[ OHM +KIRCHOFF’S LAWS]STUDIO Unit 08PHY-2054 College Physics IIDrs. Bindell and DubeyTHIS IS VERYIMPORTANT!!It is very important that you make thepredictions in these exercises BEFORE youmake any measurements. The learningtakes place when you think about what ishappening and make the predictions. Afteryou make the prediction, if you are correct,you probably understand the concept but ifyou are not correct, you can think aboutwhy you were wrong and, after youdetermine the reason, you will grasp theconcept on the second try.PART I -KIRCHHOFF’S FIRST LAW(Modified by JBB / Thacker from Lillian C. McDermott and the Physics Education Group, Physics byInquiry Volume II, John Wiley and Sons, NY, 1996)Objectives- to understand how to use an ammeter to measure current- to understand Kirchhoff’s First RuleIN A PREVIOUS UNIT WE PROVIDED THIS IMPORTANT DEFINITION:The amount of charge per unit time passing a cross-sectional area of a wire is calledcurrent. The symbol for current is I. Mathematically, tQIThe unit of current is the Ampere (A). One Ampere is equal to one Coulomb per second:1A = 1C/s.Be sure that you understand it. To check the concept, if there is a current of two amperesflowing in a circuit, how much charge passes through a cross-section of the wire in twominutes?ANSWER:KIRCHOFF’S LAWS Page - - STUDIO PHYSICS2Equipment:1 ammeter8 alligator clips1 bulb1 socket1 30cm nichrome wire1 60cm nichrome wire1 90cm nichrome wire2 45cm nichrome wire1 battery 1 battery holder1 switch1.1 We have been using the brightness of a bulb as an indicator of the amount of currentpassing through the bulb. In this section we will begin a quantitative analysis of circuits.We will measure the magnitude of the current through parts of a circuit with an ammeteran electronic device that measures current and provides a digital answer. Later in thiscourse, we will discuss how this device actually works but for the moment we will acceptthe results on faith. We will also learn more about the concept of resistance. Based on our observations, we will develop a model for current in which the current isnot “used up”. Another way of expressing this is to say that current is conserved. We willexamine the conservation of current quantitatively. In order to investigate the conservation of current, we will use linear resistors. The linearresistors we will use are either pieces of nichrome wire or commercial resistors. Yourinstructors will decide which will be used.To measure current we will use the ammeter. An ammeter, when connected in series in acircuit, measures the current through the circuit with very little change in the resistance ofthe circuit. We use the multimeter for this purpose as was discussed previously. Makesure that you use the “A” scale and remember that “m” refers to milliamperes. We have found that it is impossible from our observations to tell the direction of currentthrough the battery. We will follow the widely used convention of assuming that the flow(of positive charge) is from the positive terminal of the battery through the circuit to thenegative terminal of the battery, and from the negative to the positive terminal within thebattery. Ammeters and other electrical instruments should be connected in a circuit in thesame sense, with the terminal marked positive closer to the positive end of the battery.Note that the meter will tell you if you have it connected in the wrong direction bydisplaying a negative sign before the reading.KIRCHOFF’S LAWS Page - - STUDIO PHYSICS3Consider the following to understand what the previous paragraph stated.1. The battery (or voltage source) maintains apotential difference (voltage) across the ends of thewire.2. This voltage establishes an electric field (E) insidethe wire.3. The electric field points in the direction ofdecreasing electric potential and drives(conventional) current (I) through the wire, from thepositive terminal to the negative terminal.4. The current is in the direction of decreasingpotential (V), i.e. from the positive terminal at ahigher potential toward the negative terminal. 5. The current (positive charge motion) flows from the + terminal to the – terminal in thecircuit itself. It then flows through the battery from – to +. Discuss what this means withyour group and if you can’t follow it, ask your instructor to explain it.It might help towrite this down in a way that is clear to you.a. Hook up the ammeter in series with the bulb in a singlebulb circuit with the positive end of the ammeter closer tothe positive terminal of the battery, as in each of the twodiagrams below. Record the reading of the current in eachcase. Are the current readings consistent with yourprevious observations? KIRCHOFF’S LAWS Page - - STUDIO PHYSICS4Explain.Next, take the same readings, with the ammeter leads reversed (with the ammeterconnected in the wrong sense). How do the readings with the ammeter connected in thewrong sense, compare to the readings with the ammeter connected in the correct sense? b. Set up a circuit with a 30cm length of nichrome wire in series with an ammeter, asshown in the picture below, and record the ammeter readings. You can do this by using asingle 90 cm length but using the wire’s alligator clips to only clip off the required length.The symbol represents a resistor that, in this case, is made out of nichrome. Theresistance is small so the current will be high. Discuss the possible use of a power supplywith your instructor. This may save some battery life.KIRCHOFF’S LAWS Page - - STUDIO PHYSICS5c. Repeat part b with 60cm and 90cm lengths of nichrome wire. Predict what you expectto observe. Repeat with a second battery in series with the first.Record your results in the table. Length Current MeasuredOne BatteryCurrent MeasuredTwo Batteries30 cm60 cm90 cm.Note: you may be permitted to use a DC power supply to do this part of the experiment.Or not!d. Compare the ammeter reading of the 30cm, 60cm, and 90cm lengths of nichromewire. Does it seem reasonable? Can you make a statement