Activities ee OO ——— mi activity 1 AM: Toidentify adiode, an LED, aresistor and a capacitor fromamixed collection of such items. APPARATUS AND MATERIAL Apparatus. Multimeter. Material. Above mixed collection of items. THEORY For identification, appearance and working of each item will have to be considered, 1. Adiode is a two-terminal device. It conducts when forward biased and does not conduct when reverse biased. It does not emit light while conducting, Hence, it does not glow. 2. ALED (light emitting diode) is also a two-terminal device. Italso conducts when forward biased and does not conduct when reverse biased. It emits light while conducting, Hence it glows. 3. A resistor is a two-terminal device. It conducts when either forward biased or reverse biased. (Infact there is no forward or reverse bias for a resistor). It conducts even when operated with A.C. voltage. 4. Acapacitor is also a two-terminal device. It does not conduct when either forward biased or reverse biased. When a capacitor is connected to a D.C. source, then multimeter shows full scale current initially but it decays to zero quickly. Itis because that initially a capacitor draw a charge. ‘The components to be identified are shown in figure. 123 4 1000 nF 6V 0.47 pF ag _ Paper Rings Electrolytic () Carbon resistor (b) Capacitors () Diodes Fig. Some of the commonly available circuit elements. 181oa Comprehensive Practical Physies yy PROCEDURE 1. If the item has two terminals, it may be diode, a LED, a resistor or a capacitor. In order to differentiate proceed as ahead: Connect the battery eliminator, reversing key, the items to be identified and the multimeter, Switch on the circuit and carefully observe the movement of the pointer of the multimeter, (i) _Ifpointer moves when voltage is applied in one way and does not move when reversed ang there is no light emission, the item is a diode. (ii) Ifpointer moves when voltage is applied in one way and does not move when reversed ang there is light emission, the item is a LED. (ji) If pointer moves when voltage is applied in one way and also when reversed, the item is a resistor. (iv) If pointer does not move when voltage is applied in one way and also when reversed, the item is a capacitor. OBSERVATIONS No. of | Number oflegs Name of device No.of | Possiblecurrent | Name of device Obs. Obs. flow 1. |Two Capacitor, Diode, 2. | Unidirectional, Diode LED or resistor emits no light 3. | Unidirectional, LED emits light 4. | Both directions Resistor (steady) 5. |Initiallyhigh but —_| Capacitor decays to zero wan ACTIVITY 2 oa AIM: To use a multimeter to : (a) see the unidirectional flow of current in case of a diode and an LED and (b) check whether agiven electronic component (e.g., diode) is in working order. APPARATUS A multimeter, a diode and an LED.actites 183 THEORY (a) Adiode or an LED conducts on! ly when forward biased and in reverse biasing, there is no flow of current. (b) When a diode is in working order, it will allow the current to flow in one direction, when forward biased. oa e . PROCEDURE \ 1. Use the selector switch and put the mult & 2 Insert the metallic end of the back lead in common terminal of the multimeter and that of the red in the terminal matked P. meter in ohm range (R). y (@) Unidirectional flow of current Case of junction diode 1, Connect the +ve marked end of the ‘diode to *ve terminal of a 6 V variable D.C. battery pose R adjusted to minimum voltage, multimeter | 2. Select the D.C. current at 10 mA range by A ag using the selector switch. Insert one of the Diode —y: x metallic ends of probe in terminal P(+) and : | bl : ; is the black metallic end into the terminal . 2 marked common. current in a diode. 3. Connect the other metallize end of red * Probe to the free end of the diode and the metallic end of black probe into the -ve terminal of the battery eliminator and read the value of current by varying the output of the eliminator. § Since the diode is forward biased, it allows the current to pass. + Nowreverse the terminals ofthe diode such that the end marked -veis at higher potential and the one marked + is at lower potential. Again observe the current in multimeter on the appropriate D.C. milliampere scale. No current reading in the multimeter would indicate that the diode allows the flow of current in one direction only, ina diode. ie, the current is unidirectional 5. Nowreplace the diode by the LED and repeat the steps to establish that an LED also allows yg the flow of current only when it is forward biased as well as emits light, #*) Checking whether diode, transistor in IC is in working order A diode will conduct only in one direction ie, first connect the ends of diode to the two metal ends of the probes and reverse the connecting points. Ifit conducts in one case, working order. If conducts in both cases or does not conduct in both cases, then diodes in then it is damaged.SS 484 Comprehensive Practical Physics ‘M _ at aeriviry 3 AIM: To study effect of intensity of light (by varying distance of the source) on an LDR, APPARATUS Light source, light dependent resistors (L.D.R.s) of different variety, a multimeter (oy Metey bridge), a source of intense light (a lamp bulb with battery eliminator) and a convex lens. Light Dependent Resistor ‘The light dependent resistance are the devices for detecting and measuring electromagnet, ‘waves (light etc). Its working is based upon the principle of variation of the photoconductvity wie, radiation is incident upon it and absorbed by it. Alight dependent resistor is prepared from cadmium sulphide. Its resistance depends upon ty intensity and duration of light incident on it. A good quality L.D.R. shows a resistance variation from 1 MQ in complete darkness to about 10.9 in full day light. The intensity of light decreases inversely with increase the square of distance, PROCEDURE 1. Turn the selector switch and set it on R for the measurement of resistance in multimeter, 2. Plug the metallic ends of black probe in terminal marked common in multimeter and that of red in terminal marked as P (or +). Short the other metallic ends and adjust the ‘R adjusting’to get full scale deflection reading at zero ohm in the meter. 3. Touch the metallic probes to the two metal ends of the L.D.R. (Fig. (a)] and read the value of resistance when (a) the source is kept at a distance of 2 cm, fixing the source of light in a stand and keeping the L.D.R. vertically below it. (Moving the source to 4 cm distance from the L.D.R. (ii) Moving the source to 6, 8 and 10 cm from L.D.R. and repeating observation three more times. OBSERVATION RECORD fees source) t Electric bulb (6 V, 0.05 amp) LDR. To probes of sFnultimeter set at R To multimeter set at ohm (@ Symbol of L.D.R . (0) L.D.R. in series with a 6 V battery and exposed to light from bulb incident normally185 ities ad , | Table for Light Exposure Time and Resistance i [> Disteice ofacmda ~~ Resistance of LDR. WICTEASE in Re { from L.D.R. (cm) R(ohm) sistance (2) (3) R (ohm) | @ r 1 . | 2 4 3 6 4 8 5 Oia rime Ete eee eae eer eee eee CONCLUSION ‘When the distance between light source and L.DR. increases, the resistance of L.D.P increases. Note. Same activity can be done by varying the exposure time in steps for same source of light, same LDR and for same distance, PRECAUTIONS 1. No stray light should fall on the L.D.R. It is better to work in a dark room. 2. Connect L.D.R. carefully to the voltage source.