College Physics II Lab
College Physics II Lab PHYS 246
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This 7 page Class Notes was uploaded by Sonny Breitenberg on Monday September 28, 2015. The Class Notes belongs to PHYS 246 at George Mason University taught by Staff in Fall. Since its upload, it has received 44 views. For similar materials see /class/215197/phys-246-george-mason-university in Physics 2 at George Mason University.
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Date Created: 09/28/15
3 DC Circuits Ohm39s Law and Multimeters Theory Many are intimidated by electricity and feel it is beyond their understanding Today s lab will look at some basics of electricity in a controlled fashion so as to dispel this notion It will also show how to translate a simple circuit diagram into a functional electric circuit Three basic terms are important to a study of electricity They are basic to understanding Ohm39s Law which is Voltage Current constant This constant is defined as the Resistance of a material Current is defined as the ow of electrical charge from one place to another It is the amount of charge which passes a given point in a unit of time and is measured in units of amperes amps coulsec Voltage is a measure of potential potential di erence between 2 points in a conductor It is a measure of the amount of energy it takes to move charge from the one point to the other It is measured in units called volts Resistance is a measure of the amount of impedance a material presents to the ow of electricity Georg Ohm showed it is the ratio of the voltage across a material to the current that can then ow through it It depends upon the properties of the conducting material its nature and dimensions but for metallic conductors is usually a constant at constant temperature It is measured in units of ohms where one ohm is defined as 1 volt l ampere We will demonstrate Ohm39s Law for several combinations of resistors verifying the general rule R NIV eq where Req ZRi for resistors combined in series and lReq ZURi for resistors com bined in parallel Apparatus Three new items that this lab uses are resistors multimeters and power supplies Resistors are used in large numbers in electronic equipment to control the ow of electrical current Physically they are manu factured in a number of styles which feature either high power capacity high precision or low cost This lab will use the most common low cost carbon composite style as shown in Figure 31 The physical size determines the power rating P IV 12R V2R The 14 watt W size is about 7mm long by 2 to 3mm in diameter while the lZW 1W and 2W sizes are progressively larger The 14W resistors are designed to handle up to 14W of power before overheating The low voltages in this experiment make it safe to feel the resistor in use to check if it is overheating If it becomes hot rather than just warm reduce l9 the voltage or choose a larger resistance value resistor A quick calculation will also serve to check the power before the circuit is energized If a resistor should ever become smoking hot discard it since it will probably be damaged x quotWe 39ead color bank Figure 31 Colored bands determine the resistance of the resistor The resistance value of a resistor is determined during manufacture by the composition of the car bon mix it is made from The resistance is marked on the resistor usually by colored bands as shown in Figure 31 To read these begin with the band closest to the nonmetallic colored end and decode it and the second band using the following table These are the rst and second digits of the resistance Table 31 Color Code Resistor Bands 0 black 5 green 1 brown 6 blue 2 red 7 violet 3 orange 8 gray 4 yellow 9 white The third band when similarly decoded is the power of ten of the multiplier For example a resistor marked brown black red should have a resistance of 10 X 102 which equals 1000W or 1 kW Note that there is no decimal point between the 1 and 0 The fourth band marks the precision brown or no mark is i 20 sil ver is i 10 gold is i 5 and indicates how much the actual resistance can differ from that indicated by the colored bands A fth band sometimes indicates reliability Commercial resistors are designed to have an almost constant resistance over the range of currents and voltages within the power rating Thus a current versus voltage plot will be a straight line within parts per million Other devices are not so linear such as light bulbs diodes or LED s which have resistances which vary with current and volt age 20 Figure 32 Multimeter The Multimeter In this laboratory you will make use of a multimeter Wavetek DM28XT to measure voltage current frequency capacitance and resistance We have already used the DC voltmeter function in the electric eld experiment Figure 32 on page 20 shows the meter Each button switch and connec tor has a somewhat cryptic label that provides the expert user with important clues as to how to use the meter This is a fairly typical multimeter for electrical measurements It is important when using such a meter to understand what you are measuring and how to use the meter This is important for your safety the protection of the meter and the accuracy of your measurements The rotary switch in the center determines what is measured and also sets the range of values to be measured For example if you wish to measure electrical current that you expect to be about 15 mA you would set the rotary switch to the quot2mquot position within the A region This tells the meter that you expect a current that is less than 2 mA The result of the measurement is shown in the display and mA in the display indicates that the quantity shown is mA and not A If the actual current is greater than 2 mA then the meter will display OL indicating an overload You will then have to raise the range to 20 mA in order to get a reading When the switch is set to 20 mA you will notice that you lose one significant digit of accuracy in the reading There are four different input plugs which receive cables often called probes that are con nected to the circuit All measurements make use of the COM common input For voltage fre quency and resistance measurements the leftmost plug labeled VQ is used along with COM For low currents and capacitance the plug labeled mA CX is used along with COM For currents up to 10 A the 10 A plug is used along with COM Just below the display are four buttons The function of the ONOFF and DC AC are very important but selfexplanatory The hold button will quotholdquot the display at the current value until it is pressed a second time The MAX button causes the display to show the maximum value of the quantity being measured until it is pressed again It should be noted that AC readings are root meansquare RMS values The accuracy of the multimeter is specified for each type of measurement in the sections that follow This accuracy is expressed as a percentage plus a factor related to the number of digits in the display For example DC voltages have an uncertainty of 05 plus one least significant digit In the 2 V range the least significant digit is i 0001 V So if you were measuring a voltage of 10 V the 05 would give an uncertainty of 0005 V and you would add the least significant digit to give an overall uncertainty of i 0006 V Voltage Measurements To make a voltage measurement select AC or DC turn the rotary switch to the V area and select the appropriate range If you are unsure of the range set the rotary switch to the highest range and then lower it later Insert the probes into the V 2 and COM plugs and then connect them to the appropriate points in the circuit If you are making a DC measurement then the V 2 probe should be connected to the most posi tive voltage and the COM probe should be connected to the most negative Ifthe probes are not 21 connected correctly then the display will indicate a negative voltage but there is no need to switch the connections If you are making an AC voltage measurement the COM probe should be connected to the point closest to ground Electrical Speci cations DC VOLTS Ranges 200mV 2V 20V 200V 1000V Accuracy i 05 1 lsd Input Impedance 10 M9 Overload Protection 200mV Range 7 500350VRMS 15sec All other ranges 7 1000VDC750VRMS AC VOLTS Ranges 200mV 2V 20V 200V 750V Accuracy 50 7 500 Hz Range i 10 4 lsd 500 71000 Hz Range i 15 4 lsd Input Impedance 10 M9 Overload Protection 200mV Range 7 500VDC350VRMS 15sec All other ranges 7 1000VDC750VRMS lsd least signi cant digit Current Measurements To make a current measurement select AC or DC and turn the rotary switch to the A area and select the appropriate range If you are unsure of the range set the rotary switch to the highest range and then lower it later Insert the probes into the mA CX and COM plugs and then connect them to the circuit It is important to note that the current meter must be part of the circuit as shown in Figure 33 on page 24 which shows a typical use of voltage and current meter If you expect a current in the 10 A range then the 10 A and COM plugs should be used and the rotary switch should be set to the 20m10A position If you are making a DC measurement then the mA CX probe should be connected to the most pos itive voltage and the COM probe should be connected to the most negative voltage If the probes are not connected correctly then the display will indicate a negative current but there is no need to switch the connections If you are making an AC voltage measurement the COM probe should be connected to the point closest to ground 22 Electrical Speci cations DC Current Ranges 200uA 2mA 20mA 200mA 10A Accuracy 200uA to 200mA range i1 11sd Accuracy 10A range i 2 3 lsd 200uA to 200mA range Input Protection Fast acting ceramic fuse 500mA on mA input 20N600V on 10A input AC Current Ranges 200uA 2mA 20mA 200mA 10A Accuracy i 15 4 lsd 50500Hz Accuracy 10A range i 25 4 lsd 50500Hz Input Protection Fast acting ceramic fuse 500mA on mA input 20A on 10A input lsd least signi cant digit Resistance Measurements The multimeter can also be use to measure the resistance of a resistor or other circuit component In making resistance measurements the resistor must be completely removed from the circuit The V 2 probe should be connected to one end of the resistor and the COM probe should be con nected to the other end Turn the rotary switch to the 2 area and select the appropriate range 2009 to 2M9 range Resistance Ranges 200g 2kg 20kg 200m 2MQ 20MQ 2000MQ Accuracy i 1 41sd Accuracy 20MQ range i 2 5 lsd Accuracy 2000MQ range i 5 101sd Overload Protection 500VDCAC RMS lsd least signi cant digit 23 Capacitance Measurements The multimeter can also be use to measure the capacitance of a capacitor In making capacitance measurements the capacitor must be completely removed from the circuit The mA CX probe should be connected to one end of the capacitor and the COM probe should be connected to the other end Turn the rotary switch to the F farad area and select the appropriate range Capacitance Ranges 2 nF 200 nF 2 HF 20 ME 200 ME 2 mF 20 mF Accuracy All ranges i 3 4 lsd Overload Protection F05N250V fast acting ceramic fuse lsd least signi cant digit Experiment DC Circuits 1 Use the multimeter to measure the actual resistances of a nominal 51 kg and a lOkQ resistor separately Then connect them in series and measure the total series resistance Compare with theory using experimental uncertainties Repeat with the resistors in parallel Nb 10k A NV 1919 uuloA U ammeter Figure 33 Series Circuit 2 Use the multimeter to measure the actual capacitance of a nominal 0022 mF and a 00152 mF capacitor separately Please note that many capacitors are polarized which means that the plus side of the capacitor must be connected to the mA CX input of the multimeter Then con nect them in series and measure the total series capacitance Compare with theory using experimental uncertainties Repeat with the capacitors in parallel 3 Wire up a simple series circuit Figure 33 on page 24 consisting of a power supply a resistor and two multimeters one as a voltmeter and one as an ammeter Make measurements for 24 power supply settings from 0 to 10 volts at each recording the readings on both meters Use the spreadsheet to graph the current versus voltage From the slope of the graph determine the resistance of the resistor Compare this value with its experimental uncertainties to its nomi nal value see its colorcodes and to the value you obtain using one Multimeter as an ohmme ter 4 Now add a 51 kg resistor in series with the 10 k9 already in place Measure the voltage across each resistor and the current through each resistor Compare with theory Sketch in your report the placements of the meters in the circuit that you used 5 Move the 51 kg resistor to be in parallel the 10 k9 resistor Measure the voltage across each resistor and the current through each resistor Compare with theory Sketch in your report the placements of the meters in the circuit that you used 25
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