Unit-02 Basic Electricity I

Transcription

1 Unit-0 Basic Electricity Objective: n this experient, we would like to get you ailiar with the theore, structure and use o ultieter and learn the easureent o Alternating Current (AC) / Direct Current (DC) signals. Apparatus: Digital ultieter, power supply, unction generator, breadboard, resistance Principle: n physics experients, we usually use precision instruents to easure the physics quantities, with the ost coon being in voltage and current easureent. he ultieter is a ultipurpose tool or easuring the current, voltage, and resistance. With Oh s law, we can know the resistance o a resistor by easuring the current under speciic voltage. he design is based on D'arsonval galvanoeter. he ollowings are principles o kinds o easureents. A. D'arsonval galvanoeter he top view o D'arsonval galvanoeter is shown in Fig., when current passes through lexible coil, it generates agnetic ield. his generated agnetic ield, which interacts with the peranent agnet, would cause a clockwise torque current. t can be expressed as the ollowing equation: L, in proportion to L K K is the proportion constant his torque would cause the rotation o the coil. he twist o upper and lower springs causes the anticlockwise torque. t can be expressed as the ollowing equation: L L K K is the proportion constan When the two torques L and L hit a balance, the coil would stay at a speciic angle L or K L K National Chiao ung University Physics Experiental Handout Basic Electricity -

2 Figure. Principle o D'arsonval galvanoeter Assue K K K So, we can get K hereore, we can ind the current value by easuring the angle o the indicator. As is shown in Figure, there are two types o power supply: direct current (DC) and alternating current (AC). he current and voltage o direct current always stay constant. hey would not vary with tie. he current and voltage o alternating current would vary cyclically with tie. hus, the circuit would act dierently when you supply these two kinds o power source. Figure. AC/DC signals in relation to tie National Chiao ung University Physics Experiental Handout Basic Electricity -

3 o explicate the dierences, take electric power as an exaple: (a) Direct Current (DC) a resistor R is supplied by direct current, the average power P would be (b) Alternating Current (AC) P a resistor R is supplied by alternating current sin t / would be P R R R t R sin dt 4t cos dt 0 0 4t sin t 8 = 0 R R R rs, the average power P is the axiu current and is the period n igure 3, we can see that the average power o alternating current has an additional actor /. n order to uniy the orula o alternating current and direct current, we deine an eective current rs (root ean square o current) to describe the total eect o alternating current. Siilarly, we can deine an eective voltage (root ean square o voltage). Figure 3. Root ean square o voltage National Chiao ung University Physics Experiental Handout Basic Electricity -3

4 are When easuring the alternating voltage and alternating current, the values on ultieter. V rs and rs Root ean square o voltage V rs V 0. ax 707 V ax Root ean square o current rs ax ax B. Aeter current easureent As shown in Figure 4, when a galvanoeter is used as an aeter, a shunt resistor R is connected in parallel with the galvanoeter. Assue the internal resistance o galvanoeter is Rg, the axiu direct current which passes through the aeter is, and the axiu direct current which passes through the galvanoeter is. According to Oh s Law, the value o the shunt resistance S is deterined by R g S S Rg hus, the easureent gear o the DC aeter varies in accordance with shunt resistance S. Figure 4. DC Aeter For AC aeter, it is ored with a galvanoeter, a sall resistor, and a rectiier. (A diode has the characteristic o low resistance under orward bias and high resistance under reverse bias.) Since D'arsonval galvanopeter can only work with direct current, a rectiier is required to transor alternative current into direct current. nsert a rectiier into the let side o node a in igure 5 and it can be an AC aeter. But please reeber that the dial scale o an AC aeter is the peak current ties 0.707( ). National Chiao ung University Physics Experiental Handout Basic Electricity -4

5 Figure 5. AC Aeter C. Volteter voltage easureent As shown in Figure 6, when a galvanoeter is used as a volteter, a resistor R is connected in series with the galvanoeter. Assue the highest voltage easured by galvanoeter is V, the current at this tie is, the internal resistance o the galvanoeter is, the resistor R we need to connect in series V V N Rg and the highest voltage to easure is can be deterined by V R g V NV ( R R ) g then NR ( R R ) g g R ( N ) R g For AC volteter, it would be the sae as DC volteter except that it requires one ore rectiier and the dial scale is deined by ultiplying Figure 6. DC Volteter National Chiao ung University Physics Experiental Handout Basic Electricity -5

6 D. Resistance easureent With Oh s law, we can know the resistance by easuring the current under speciic voltage. he aount o resistance can be deterined as the ollowing R V (Oh s Law) Rearks:. he rotary switch should be placed in the right position and no any changeover o o range shall be ade during easureent is conducted to prevent daage o the Meter.. When easuring voltage, the ultieter should be in parallel with the circuit. 3. When easuring current, the ultieter should be in series with the circuit. 4. Never easure the voltage by current gear. you do, it will cause ire and people ight get hurt. 5. o avoid daage to the equipent, the output voltage o the power supply ust not exceed 0.0 V. Procedure: Preparation. Beore easureent, read user guide o power supply, unction generator and breadboard.. Scale switch:switch to the proper gear when easuring voltage or current. You should start with the axiu gear down to the iniu. 3. urn the ultieter o when it is not in use to prevent the battery ro dying. A. DC voltage, current and resistance easureent. Choose any two resistances and connect in series on the breadboard. Record R and R resistance values.. Set the ultieter to resistance gear, ake sure that the sensor is properly connected, use the senor to touch the two ends o the resistance, and record the values o R and R based on color code table. 3. Set up the circuit in series and parallel on Breadboard, as indicated in Figure Supply direct voltage power (< 0.0V) to series circuit and parallel circus respectively. 5. urn on the ultieter and press blue button, and then switch to DC easureent. 6. Set the ultieter to voltage gear and record the total voltage and the shunt voltage with the Meter connected in parallel. 7. Set the ultieter to current gear (ro high to low) and record the total current and the shunt current with the ultieter connected in series. National Chiao ung University Physics Experiental Handout Basic Electricity -6

7 8. Calculate the total voltage, shunt voltage, total current, and shunt current by Oh s Law. Figure 7. Series and parallel circuit [Note] Color Code Black Brown Red Orange Yellow Green Blue Violet Grey White Gold Silver ±5% ±0% B. AC voltage easureent. urn on the Meter and press Function (AC/DC) (blue button), and then switch to AC easureent.. Connect two leads separately to anode and cathode o Meter, and then connect the other side o both leads to the anode and COM o unction generator. 3. Fix output requency 60 Hz sine wave, adjust the aplitude (AMPL switch), and observe the variation o AC voltage. 4. Fix the output aplitude, adjust output requency (60 Hz, 0 Hz, khz, 0 khz, 00 khz, MHz) and easure voltage. Questions:. Are the easured values and calculated values the sae in DC voltage easureent? What is the reason or the dierence? Please explain.. Are the easured AC voltage values the sae when the output aplitude is ixed with varying output requency? What is the reason or the dierence? Please explain. National Chiao ung University Physics Experiental Handout Basic Electricity -7