Physics Work with your neighbor. Ask me for help if you re stuck. Don t hesistate to compare notes with nearby groups.

Transcription

1 Physics Work with your neighbor. Ask me for help if you re stuck. Don t hesistate to compare notes with nearby groups. Today we ll build on what we did Monday with batteries and light bulbs. Some new things we ll learn to work with today are a volt meter (for measuring potential difference), an ammeter (for measuring current), a breadboard (for conveniently connecting wires together), and resistors (whose resistance is more constant than that of a light bulb, because a resistor s temperature is normally much more constant than that of a light bulb s filament). (1) On your desk are two different kinds of light bulbs. The more spherical kind we ll call wide light bulbs, and the more oblong kind (like an American football) we ll call narrow light bulbs. Use one narrow light bulb, one wide light bulb, and two D-cell batteries. Screw the bulbs into sockets. Put the batteries into sockets. Connect everything in series, as shown in the above schematic diagram. You should see the narrow bulb light up but the wide bulb stay dim. Is current flowing through the wide bulb even though it is dim? Try unscrewing the wide bulb to see if its absence puts out the narrow bulb. (It should unless the two sides of the bulb socket are accidentally touching each other.) Which of the two light bulbs do you think has the larger resistance? (Hmmm! It can take a while to puzzle through this. We ll make some measurements below to check your guess.)

2 (2) Use the red Amprobe hand-held volt meter to measure the two battery emfs and the two light bulbs voltage drops. A volt meter needs to be connected in parallel with the circuit element whose voltage drop it is measuring: it is measuring the potential difference between two points in your circuit. The meter will read a positive voltage if the red lead (connected to the meter s V input) is at higher potential than the black lead (connected to the meter s COMmon input). To measure DC volts, turn the knob two clicks to the right to the V setting with the straight (non-wavy) line above it. If the meter shuts itself off after being left idle, rotate back to OFF, then back to the V setting. As an example, the above schematic diagram shows the volt meter connected to measure the voltage drop across the first light bulb. Work your way around the four circuit elements to measure the two battery emfs (E 1 and E 2 ) and the two lightbulbs voltage drops (IR 1 and IR 2 ). Check that the four values are consistent with the loop rule: E 1 + E 2 IR 1 IR 2 = 0. The two bulbs are in series with one another (after either removing or neglecting the volt meter through which negligible current flows), so the two bulbs currents are equal. Looking at your two voltage-drop values (IR 1 and IR 2 ), now which bulb do you think has the larger resistance?

3 (3) Use the orange Tenma bench-top current meter to measure the current flowing through your circuit. Since the circuit has only a single branch (when you either remove or neglect the volt meter), there is only one current to measure. In reality, both of your meters are capable of measuring voltage, current, resistance, and much more. But to save time, I ve wired up your red meter to measure voltage (volts) and I ve wired up your orange meter to measure current (amps). CAUTION! It is easy to blow the fuse in a current meter, thus rendering your current meter unusable until someone replaces the blown fuse. Never connect the two leads of a current meter directly across a battery or other voltage source: the current meter s internal resistance is tiny (usually something like an ohm), so connecting a current meter directly across a voltage source (e.g. a battery) creates a short circuit, which the meter s fuse protects against. If a current larger than 10 amps flows through the meter, its fuse will blow. Changing the fuse is not a big deal, but it s a hassle (it takes a few minutes), so be mindful when connecting a current meter. To measure the current flowing through a given circuit element, you need to connect the current meter (a.k.a. ammeter) in series with that object. To measure the current in a given branch of your circuit, the ammeter needs to become part of that circuit branch, such that the entire current from that branch passes through the meter. So you always need to interrupt a circuit (temporarily) to measure a DC current, while you can measure a voltage without taking anything apart. So measuring currents is usually much more of a hassle than measuring voltages. (For AC currents, there is a trick using magnetic fields, which lets you avoid interrupting the circult, if you have a clamp -type current meter.) Put the current meter in series with your circuit, as shown in the schematic diagram below. (You can keep the voltmeter in place as well, or remove it, as you prefer.) How much current is flowing through the narrow bulb? Do you need to measure anything else to know how much current is flowing through the wide bulb, or do you already have the information you need?

4 (4) Now connect the two bulbs in parallel, as shown in the above schematic diagram. Because the wires are so stiff, and because the clips on the bulb sockets can rotate, it may be tricky to do this without short-circuiting one or the other light bulb. Notice now which light bulb is the brighter one. Whoa! Mysterious! Is this observation consistent with your conclusion above about which bulb had the larger resistance? Remember that the power dissipated in a given circuit element is P = IV (where I is the current flowing through that circuit element, and V is the potential difference across the two ends of that element) and that for a resistor (or a filament), V = IR. You can combine these two equations in two different ways, to get P = I 2 R or P = V 2 /R. When the two bulbs are in series, is the current through the two bulbs the same, or is the voltage drop across each bulb the same? If the bulbs have different resistances, which one do you expect to be brighter when they are in series? When the two bulbs are in parallel, is the current through the two bulbs the same, or is the voltage drop across each bulb the same? If the bulbs have different resistances, which one do you expect to be brighter when they are in parallel? Make sure your reasoning for the above two paragraphs agrees with your observations of which bulb was brighter and your measurement that showed which bulb had the larger resistance.

5 (5) Continue with the parallel circuit you built on the previous page. Use the volt meter to measure the battery emfs and the light bulbs voltage drops. This circuit is no longer a single-branch circuit: there are two parallel branches. What does the loop rule tell you to expect about the relationship between the battery emfs and the voltage drop across the wide bulb? (It s a really simple result.) What does the loop rule tell you about the relationship between the battery emfs and the voltage drop across the narrow bulb? (Once you ve answered the previous question, this is such a simple result that you might think it s a trick question.) Now use the current meter to measure the current flowing out of the battery. Then measure the current flowing through the narrow bulb. Then measure the current flowing through the wide bulb. What relationship does the junction rule tell you to expect between these three currents? (My numbers worked out within a few percent. The main reason they may not agree perfectly is that the current meter s internal resistance is not completely negligible in comparison with that of the narrow light bulb.)

6 (6) If you re not yet out of time, try wiring up your series and parallel light-bulb circuits using the breadboard (the large angled one on your desk) to make connections between wires. It is actually somewhat awkward to connect the light bulbs and the batteries on the breadboard, but lighting up the light bulbs gives you a chance to make sure that you understand how connections are made on the breadboard, since success will cause things to light up in the expected way. The above photo shows a miniature breadboard. Your big breadboard works the same way: each little column of 5 holes is internally wired together. So choose one little 5-hole column for each point in your circuit at which two or more wires/components are connected. The thick wire that you have been using so far is too wide to fit into the holes in the breadboard. Instead, you will need to use the rolls of 22-gauge wire at your desk and the wire strippers. Use the scissors-like cutting edge of the wire strippers to cut wires to a comfortable length. Then use the slot marked 22 to strip about 1 cm of insulation off each end of each wire. Since there is only one of me in the room, this is a point where it will help to find someone nearby who has already figured this out, to help you to figure it out as well. Another opportunity to work with the breadboard is at the end of the two-page worksheet that is also on your desk: connecting and measuring your two batteries with three 100 Ω resistors as shown on the front page, or (more complicated!) five 100 Ω resistors as shown on the back page.

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