’s Law. Resistance of Intensity of Time for which | Heat generated Change in conductor R (©) Current I (A) [current flows t (s) PRt(J) Heat (H) 2R I t 2 PRt Twice (2H) R 21 ton | oooooooeeooeenrcs | eooeroneeoareeanece R/2 I to | ooooeeooeeooeeencs | ceceeeeceeeeeeee ees R 1/2 tro | ee eeeeeeeeeeeeeee | eoooooneeoaneeannce R I DU TE| ceeeeeeeeeeeeeeeees R I 2 | EE ENT Table 1.2 Analyse the table and find out the factor that influences heat the most. Don't you now understand how the change in current, resistance and the time for which electricity flows influence the amount of heat developed? Joule’s Law is useful in devices that make use of heating effect of electricity. Let’s solve some mathematical problems which are related to Joules Law. e How much will be the heat developed if 0.2 A current flows through a conductor of resistance 200 Q for 5 minute?. H=PRt R = 200 Q = (0.2 x200x 300 I =02A = 2400 J t =5 x60s = 300s -. Heat generated = 2400 J If 4.2 J is one calorie then H =........ calorie H = FRt is used to find out the heat developed when current flows through a conductor. Let’s try to write down the equation in some other forms as well. According to Ohm’s Law, I= V/R. If we substitute this in the equation according to Joule's Law H = Rt, we get H = (V/RYRt © Let's find out the heat developed in 3 minute by a device of resistance 920 Q working under 230 V V=230V Try to solve the problem R = 920 Q in another way t=3x60s V = 230 V, R = 920 Q On using the given values [= a = euaoronenaoenannece H= vit H=FRt R _ 230*x3x60 _ — 90 TT 000500000000000000000000000 H = 10350 J HE=........coMn J Is there any difference in the amount of heat energy thus obtained? Note down in the science diary how this problem can be solved using the relation, H = VIt. Let’s calculate the heat developed when 3 A current flows through an electric iron box designed to work under 230 V. Which equation will help us to solve the problem easily? Solve the problem. Details of two electric heaters are given below. How much will be the heat developed if they are made to work for 5 minute each? Heater - A Heater - B Working voltage : 230V Working voltage : 230V Resistance : 1150Q Resistance : 460Q Working time : 5minute | Working time : 5 minute Vt V*t H = — H = — R R 2307x300 _ 230° x300 1150 460 = 13800J = 34500J Table 1.3 e Why does the heater having low resistance get heated more? e In which way does the change in resistance influence the heat developed? e Find out the current in the heaters A and B and compare the heat developed. e How do the resistors bring about a change in the current in the circuit? Let’s take a look at how the voltage and current changes when resistors are arranged in different ways in circuits. Arrangement of Resistors in Circuits Two different circuits which can be constructed using a 6 V- 2 A battery, 3 W-6 V bulbs and a switch are given (Fig 1.4). Construct these circuits and operate it. Write down the answers based on your observations. 3W ©) NS 3. (©)2: a [Vv  < “To Fig. 1.4 (b) © [In which circuit does the bulb glow with high intensity? e Remove one bulb from each circuit. What do you observe? In fig1.4 (a) vue eeeeeeeeeeeeeens In fig 1.4 (b) Cc...eee e Why do the bulbs in Fig1.4 (a) glow with maximum brightness? Draw a suitable circuit with ammeter and voltmeter in the circuit replacing the bulbs by 1 Q, 2.2 © resistors. Compare the circuit you have drawn with the one given below and construct it properly. Record the readings in the table. ee Aw SYHTUH  Voltage obtained Current in Effective Resistance in resistance (V) resistances (I) (by analysing the current) Mode of ' 2.2Q | 1Q |Effectivel! 2.2Q | 1Q A Increases/ decreases connection 0 V, Vv, | voltage | 1, 1, I resistances V 2.2Q 10 NNN ——W- 2,20 me 1Q Table 1.4 Analyse the table and tick (“) the best suited. Mode of Effective Voltage obtained Current connection of Resistance in each through each resistances resistance resistance 2.2 Q 1Q increases/ same / same / AWA —VA- :. decreases different different RAK increases/ same / same / decreases different different 1Q Table 1.5 Series Connection When a circuit is completed by connecting the resistors one after the other, it is called series connection. When resistors are connected like this, the effective resistance increases. I R, R, NN NAN Fig. 1.6 When resistors are connected in series, the potential difference gets divided. V=V, +V, The current through each resistor will be the same. Hence V,= IR,,V, =IR, In a series circuit the voltage across the higher resistane will be greater. According to Ohm's Law, V = IR. Here R indicates the effective resistance. Hence ether t th tte teh th hl  Now our journey is impossible, The value of resistance of carbon resistors available in the market has their resistance directly marked on them or indicated by colour codes. Usually rings of four different colours are used. The first two rings indicate the two digits, the third one indicates the number of zeroes and the fourth ring indicates tolerance Oh, We have to cross © (deviation). Silver + 10%, gold three bridges to reach the & + 5%, and if there is no fourth other side. Very difficult. Just as the resistors in line, then + 20% deviation will series. be there. [ For example, if the first two rings IR IR = = IR +IR, I(R+R) are red and violet, then the first two R = R#+R digits are 2 and 7. The third one Effective resistance 1s the sum of the resistance of all the denotes the number of zeroes. If resistors when they are connected in series. If the resistors this is orange, then there are three are of the same value, then the effective resistance can be zeroes. Now the value is 27000 Q. obtained by multiplying the resistance of a resistor with If the silver ring is also considered the number of resistors. then the value = 27 kQ+ 10%. Parallel Connection R Colour | Number No. of bat “ ANY AAA “7 B Zeros I Black S So Brown Fig 1.7 R, | — Red of So Orange The current completes the circuit by getting divided into w] w] Yellow each branch since the resistors are connected in parallel. BY] BY] Green Ca] Cf The total current in the circuit is the sum of the current Blue DY] DY Violet through all the branch circuits. CO] N] NY Grey I =1, +I, oo] White \©| ©] Since R is the effective resistance, By y Ohm's Ohm’s L Law viv, RT R TR, ee a th a ce dh ht =  1 1 1 —=— +— R Ri R, RR, R= R +R, r If resistors of the same value are connected in parallel, then R = a where n is the number of resistors and r is the resistance of one resistor. Complete Table 1.6 by analysing Tables 1.4 and 1.5. Resistors in series Resistors in parallel « Effective resistance increases. = « Thecurrent through each resistor is different. It gets divided as per the value of resistors. The potential difference across each resistor is different. It gets divided as per the value of resistors. « Each resistor can be controlled by using separate switches. Table 1.6 _ e What is the current if 4 Q and 2 Q resistors are connected in series and 6 V 5 potential difference is applied ? > V = 6V WW © R = R, - R, a = 4+2=6Q :> _oT Q — 6 AMN I 2 6 [LY Current I = ==I1A > 6 = tte tt th het nt te teeth rn What is the current if 12 Q and 4 Q resistors are connected in parallel and 12 V potential difference is applied? R,= 12Q,R,= 4Q,V=12V V = 12V 1 —=— 1 + — 1 R _ RR, —_—_— R R R, R,+R, 1 iil _4+12 _16 12x4 R 12 4 12x4 48 2 12+4 r= -39 _ B_ 30 16. 16. current I YL, =R = 3 = C urrent I=LEA 5 = 3 10 resistors of 2 Q each are connected in parallel. Calculate the effective resistance. When the voltage is constant, current will decrease with an increase in resistance. Won’t this help you in explaining why the heat decreases even after increasing the resistance? We can bring about change in current and voltage by connecting resistors in different ways. Now you would have understood why it is stated in Joule’s Law that heat is directly proportional to resistance only when the current and time are constant? Heating Effect of Electricity- Uses ae Electric heating appliances are instruments that make use of the heating effect of electricity. Electrical energy is converted into heat energy in them. Fig 1.8 A few heating appliances are shown in the figure. Examine any one of them and answer the following questions. Record the answers in the science diary. e Name the part in which electrical energy changes into heat energy. e Which material is used to make this part? e What are the peculiarities of such substances? e high resistivity ability to remain in red hot condition for a long time without getting oxidised  Heating coils are made of nichrome. Nichrome is an alloy of nickel, chromium and iron Let’s see what advantages of nichrome are made use of in electric heating appliances. © High resistivity Microwave = | os D © High melting point _— © Ability to remain in red hot condition for a long time without getting oxidised. Microwave oven and induction cooker are appliances which do Safety fuse not use a heating coil. In a Safety fuse is a device that works on the heating effect microwave oven, microwaves of electric current. Let's see how it works. are used and induction cooker uses eddy currents. Fuse wire, an alloy of tin and lead, is the main part of safety fuse. Alloys are used to make fuse wire. Fuse wire has a relatively low melting point. In each circuit = the fuse wire should be used in accordance with the = | i Ps. va current flowing through it. e Which are the circumstances that cause high » electric current, leading to the melting of fuse wire? © How is the fuse wire connected to a circuit? In Fig 1.9 series/ parallel? Short Circuit and You know that according to Joule’s Law, more heat Overloading will be produced when electric current is increased. If the positive and the negative What happens to the fuse wire due to this? terminals of a battery or the two | wires from the mains come into | contact without the presence of.. melt? aresistance in between, they are Said to be short-circuited. rrr rrr rcne A circuit is said to be overloaded e When the fuse wire melts, the circuit is broken. if the total power of all the What happens to the current in the circuit? appliances connected to it is more than what the circuit can Why is the fuse used in a circuit called safety fuse? withstand. Explain. © a ee During the entire time of the passing of current through a circuit, a small amount of heat is generated in the fuse wire. But this heat will be transmitted to the surroundings. When the current that flows into the circuit exceeds the permissible limit, the heat generated becomes excessive. Since more heat is generated in unit time than the heat transmitted, the fuse wire melts. Safety fuse is a device which protects us and the appliances from danger when an excess current flows through the circuit. Is the current passing through different circuits the If same? Intensity of electric current differs from one appliance to another. Hence fuse wires of appropriate amperage should be selected. Gauge is the reciprocal of the diameter of a conductor. As the When a fuse wire is included in a household wiring, what gauge increases, the thickness of the are the precautions to be taken? Let’s see. conductor decreases and hence the © The ends of the fuse wire must be connected firmly amperage also decreases. at appropriate points. e The fuse wire should not project out of the carrier base. Amperage e Amperage (A) is the ratio of the Electric power power of an equipment to the voltage applied. Amperage You might have noticed the marking of 500 W on an | increases with the thickness of the electrical appliance. What does it indicate? An electrical | conductor. appliance works by making use of electrical energy. Hence it has a power. You have studied in the earlier class that power is the work done per unit time. The amount of energy consumed by an electrical appliance in unit time is its power. | WwW Power is calculated using P = (] e What is the unit of power? e According to Joule's Law, the heat generated (H) in an electrical circuit in an interval of time t second or the work done is H = FRt Then, how is the power calculated?  Work done H = FRt Time = t Work H Power, P =—: = — time t I'Rt Power P= n P = FR V By Ohm’s Law, I = ry P = PR vy V? ~ (2) "R 2 Thus, P= — V If R= T what will be P? P = PFRE=IX.... =. The unit of electric power is watt (W). e An appliance of power 540 W is used in a branch circuit. If the voltage is 230 V, what is its amperage? Wattage W Amperage = Voltage v pan -L -2344 24A v 20 a © A heating appliance has a resistance of 115 Q. If 2 A current flows through it, what is the power of the appliance? R = 115Q I = 2 A Power P = FR = 22x115=460 W e A current of 0.4 A flows through an electric bulb working at 230 V. What is the power of the bulb? Lighting effect of electric current Filament lamps were in wide use in the early days. Observe the parts of a filament lamp shown in Fig 1.10. Glass cover [=] = JZAKKS _ Fig 1.10 Incandescent lamps In normal voltages, the filament becomes white hot and gives out light. Such bulbs are the incandescent (glowing with heat) lamps. Filaments made of the metal tungsten are used in them. Tungsten can become white hot and emit white light for a long time. In order to avoid oxidation of tungsten, the bulb is evacuated. Vaporisation can be reduced by filling some inert gas at low pressure inside the bulb. Nitrogen is usually used for this purpose now. JEEY © Why is the bulb filled with an inert gas/ nitrogen? — OPPPPPPPPP PPP PPP PPP PUP DDDDDODODD At normal temperature and © What properties of tungsten make it suitable for | pressure, nitrogen behaves like an being used as a filament? inert gas. Small increase in temperature does not influence © high resistivity the expansion of nitrogen. The ready availability of nitrogen is one e high melting point of the reasons for it being used in e high ductility bulbs.In the absence of air within e ability to emit white light in the white hot | the bulb, this gas behaves condition completely as an inert gas. © Dhth th td  © Nichrome is not used as filament in incandescent lamps. Why? e Touch a filament lamp after it has been lit for a short period of time. What do you feel? A major part of the electrical energy supplied to an incandescent lamp is lost as heat. Hence the efficiency of these devices is less. Haven't you understood that a major part of the electricity supplied to an incandescent lamp for obtaining light is lost as heat? By now it may be clear to you why the use of incandescent lamps is to be restricted. What are the other types of lamps working on electricity? List them. e Discharge lamp e Fluorescent lamp Discharge lamps =ma 97AGZF Sodium vapour lamp Arc lamp Fluorescent lamp Fig 1.11 Discharge lamps are glass tubes fitted with two electrodes. They emit light as a result of discharge of electricity through the gases filled in tubes. When a high potential difference is applied the gas molecules get excited. Excited atoms come back to their original states for attaining stability. During this process the energy stored in them will be radiated as light. Depending on the difference in the energy levels lights of different colours and other radiations are emitted. e What are the advantages of using discharge lamps instead of incandescent lamps? e What are the factors to be considered when you select a bulb? Which are the lamps that are mostly used? Why? LED bulbs are devices that give more light than discharge lamps and incandescent lamps. They work using low power. What are their peculiarities? LED bulb (Light emitting diode bulb) e LEDs are Light Emitting Diodes. © As there is no filament, there is no loss of energy in the form of heat. _—" © Since there is no mercury in it, it is not harmful to environment BP oes ee 92 JI0235 LED Bulb Fig 1.12 2 LED Bulbs aw F=| (Construction, repair, reuse and disposal) It is the research for high energy efficient and less polluting bulbs that led to the invention of LED bulbs. LED bulbs are more efficient than other bulbs. Low power consumption, high efficiency and high longevity are the advantages. Production of LED bulbs at low cost must be encouraged to enhance their use. At the same time we should know how to do minor repairing of the bulb to facilitate  its reuse and also the scientific methods of its disposal to minimise the related environment problems. Let's do some activities to understand about LED bulbs. Let's familiarise with the parts of the bulb. Base unit E22 This is the metallic Power Supply Function of this is to part that connects the board (LED convert AC into DC aes bulb to the holder driver) and supply necessary SJ output voltage ( The TL same board can be S =D —— used for 5W, 7W and BASE UNIT OE. 9 W bulbs.) ia} Heat sink The part close to the Printed Circuit LEDs are fixed on = base unit of the bulb. Board (LED Chip ||this board. In this the / It is an arrangement positive and negative for absorbing heat polarities are marked. from the base. Base plate Metal plate that fixes it to the holder. Back conductor Screws for fixing Diffuser cup This is the part from wires from LED which light comes out Screws. drive to the base unit. of the bulb. 244 sé DIFFUSER Figure Showing a Completed LED Bulb Circuit  Other accessory tools required to construct an LED bulb Stn 2 Insulation tape Pliers Heat sink compound Wire stripper — Soldering iron Solder Lead Soldering wax Construction e Fix the base unit by punching it on to the heat sink e Cover the power supply board using insulation tapes in such a way that the input and output supply wires will be visible out but will not get covered with dust or moisture. © Fix the wires seen on the input part of the power supply board to the back conductor, by passing it through heat sink and terminal holes. © Fix the red wire in the output into the part marked positive in the board and the black wire into the negative marking. © Apply heat sink compound at the back of the LED Printed Circuit Board and then fix it on the base plate. © Press and close the heat sink by using the diffuser cup. Make sure that the LED bulb thus made emits light when fixed to the bulb holder. Repairing the damages in the LED bulbs © An LED bulb is a combination of many LEDs connected in series. The bulb will fail to emit light if any one LED is damaged or if the contact of any one LED is lost. e LED bulb will not emit light if the rectifier or load resistor or filter capacitor is damaged. © Even a minor damage in an LED bulb will lead to complete failure in the working of the bulb. How can these errors be rectified? Te a t th tte teh a  The Main Components of an LED bulb Examine a damaged LED bulb and find out the following parts (Rectifier, Load resistor, Filter capacitor, LED Chip, Heat Sink) Let’s familiarize with the tools required to repair LED bulbs. a) Soldering iron b) Multimeter c) Tester d) Screw driver e) LEDChip f) Nose pliers g) Tweezer e Open anon functioning bulb and examine each part using a multimeter to know whether the parts are functioning or not. Find out which of the following parts are damaged in an LED bulb and replace them with new ones. e Rectifier e Load resistor e Filter capacitor LED Chip How can the LED bulbs be disposed of scientifically? © Segregate the plastic, electronic and metal components of LED bulb and transfer them to their respective disposal units. e Don’t you have to enhance the use of LED bulbs which are environment friendly and suitable for energy conservation? Energy saved is equivalent to energy produced i a a a el a ae et tee] fe Let us assess 1. Fuse wire is to be used by understanding the amperage correctly. Write down the amperage of the fuse wires that are currently available in the market. 2. 0.5 Acurrent flows though an electric heating device connected to 230 V supply. (a) the quantity of charge that flows through the circuit in 5 minute is (i) 5C (ii) 15 C (iii) 150 C (iv) 1500 C (b) How much is the resistance of the circuit? (c) Calculate the quantity of heat generated when current flows in the circuit for 5 minute. (d) How much is the power of the heating device connected to the circuit if we ignore the resistance of the circuit wire? 3. According to Joule’s Law the heat generated due to the flow of current is H = FRt. Will the heat developed increase on increasing the resistance without changing the voltage? Explain. 4. The table shows details of an electric heating device designed to work in 230 V. Complete the table by calculating the change in the heat and power on changing the voltage and resistance of the device. Analyse the table and answer the following questions. voltage the device (R) | in the device per second by the device the change I=V/R Heat, H=VxIxt | P=VxIor P=H/t} in power 230 V 57.5 Q 4A 920 J 920 W 230 V 115 Q 230 V 230 Q 115 V 57.5 Q 460 V 57.5 Q (a) How does the voltage under which a device works affect its functioning? © ninth ttt ttt ——|  (b) What change happens to power on increasing the resistance without changing the voltage? (c) What change is to be brought about in the construction of household heating devices inorder to increase their power? 5. (a) Complete the table based on the amperage of the fuse wire. Electrical device Operating Power of Current The amperage voltage (V) the device (P) | through the circuit | of the fuse to be I=P/V used in the circuit (A) Water heater 230V 4370 W 19A 20A Air conditioner(AC) 230V Inn n nn nnn 14.5A enema n nnn nanan Television (LED - TV) 230V 57.5 W ——— ET Computer (Laptop) 230V wawnnnnnn nnn mmnnn= 0.125A wanna nana EC b) The amperage of the fuse wire used in a circuit that works on 230 V is 2.2 A. If so the power of the device is (1) less than 300 W (ii) 300 W to 500 W (111) between 500 W and 510 W (iv) more than 510 W 6. A230 V, 115 W filament lamp works in a circuit for 10 minute. (a) What is the current flowing through the bulb? (b) How much is the quantity of charge that flows through the bulb in 10 minute? 7. An electric heater conducts 4 A current when 60 V is applied across its terminals. What will be the current if the potential difference is 120 V? 8. Three resistors of 2 Q, 3 Q and 6 Q are given in the class. (a) What is the highest resistance that you can get using all of them? (b) What is the least resistance that you can get using all of them? (c) Can you make a resistance 4.5 Q using these three? Draw the circuit. 9. A girl has many resistors of 2 Q each. She needs a circuit of 9 Q resistance. For this draw a circuit with the minimum number of resistors. 10. If a bulb is lit after rejoining the parts of a broken filament, what change will occur in the intensity of the light from the lamp? What will be the change in the power of the bulb? 11. Which of the following does not indicate the power of a circuit? (a) PR (b) VI (c) IR? (d) V?/R 12. How much will be the power ofa 220 V, 100 W electric bulb working at 110 V? (a) 100 W (b) 75 W (c) 50 W (d) 25 W 13. Which of the following should be connected in parallel to a device in a circuit? (a) voltmeter (b) ammeter (c) galvanometer 14. When a 12 V battery is connected to resistor, 2.5 mA current flows through the circuit. If so what is the resistance of the resistor? 15. If 0.2 Q,0.3 Q, 0.4 Q, 0.5 Q and 12 Q resistors are connected to a 9 V battery in parallel, what will be the current through the 12 Q resistor? 16. How many resistors of 176 Q should be connected in parallel to get 5A current from 220 V supply? a) 2 b) 3 c) 6 d) 4 17. Depict a figure showing the arrangement of three resistors in a circuit to get an effective resistance of (1) 9 Q (ii) 4 Q Extended activities 1. Analyse and describe the working of a microwave oven. 2. How does an arc lamp help in rescue operations? 3. With the help of teachers and the Internet find out the following  (a) What is the percentage of nickel, chromium and iron in Nichrome? (b) How much is the melting point of nichrome in degree celsius? (c) How much is the resistivity of Nichrome? (d) Does the result of your observation justify the use of nichrome as a heating element ? 4. Analyse the merits and demerits of the following lamps and find out which is best in the group. Justify your answers. (a) filament lamp (b) fluorescent lamp (c) arc lamp (d) CFL (e) LED bulb