# Week 27 (Feb. 26 - Mar. 1)

Reading: Direct current circuits (Chap. 28)

Key Topics:
electromotive force, kirchoff's circuit rules, resistors in parallel and series, ammeters and voltmeters, household wiring
Laboratory: This week, we will learn (i) how to draw simple electronic circuit diagrams, and (ii) how to analyze electronic circuits using Kirchhoff's rules. We will use an adjustable DC power supply (0 - 6 Volts, 1 Amp), assorted resistors, a tungsten filament light bulb, and two digital multimeters that can measure electric potential (volts), current (amps) and resistance (ohms). The lab consists of three parts; these parts are described in detail in the DC Circuits laboratory . Here is a brief summary of the three parts. I'd recommend doing A and B in order, but part C can be done anytime.
1. Ohm's law for a resistor. You will measure the current, I, passing through a resistor when a voltage V is applied across the resistor. Select a resistor of about 1000 Ohms. Be sure to set up your multimeters correctly so that one of them measures the voltage across the resistor (it acts as a voltmeter) and the other is in series with the resistor (it acts as an ammeter). Once you collect several data points (the more the better), make a plot of V vs. I. (Put V on the vertical and I on the horizontal axis) Is your data linear? If so, what is the slope? Is the slope equal to the resistance? There is a slight complication here: not all of the current measured by your ammeter is actually going through the resistor; a small portion of the current is being diverted into the voltmeter, which is in parallel with your resistor. How does this affect your measurements? In particular, are you over (or under) estimating the current passing through your resistor? And how does this affect the slope of your graph derived from your measurements?
2. Ohm's law for a tungsten filament: Repeat the previous measurements, but now use a small tungsten filament light bulb. To avoid burning out the bulb, you should put a resistor (try 50 or 100 Ohms) in series with the light bulb. The light bulbs are rated at 150 mAmp, so do not apply more than about 125 mAmps or they will burn out. Again, plot V vs I for your filament bulb. Is your plot linear? What does this imply? Finally: for the tungsten filament, plot the power P as a function of the current I. This is the rate of heat generation in the tungsten filament (in joules per second). P can be calculated from the Joule heating formula: P = I V.
3. Resistors in series and in parallel: Now we will use a multimeter as an ohm-meter. Turn the knob to the appropriate setting. Select three resistors between 10 and 90 ohms. First, measure the resistance of each by itself. Does your reading correspond to the color code on the resistor? Second, put the resistors in series and measure the resistance. Is it what you'd expect? Finally, put the resistors in parallel and measure the resistance. Is it what you'd expect?
General College Physics