Resistance! (Is Not Useless) 1. Measuring Resistance The resistance between two points is dened as the ratio of the voltage difference between those points to the current owing between 1hem. That is. R = WI This denition is often confused with Uhm's Law (M? = R I} for ohmic devices. in which R is assumed to be constant for all values of 1? and I. For many resistive devices this is true, if the temperature is relativer constant. But if the current varies signicantly. so will the temperature and hence the resistance. The light bulb is such a case. In this section, you will examine a standard tungsten lament light bulb and a special bulb with a carbon lament. The power consumption of the bulb lament [which is then given up as heat and light] is given by P=IV The Set-Up The circuit shown below uses a variable AC transformer {variac} as the voltage source. Think of it simply as a variable voltage battery. it has a standard AC outlet with two \"slots\". one of which can be thought of as the negative terminal and the other as the positive terminal [we will study the true nature of AC potentials later]. An ammeter {A} is placed in series with the variac and the light bulb and measures the current in the circuit. The ammeter has very little resistance and therefore has very little effect on the circuit. The voltmeter {V} is connected in parallel with the light bulb and measures the potential drop across it. The voltmeter has very high resistance and therefore, like the ammeter but for the opposite reason. has very little effect on the circuit. {Either or both the ammeter and voltmeter may be a multimeter. Your instructor will show how to set them up} Tungsten Filament The standard household light bulb uses a tungsten lament. Screw the standard bulb into the socket board and wire the ammeter. voltmeter and variac together as shown in the diagram. Be sure that the instructor has examined your work before tuming on the variac. 1. Choose increments of 1:1) volts for a range of 2'v' to 12m; by adjusting the variac voltage supply and reading the values from the voltmeter and ammeter. Both the voltage and current readings are in rms AC units. but may be treated the same as DB values. Record these values in a table. [Leave room for two additional columns in this table.) 2. Calculate the resistance of the lament at each voltage and record the values in the table. Calculate the power consumption of me lament at each voltage and record mesa values in the table. 3. Plot voltage vs the current. 4. Plot resistance vs the power consumption. Carbon Filament Repeat the procedure above for the special carbon light bulb provided. It can be easily identied by the cleady visible double loop lament. The carbon lament is similar to the original lament used by Edison. 1ll'lul'ithin a few years it was replaced with the cheaper and more durable tungsten lament. Report In your report, discuss the voltage vs current plot. Particularly. what does the shape of the plot indicate about the resistance of the lament. Similarly. discuss the resistance vs power consumption plot. Describe qualitatively how the carbon and tungsten laments differ. What can you say qualitatively about the temperature dependence of the resistance for each lament'? (Temperature increases with increasing power consumption.) 2. A Row of Bulbs The power rating of a light bulb is the power consumption (and output) of the bulb at 120 V. The light bulb platform contains four bulbs of unknown power ratings. Using the jumper leads, you may connect the bulbs in any series combination. Connect the bulbs in all possible series combinations and record your observations of the relative brightness of the bulbs in each case. (Note, it is not really necessary to do all combinations, but it might help.) Report From these observations how would you rank the bulbs in terms of the their power ratings? Justify the method by which you ranked the bulbs